π d2 A= π r2 = Slot 4 Head D Shank RECESSED HEAD OR PHILLIPS SCREW Thread RROurUalN sDtru HctEurAesD in the troRpAicIsSED d e s i g n a n d d e v e Cl oOpUm Ne nTEt RSUNK HEAD Core 4 × 573 Point d = = 27 mm (min) COUNTERSUNK WOODEN PLUG HEAD SCREW π The plug in the drilled hole will expand as the screw is driven PLASTIC PLUG Squared Hexagonal Cup head head head Square neck WASHER WASHER Hexagonal Square nut nut LUG or MACHINE BOLT COACH BOLT COACH SCREW PLAIN WASHER SPRING WASHER 2/3 L Geoffrey C. Mrema Rural Infrastructure and Agro-Industries Division, FAO Lawrence O. Gumbe University of Nairobi Hakgamalang J. Chepete Botswana College of Agriculture Januarius O. Agullo University of Nairobi Rural structures in the tropics d e s i g n a n d d e v e l o p m e n t FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome, 2011 About CTA The Technical Centre for Agricultural and Rural Cooperation (CTA) was established in 1983 under the Lomé Convention between the ACP (African, Caribbean and Pacific) Group of States and the European Union Member States. Since 2000, it has operated within the framework of the ACP-EU Cotonou Agreement. CTA’s tasks are to develop and provide products and services that improve access to information for agricultural and rural development, and to strengthen the capacity of ACP countries to acquire, process, produce and disseminate information in this area. CTA is financed by the European Union. CTA Postbus 380 6700 AJ Wageningen The Netherlands www.cta.int Correct citation FAO. 2011. Rural structures in the tropics. Design and development. Rome. The designations employed and the presentation of material in this information product do not imply the expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations (FAO) concerning the legal or development status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. The mention of specific companies or products of manufacturers, whether or not these have been patented, does not imply that these have been endorsed or recommended by FAO in preference to others of a similar nature that are not mentioned. The views expressed in this information product are those of the author(s) and do not necessarily reflect the views of FAO. ISBN 978-92-5-107047-5 All rights reserved. FAO encourages the reproduction and dissemination of material in this information product. Non-commercial uses will be authorized free of charge, upon request. Reproduction for resale or other commercial purposes, including educational purposes, may incur fees. Applications for permission to reproduce or disseminate FAO copyright materials, and all queries concerning rights and licences, should be addressed by e-mail to copyright@fao.org or to the: Chief, Publishing Policy and Support Branch Office of Knowledge Exchange, Research and Extension FAO Viale delle Terme di Caracalla 00153 Rome, Italy © FAO 2011 iii Contents Preface x CHAPTER 3 Acknowledgements xi Graphical techniques 21 About the authors xii Introduction 21 Computer-aided design and drafting (CADD) 21 CHAPTER 1 CADD hardware and software 21 CADD design applications 22 Introduction 1 Projections 22 Scope of the textbook 2 Isometric projection 23 Oblique projection 23 Further reading 3 Axonometric projection 23 Perspective 23 CHAPTER 2 Three-Dimension drawing and modelling Planning farm and rural structures 5 in CADD 24 Introduction 5 Printing and plotting process 25 What is planning? An overview 5 Selecting a scale for drawings 25 Forms of planning 6 Standard paper sizes used for plotting 25 Title box 26 Regional planning 6 Architectural symbols 26 Urban planning 6 Rural planning 7 Documentation for a building project 28 Infrastructure planning 7 Environmental planning 8 Site plan 28 Economic planning and feasibility 8 Plan of external service runs 28 Economic planning of the farm operation 8 Foundation plan 28 Plan view 28 An approach to building planning 11 Section 28 Elevation 29 Background information 11 Details 29 Calculations 11 Plan of electrical installations 29 Analysing the activities 11 Plan of water and sanitary installations 29 Room schedule 13 List of drawings 29 Communication schedule 13 Technical specifications 29 Functional design of the building 13 Functional and management instructions 29 Finalization of sketching 14 Bill of quantities 29 Final design 14 Cost estimate 29 Farmstead planning 14 Time schedule 30 Zone planning 14 Model buildings 30 Farmstead planning factors 15 Physical model 30 Safety and fire protection 15 Computer-generated models 31 Fire resistance in materials and construction 15 Review questions 31 Classification of fire hazards 15 Fire separation 15 Further reading 32 Evacuation and fire extinguishers 15 Bushfire 16 Project planning and evaluation techniques 16 CHAPTER 4 Project planning 16 Geospatial techniques 33 Project evaluation and techniques 16 Introduction 33 Environmental management 17 Survey of a building site 33 Working project 18 Distances 33 Angles 34 Further reading 19 Vertical alignment 35 Leveling 35 Chain surveying 35 Setting out the building work 37 iv Excavation depth control 38 Mixing 74 Volume of earth to be removed 39 Placing and compaction 75 Formwork 76 Modern geospatial technologies 40 Curing concrete 77 Remote sensing 40 Finishes on concrete 78 Global Positioning System (GPS) 40 Reinforced concrete 78 Principle of GPS positioning 41 Geographic Information Systems (GIS) 42 Concrete blocks, sand and cement blocks 79 Digital mapping 42 Block manufacturing 79 Decorative and ventilating blocks 79 Review questions 43 Mortar 79 Further reading 43 Finishing mortar 81 Plastering and rendering 81 CHAPTER 5 Ferrocement 81 Construction materials 45 Fibre-reinforced concrete 81 Asbestos cement (AC) 82 Introduction 45 Sisal-fibre-reinforced cement (SFRC) 82 Making corrugated reinforced roofing sheets 83 Wood 45 Walls using the sisal-cement plastering technique 83 Hardwoods versus softwoods 45 Wood characteristics 45 Metals 84 Defects in wood 46 Corrosion 84 Corrosion-inhibiting coatings 84 Poles and timber 46 Wooden poles 46 Building hardware 84 Sawing timber 47 Nails 84 Seasoning of timber 48 Screws and bolts 85 Grades and sizes for timber 48 Hinges 86 Strength of wood 49 Locks and latches 86 Timber preservation 52 Glass 86 Wood preservatives 52 Plastics 87 Manufactured building boards 53 Thermoplastics 87 Plywood 54 Thermosetting plastics 87 Other manufactured boards 54 Plastics used for seepage protection in dams 87 Plastic components used with dam liners 88 Other wood products 55 Rubber 88 Other organic materials 55 Bamboo 55 Bituminous products 88 Natural fibres 57 Paints 88 Natural stone products 57 Painting 89 Estimation of quantities of paint required 89 Earth as a building material 58 Oil- and resin-based paints 89 Soil classification 58 Water-based paints 89 Soil-testing methods 60 Soil stabilization 61 Review questions 90 Cob 62 Wattle and daub (mud and wattle) 62 Further reading 91 Clay/straw 63 Rammed earth 63 Adobe or sun-dried soil (mud) blocks 63 Stabilized-soil blocks 64 CHAPTER 6 Comparison of masonry units made of various materials 66 Basic mechanics 93 Burnt-clay bricks 66 Basic principles of statics 93 Brickmaking 66 Static equilibrium 93 Force 93 Binders 68 Resolution of a force 94 Lime 68 Loading systems 99 Cement 69 Shear force and bending moment of beams 100 Pozzolana 69 Forces in pin-jointed frames 103 Concrete 70 Mechanics of materials 104 Properties of concrete 70 Direct stress 104 Ingredients 71 Strain 106 Batching 72 v Elasticity 106 Pressure exerted by retained material 143 Factor of safety 106 Designing for earthquakes 146 Structural elements and loading 107 Applied loads 107 Review questions 147 Principle of superposition 107 Effects of loading 108 Further reading 147 Structural elements 108 Properties of structural sections 110 CHAPTER 8 Area 110 Centre of gravity or centroid 110 Elements of construction 149 Moment of inertia 110 Section modulus 111 Introduction 149 Radius of gyration 112 Loads on building components 149 Review questions 113 Footings and foundations 150 Further reading 113 Soil bearing 151 Site drainage 151 Foundation footings 152 CHAPTER 7 Footing trenches 153 Types of foundation 153 Structural design 115 Foundation materials 154 Foundation construction 154 Introduction 115 Structural design process 115 Concrete foundations 155 Philosophy of designing 115 Protective elements for foundations 158 Design aids 115 Design codes 116 Walls 160 Design of members in direct tension Types of building wall 161 and compression 116 Floors 169 Tensile systems 116 Solid or grade floors 171 Short columns 117 Suspended or above-grade floors 172 Floor finishes 173 Design of simple beams 118 Bending stresses 118 Roofs 173 Horizontal shear 119 Types of roof 174 Maximum horizontal shear force in beams 120 Roofing for pitched roofs 179 Deflection of beams 120 Rainwater drainage from roofs 190 Design criteria 121 Bending moments caused by askew loads 122 Doors 191 Universal steel beams 123 General characteristics of doors 191 Continuous beams 124 Types of door 191 Standard cases of beam loading 124 Door frames 193 Simple locks for barn doors 193 Composite beams 124 Windows 195 Built-up timber beams 126 Stairs and ladders 196 Columns 126 Buckling of slender columns 126 Electrical installations 199 Axially loaded timber columns 129 Electricity supply 199 Axially loaded steel columns 130 Earthing and bonding 200 Axially loaded concrete columns 131 Distribution circuits 200 Eccentrically loaded timber and steel columns 131 Artificial lighting 201 Plain and centrally reinforced concrete walls 132 Electrical motors 202 Trusses 133 Lightning conductors 202 Review questions 202 Frames 136 Further reading 203 Connections 138 Timber structure 138 Connections in steel structures 140 CHAPTER 9 Stability 140 Building production 205 Retaining walls 142 Introduction 205 Wall failure 142 vi The building production process 205 Animal moisture and heat production 228 Climatic factors 228 Methods of construction 205 Effect of climatic factors on livestock performance 230 Traditional buildings 207 Microbiological environment 231 Post-traditional building 208 Other environmental factors 231 System building 208 Cattle housing 232 Prefabrication 208 Herd profiles 232 On-site prefabrication 208 General housing requirements 232 Off-site prefabrication 208 Calf pens 235 Housing for the small herd 237 Dimensional coordination and standardization 209 Housing for medium to large herds 237 Milking and milk handling 239 Building legislation 210 Milking parlour for a medium-size herd 242 Milking parlour 242 Feeding equipment 245 Construction costing 211 Watering equipment 245 Quantity surveying 211 Feed handling 248 Costing 214 Manure handling 249 Cattle dips 250 Economic feasibility 215 Cattle spray race 252 Building life (depreciation period) 215 Interest 216 Pig housing 253 Repairs and maintenance 216 Management improvements 253 Insurance and taxes 216 Management systems in intensive Annual cost 216 commercial pig production 254 Cash flow and repayments 217 Determining the number of pens and stalls required in a pig unit 255 Organization for construction of small buildings 217 Space requirement 256 Forms of organization 217 General requirements for pig housing 258 Forms of payment 218 Housing for a small-scale pig unit 258 Housing for the medium-scale pig unit 259 Tendering 218 Housing for the large-scale pig unit 262 The tender procedure 219 Special arrangements for warm climates 264 Methods of tendering 219 Feed troughs and feed storage 265 Evaluation of tenders 219 Watering equipment 265 Manure handling 267 Contracts 219 Poultry housing 267 Specifications 220 General housing requirements for chickens 267 General specification 220 Housing systems for layers 268 Planning for continuous production 276 Progress chart 220 Housing for breeders 276 Brooders 276 Inspection and control 222 Housing for pullets and broilers 277 Equipment and stores 278 Feeders 279 Safety at building sites 222 Duck housing 281 Geese housing 283 Building maintenance 222 Housing for turkeys 283 Review questions 222 Sheep and goat housing 284 Management systems 284 Further reading 223 Housing 284 Parasite control 286 CHAPTER 10 Rabbit housing 287 Management systems 287 Livestock housing 225 Hutches 288 Equipment and store 289 Introduction 225 Slaughter slabs and slaughterhouses 291 Animal behaviour 225 Gantry hoist 291 Introduction 225 Pig slaughter 292 Behaviour patterns 225 Poultry slaughter 294 Social rank order 226 General recommendations for design and construction 294 Animal behaviour studies 226 Animal behaviour and building design 226 Review questions 297 Animal environmental requirements 227 Further reading 297 Heat regulation 227 vii CHAPTER 11 Psychrometry 319 Rural buildings 299 Properties of moist air 319 Psychrometric chart 320 Introduction 299 Air–water-vapour mixture processes 320 Adiabatic mixing of two air streams 322 Space requirements 299 Moisture transmission 323 Family cultural and social requirements 299 Vapour barriers 324 Special requirements of rural dwellings 300 Condensation on surfaces and within walls 324 Heating and cooling loads 324 Categories of rural houses 300 The cooling load 324 The heating load 324 Function and communication schemes 300 Methods of estimating cooling and heating loads 325 Contemporary designs 301 External-access type 302 Overview of heating, ventilation and Courtyard type 302 air-conditioning systems and equipment 325 Corridor type 302 Heating systems 325 Central-room type 302 Air-conditioning systems 325 Functional requirements for different Ventilation and air-handling systems 325 Electrical systems 326 rooms and spaces 302 Sleeping 302 Review questions 326 Meeting and rest 304 Taking meals 304 Further reading 326 Preparing and cooking food 304 Storage 305 Washing 306 Reading and writing 307 CHAPTER 13 Entrance 307 Ventilation 329 Improvement of existing dwellings 307 Introduction 329 Contemporary farm dwellings 308 Climatic zones 329 Farm workshop facilities 308 Ventilation process 329 Determination of ventilation rates 330 Machinery and implement storage 310 Heat balance for determination of maximum ventilation rate 330 Fuel and chemical storage 311 Moisture balance for determination of minimum Storage of hazardous products 311 ventilation rate 331 Storage of fertilizers and other non-hazardous materials 312 Natural ventilation 332 Review questions 312 Mechanical ventilation 334 Fans and blowers 335 Further reading 312 Ventilation system design: cool climates 337 Air distribution 337 CHAPTER 12 Ventilation controls 338 Ventilation design example 339 Fundamentals of heating and cooling 313 Cooling 339 Heat terminology 313 Evaporative cooling 339 Refrigeration 340 Heat transfer 313 Conduction 313 Review questions 341 Convection 313 Radiation 314 Further reading 342 Thermal resistance of building components 314 Insulating materials 315 CHAPTER 14 Selecting insulation 315 Surface resistances 315 Greenhouses 343 Thermal resistance of pitched roof spaces 315 Overall heat transfer coefficients 315 Introduction 343 Location of the greenhouse 343 Rate of overall heat loss or gain from a building 318 Greenhouse design parameters 345 Solar load 318 Calculating greenhouse cooling requirements 348 Example of heat loss from buildings 318 viii Heating 349 Review questions 385 Methods of heat conservation 349 Further reading 385 Air quality in greenhouses 349 Review questions 350 CHAPTER 17 Further reading 350 Rural roads 387 Introduction 387 Road location 387 CHAPTER 15 Gradients 387 Handling semi-perishable Curves 388 and perishable crops 351 Slopes 388 Camber 388 Semi-perishable crops 351 Cross-section of a simple earth track 388 Cross-section of an upgraded earth road 388 Properties 351 Storage requirements for potatoes and other Erosion of earth roads 389 horticultural crops 351 Storage without buildings 352 Side drains 389 Storage in multipurpose buildings 353 Mitre drains 390 Naturally ventilated stores 353 Diversion banks 390 Larger stores 353 Catchwater drains 390 Grading and handling facilities 355 Road construction 390 Perishable crops 357 Stumping and clearing 390 Storage requirements 357 Construction of side drains 390 Storage structures for perishables 359 Road maintenance 391 Common cooling methods for produce 360 Transportation of horticultural crops 360 Minor river crossings 391 Refrigerated trailers 360 Splashes and drifts 391 Open vehicles 360 Embanked drifts 392 Culverts 393 Review questions 361 Simple bridges 393 Further reading 361 Vehicle access to farmsteads 395 Vehicle dimensions 396 Planning space for vehicles in farm drives and courtyards 396 CHAPTER 16 Review questions 397 Grain crop drying, handling and storage 363 Further reading 397 Introduction 363 CHAPTER 18 Grain drying 363 External facilities 399 Properties of grains 363 Requirements for safe storage 363 Fencing 399 Drying theory 364 Security 399 Drying systems 366 Improved livestock management 399 Natural drying 367 Artificial drying 368 Types of fences 399 Drying problems 374 Instruments 375 Wire fences 400 Fencing posts 400 Grain storage 375 Plastic poles 401 Wire fence construction 401 Parameters 375 Other types of fence 403 Solid-wall bins and silos for bulk storage 376 Improved traditional bins 376 Fencing accessories 404 Bag storage 378 Insect control 380 Wire gates 404 Rodent and bird control in stores 382 Pole-and-chain gate 404 Storage management, hygiene and safety 383 Field gates 404 Stiles 404 Grain-handling equipment 384 Person-pass 404 Cattle grid 405 Belt-and-bucket elevators 384 Wheel splashes 406 Auger (screw conveyors) 384 Flat-belt conveyors 384 Animal-handling facilities 406 Chain-and-slat conveyors 384 Sack elevators 384 Main yard 407 Dumping pits 384 Cattle races and crushes 407 ix Loading ramps 408 CHAPTER 20 Sorting alley 409 Sales yard 409 Rural energy 447 Review questions 411 Introduction 447 Further reading 411 Energy sources 447 Rural energy choices 447 Rural energy supply routes 448 CHAPTER 19 Biomass energy 448 Water supply and sanitation 413 Electricity 448 Water requirements: quantity and quality 413 Rural electrification 449 Quantity for domestic use 413 Quantity for livestock 413 Fossil fuels 449 Quality of water 413 Hydroelectric power 450 Water storage 414 Small hydro and micro hydropower 450 Catchment areas 414 Roof catchments 414 Cogeneration from agricultural industry 450 Partial run-off catchments 415 Storage requirements 415 Solar energy 450 Selection of tank size 415 Solar flux 451 Calculation of tank and reservoir volumes 417 Application of solar energy 451 Sand dams 419 Solar collectors 452 Photovoltaic cells 453 Development of sand dams 419 Structural design criteria 420 Biogas 454 Methods for abstracting water from sand dams 420 Biogas digesters 455 Quality status of water from sand dams 420 Rock catchment dams 421 Wind power 456 Power content of wind 456 Wells 421 Wind turbine power 456 Analysis of wind regime 456 Location of well site 421 Wind turbine topologies 458 Types of well casing 421 Generation of electrical energy from wind turbines 458 Lift for wells 422 Hybrid power systems 459 Pumps 423 Hand pumps 423 Energy efficient rural buildings 460 Power-driven pumps 423 Choosing a pump 425 Energy audits 460 Pump storage tanks 426 Pipe flow 427 Water-system problem 428 Energy economics 460 Water system design features 429 Review questions 461 Water treatment 430 Boiling 430 Further reading 461 Chlorination 430 Water treatment by solar disinfection (SODIS) 431 APPENDICES Open channel flow 431 I. SI base units 463 Rural sanitation 433 Wastewater treatment and sanitation 434 II. Conversion tables 465 Pit latrines 435 Aqua privies 437 III. Greek alphabet 467 Septic tanks 438 IV. List of symbols 469 Waste management 439 Sources of waste 439 V. Design tables and charts 471 Waste collection 440 Waste storage 441 Transportation of waste 441 VI. Number of pens and stalls Waste treatment 441 required in breeding pig units of various sizes 481 Review questions 444 Further reading 445 Index 483 x Preface There is a growing awareness of the need for better rural structures and services in many developing countries. For many years, rural buildings and structures in numerous countries have been built either traditionally with few improvements, or in an inadequate and often overly expensive way, guided by people with insufficient knowledge of the special technical, biological and socio-economic problems involved. Rural buildings and structures have become an important part of integrated rural development programmes. As a large proportion of the food grain produced in Africa is stored on-farm, it is very important to develop effective storage methods and structures, especially for the modern, high-yielding grain varieties being adopted by farmers, which are more susceptible to pests than traditional types. Improved management and breeding programmes to increase livestock production have also created a need for more appropriate animal housing. The subject of rural structures and services needs to be included at all levels of the agricultural education system to assist the rural population still further in raising their standard of living. Specialists in rural structures and services need to have a thorough knowledge of farming systems, crop and livestock production systems and climate factors, as well as a genuine understanding of rural life and the farmer’s social and economic situation. They should also be familiar with the full range of building materials and types of construction, from traditional indigenous to industrially produced, as they apply to rural structures. They must be able to select appropriate installations and equipment for rural buildings. This knowledge will enable them to produce specifications, in cooperation with the farmer, for functional building designs that provide a good environment and durable construction, thereby contributing to efficient and economically sound farm operations. Further important tasks for specialists in rural structures and services are interpreting and explaining the drawings and technical documentation to farmers, as well as supervising the construction work. However, they should be aware of the need to consult other specialists in related fields where necessary. This book is an effort by FAO to compile an up-to-date, comprehensive text on rural structures and services in the tropics, focusing on structures for small- to medium-scale farms and, to some extent, village-scale agricultural infrastructure. The earlier edition, entitled Farm structures in tropical climates. A textbook for structural engineering and design, was published in 1986, and was based on material developed as part of the FAO/SIDA Cooperative Programme: Rural Structures in East and South-East Africa. The programme was established to help member countries to develop functional, low-cost rural structures using locally sourced construction materials and skills wherever possible. For over two decades, the earlier edition has been used as a standard textbook for teaching undergraduate and postgraduate courses on rural structures and services in universities throughout sub-Saharan Africa. As part of its normative programme on rural infrastructure development, the FAO Rural Infrastructure and Agro-Industries Division (AGS) commissioned a team of three professional engineers who participated in teaching courses on rural structures and services to review and rewrite the earlier edition, whilst examining the socio-economic and technological developments that have taken place over the past 25  years. This team, which worked during the period 2010–2011 under the direct supervision of former AGS Director, Professor Geoffrey C. Mrema, comprised Professor  Lawrence  O. Gumbe and lecturer Januarius  O. Agullo from the University of Nairobi, Kenya, and Dr Hakgamalang J. Chepete from Botswana College of Agriculture. We trust that this second edition will help to improve teaching – at all educational levels – on the subject of rural buildings in developing countries of the tropics and that it will assist professionals currently engaged in providing technical advice on rural structures and services, from either agricultural extension departments or non-governmental rural development organizations. We also trust that this book will provide technical guidance in the context of disaster recovery and rehabilitation, for rebuilding the sound rural structures and related services that are key to development and economic sustainability. While this book is intended primarily for teaching university- and college-level agricultural engineering students about rural structures and services, it is our hope that resources will be made available to produce textbooks based on this material for teaching at other educational levels. Although parts of the background material relate specifically to East and Southeast Africa, the book’s principles apply to the whole of tropical Africa, Latin America and South Asia because, while building traditions may vary, the available materials are similar. xi Acknowledgements This book is based on the FAO publication Farm structures in tropical climates, published in 1986. The original material and documentation was produced by the FAO/SIDA Cooperative Programme: Rural Structures in East and South-East Asia. The lecturers underwent two six-month intensive training courses by the SIDA programme between 1981 and 1983. Our grateful thanks go to the University of Nairobi and the consulting firm LOG Associates, Nairobi, Kenya. In the University of Nairobi, we wish to thank: Prof. Francis W.O. Aduol, Principal, Kenya Polytechnic University College; Prof.  Bernard N.K.  Njoroge, Principal, College of Architecture and Engineering; Prof.  Washington H.A. Olima, Department of Real Estate and Construction Management, College of Architecture and Engineering; Dr Duncan O. Mbuge, lecturer, Department of Environmental and Biosystems Engineering, College of Architecture and Engineering; Robert M. Mathenge, technologist, Department of Environmental and Biosystems Engineering, College of Architecture and Engineering. Further special thanks go to: Josef Kienzle (FAO) for overall project coordination; Rachel Tucker (FAO) and Jenessi Matturi (Technical Centre for Agricultural and Rural Cooperation – CTA) for facilitating the copublishing agreement between FAO and CTA; Andrea  Broom, Jim  Collis and Madeline Grimoldi for language editing; Larissa D’Aquilio (FAO) for production process management; Francesca Komel for redrawing most of the original illustrations in vector format, and Simone  Morini for reproducing drawings in high-resolution, for desktop publishing and cover design. xii About the authors GEOFFREy C. MREMA Prof Geoffrey C. Mrema, holds a B.Sc. degree in Engineering (Hons) from the University of Nairobi, Kenya; an M.Sc. in Agricultural Engineering from the University of Newcastle-upon-Tyne, UK; and a Ph.D. in Agricultural Engineering from the National University of Ireland. From 1973 to 1984, he lectured at the Department of Agricultural Engineering and Land Planning of the Faculty of Agriculture of the University of Dar-es-Salaam, Morogoro, Tanzania. This Faculty of Agriculture became, in July 1984, the Sokoine University of Agriculture (SUA). In addition to advancing from Lecturer in 1979 to Professor by 1987, he also was head of the Department of Agricultural Engineering and Land Planning (1979–1987) as well as Associate Dean of the Faculty of Agriculture (1985–1987) until he was appointed the inaugural Dean of the Faculty of Agriculture of the University of Botswana and Professor of Agricultural Engineering, where he served during the period 1987–1995. From 1995 to 2001, he was First Executive Secretary of the Association for Strengthening Agricultural Research in Eastern and Central Africa (ASARECA) in Entebbe, Uganda. In 2001, he joined FAO as Director of the Agricultural Support Systems Division at FAO Headquarters in Rome, Italy. During the period 2005/06 he served as the Subregional Representative of FAO in Eastern and Central Africa based in Harare, Zimbabwe. He returned to Rome in August 2006 as Director of the Rural Infrastructure and Agro-Industries Division (AGS), and he held this position until 31 March 2011, when he retired from FAO service on reaching the UN mandatory retirement age. He is now based at the Department of Agricultural Engineering and Land Planning of SUA, where in addition to lecturing he undertakes consultancy missions for international and national agencies. LAWREnCE O. GuMBE Prof Lawrence O. Gumbe is a professor in the Department of Environmental and Biosystems Engineering at the University of Nairobi, Kenya. He holds a Ph.D. degree from Ohio State University, USA; a M.Sc. degree from Cranfield University, UK; and a B.Sc. degree from the University of Nairobi, Kenya. He is a Registered Consulting Engineer with the Engineers Registration Board of Kenya. He has been a member of several learned societies including the Institution of Engineers of Kenya (IEK); the Association of Consulting Engineers of Kenya (ACEK); the Architectural Association of Kenya (AAK); the American Society of Agricultural Engineers (ASAE); the Kenya Society of Agricultural Engineers (KSAE); the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE); and the Kenya National Academy of Sciences (KNAS). He has been a visiting scholar in several universities and has worked as a consultant in Kenya, Malawi, Uganda, Tanzania, Sudan, Ethiopia, Rwanda, Zambia, the UK and USA. HAkGAMALAnG J. CHEPETE Dr Hakgamalang J. Chepete is a Senior Lecturer in the Department of Agricultural Engineering and Land Planning at the Botswana College of Agriculture, an associate institute of the University of Botswana. He holds both Ph.D. and M.Sc. degrees in Agricultural Engineering from Iowa State University in USA, and a B.Sc. in Agriculture from the University of Botswana. His area of concentration is Agricultural Structures and Environment. He is a member of the American Society of Agricultural and Biological Engineers (ASABE) and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). He has been a visiting research scientist at Iowa State University, USA. JAnuARIuS O. AGuLLO Januarius O. Agullo lectures in the Department of Environmental and Biosystems Engineeering of the University of Nairobi. He has B.Sc. and M.Sc. degrees from the University of Nairobi and is a Registered Graduate Engineer and a member of the Kenya Society of Agricultural Engineers. He has taught courses in thermodynamics, solid mechanics, structures, information technology, engineering graphics and post-harvest technology. He has also consulted in the areas of irrigation engineering, data analysis and systems optimization. 1 Chapter 1 Introduction The subject of this book is rural structures and services improved livestock housing and services. For instance, in tropical regions. Although it has been written with in parts of eastern Africa many small-scale farmers have a special focus on the situation in eastern and southern invested in intensive dairy projects where the dairy Africa, the principles apply to most of the tropics, albeit cows are zero-grazed. In addition, general development, with some modifications to cater to local conditions. In which has led to improved standards of living for many parts of Africa rural structures, including farm the rural population, has led to increased demand buildings, have been built using traditional methods for durable, comfortable and healthy dwellings with and materials. However, rural development and clean water, sanitation facilities, telecommunications globalization are bringing about significant changes in and community infrastructure. Most of these facilities rural areas of Africa. would normally be owned by the individual farmer, Whereas in the past it was common to find only while some may be communally owned. small thatched and/or mud houses, nowadays brick- All these rural structures and services play a walled houses roofed with corrugated iron sheets and/ significant role in increasing agricultural production, or tiles are quite common in many rural areas of eastern helping countries to achieve food security. These are and southern Africa. Some of these new structures some of the most important investments that rural are an improved traditional design constructed using dwellers will make in their lifetime. FAO projections industrial building materials. Others are replications show that, over the next 40 years, significant investment of urban building designs and often fail to cater to the must be made in rural infrastructure if the world is to special technical, biological, physical and economic feed a global population of more than nine billion. characteristics of rural areas, where in most cases The general thinking on the future of rural areas, agricultural production is combined with processing and countries in general, is reflected in the various and dwelling. policies of developing countries, such as: Vision 2030 in There is therefore a growing need for improved Kenya; Vision 2025 in Tanzania and Uganda; Growth, rural structures in most parts of Africa. In urban Employment and Redistribution (GEAR) in South areas, there are town/municipal engineers responsible Africa; Vision  2016 in Botswana; and Vision  2020 for enforcing the building code, while in rural areas, in Nigeria. All these policy statements include the rural development officers are normally called upon to primary objective of accelerating rural development. provide technical advice on improving rural structures. As an example, the aim of Vision 2030 is to transform Rural dwellers, who in most parts of Africa are either Kenya into a newly industrializing, “middle-income crop and/or livestock farmers, rely on the technical country providing a high quality life to all its citizens advice of agricultural extension workers who, in most by the year 2030”. The vision aspires to a country firmly countries, also serve as general rural development interconnected through a network of roads, railways, practitioners. Improved rural structures and services ports and airports by the year 2030. According to the are becoming increasingly important parts of the rural vision statement, it will no longer be possible to refer to development agenda. any region of Kenya as “remote” as investments in the Rural structures also play a major role in increasing nation’s infrastructure will be given the highest priority. agricultural productivity and overall production. Rural Increasing value and incomes in rural areas by buildings are used not only for storing agricultural inputs transforming agriculture and making it more such as fertilizers, but also for preserving agricultural productive is a major objective of most planners and outputs. In Africa this is particularly important because policy makers in developing countries, especially a significant percentage of grain production is stored on those in eastern and southern Africa. They all aim to the farm for own consumption. It is therefore important raise incomes in crop, livestock, forestry and fishery to develop methods and structures for effective storage, production, as industrial production and the service especially for the modern high-yielding grain varieties, sector expand. This will be achieved partially through which tend to be more susceptible to pests than the processing, which will add value to agricultural traditional types. produce before it reaches the market. Improved management and breeding programmes to All countries of the region want agricultural increase livestock production have also created a need for producers to be competitive not only nationally, but 2 Rural structures in the tropics: design and development also regionally and globally. The aim is to accomplish climate factors and a genuine understanding of rural life this by means of an innovative, commercially oriented, and the farmer’s social and economic situation. They modern style of agriculture that includes the crop, should also be familiar with the full range of building livestock, forestry and fishery sectors. Agricultural materials and types of construction, from traditional production should also be sustainable and every effort indigenous to industrially produced, as applied to rural should be made to protect the environment. structures. They must be able to select appropriate The transformation of agriculture and other rural installations and equipment for such buildings. enterprises and livelihoods will require the planning, This knowledge will enable them to produce design, construction, operation and maintenance of specifications, in cooperation with the farmer, a broad range of rural structures and infrastructure. for functional building designs that provide a good Innovation is a key factor in the success of this environment and durable construction, thereby endeavour because it will lead to the efficient and contributing to efficient and economically sound farm effective design and construction of rural structures. operations. Further important tasks for the engineers This is essential as there are major challenges, in responsible for rural structures and services are particular limited financial resources. interpreting and explaining the drawings and technical The development of rural structures may be divided documentation to farmers, as well as supervising into four phases: planning; design; construction; construction work. However, engineers should be operation and maintenance: aware of the need to consult specialists in related fields • Planning: This phase involves consideration of where necessary. the various requirements and factors that affect This textbook is intended for the design and the general layout and dimensions of the desired development of rural structures for agricultural structures. It leads to the selection of one, or production in the tropics. This single volume covers the perhaps several, alternative types of structure that basic procedures for planning, designing, constructing, provide the best overall solution. operating and maintaining rural structures. Other The primary consideration is the structure’s topics include rural water supply, rural sanitation, rural function. Secondary considerations include energy and minor rural roads. In line with current aesthetics, sociology, law, economics and the and future requirements, the book presents modern environment. In addition, structural and methods of developing structures and infrastructure. constructional requirements and limitations may affect the type of structure to be selected. SCOPE OF THE TExTBOOk Equipment and machinery to be installed in the This textbook is intended as a resource for practitioners structures also need to be factored in during the engaged in the planning, design, construction, operation planning phase. and maintenance of rural structures and services in • Design: This phase involves the detailed support of agricultural production. It focuses mainly consideration of the different options involved on the structures and services required by smallholder in the planning phase. It leads to the definition farmers in rural areas of Africa. It is also designed to of the most suitable proportions, dimensions serve as a textbook for students enrolled in agriculture and details of the structural elements and and engineering courses at colleges and universities. connections required for constructing each option The book is divided into four main parts. Part one under consideration. Details of equipment and deals with the fundamentals required by a professional machinery to be installed in the structure also responsible for providing technical advice on rural form part of the designs. structures and services, such as graphical and geospatial • Construction: This phase involves the techniques and the properties of construction materials. procurement and transportation to the site of Part  two deals with the principles of designing rural materials, equipment, machinery and personnel, structures and services, including basic mechanics and as well as actual field erection. During this structural design. phase, some redesigning may be required due to Part  three deals with the elements of actual unforeseen circumstances such as unavailability of construction and building production. The final part, specified materials or foundation problems. which is the largest, deals with the specifics of designing • Operation and maintenance: During this period, and constructing different types of rural structure the structure is in use. It requires planned and (structures for livestock production, rural dwellings, unplanned maintenance. Experience gained in this structures for produce handling, conditioning and phase leads to the design of better structures in storage; rural infrastructure, such as rural roads, water the future. and sanitation, and external services, such as fencing). Engineers who specialize in designing rural buildings and services need to have a thorough knowledge of farming systems, crop and livestock production systems, Chapter 1 – Introduction 3 FuRTHER REAdInG De Janvry, A., Key, N. & Sadoulet, E. 1997. Agricultural and rural development policy in Latin America: new directions and new challenges. University of California at Berkeley. Federal Republic of Nigeria. 2010. Vision 20: 2020 (available at http://www.npc.gov.ng). FAO. 2009. Capital requirements for agriculture in developing countries to 2050. Rome. Knight, R. 2001. South Africa: economic policy and development (available at http://richardknight. homestead.com/files/sisaeconomy.htm). Nelson, G.L., Manbeck, H.B. & Meador, N.F. 1988. Light agricultural and industrial structures: analysis and design. AVI Book Co. Republic of Botswana. 1997. Vision 2016: Towards prosperity for all (available at http://www.vision2016. co.bw). Republic of Kenya. 2007. Vision 2030 (available at http://www.brandkenya.co.ke/downloads/ Vision2030_Abridged%20version.pdf). Republic of Uganda. 1998. Uganda vision 2025: A strategic framework for national development National Long Term Perspective Studies Project, Ministry of Finance, Planning and Economic Development (Kampala). United Republic of Tanzania. 2020. The development vision 2025 (available at http://www.tanzania.go.tz/ vision.htm). 5 Chapter 2 Planning farm and rural structures InTROduCTIOn a consensus. Various scholars have come up with A major constraining factor in the design and different definitions, such as: construction of farm and rural structures in the tropics is the need to implement such projects in an environment “…. Planning is the making of an orderly sequence where most farms are small and fragmented. Additional of action that will lead to the achievement of stated limiting factors include severe financial constraints goals” (Hall, 1974). and the need for agricultural mechanization and rural transformation. These challenges can be met, in part, by “…. Planning is an activity by which man in society producing standard designs and case studies for target endeavours to gain mastery over himself and shapes his groups. These case studies can be modified thereafter to collective future through conscious reasoned effort” suit each individual need. (Friedmann, 1966). As buildings and other rural structures are fixed assets that have a relatively long lifespan and require The above definitions notwithstanding, there are also a relatively large amount of resources to construct, it other schools of thought on planning. These include: is important that they are planned and designed for efficient and profitable use throughout their life. Once (i) Planning as a basic human activity a building is erected, however, it is expensive to make This looks at planning as a basic activity that pervades changes. A plan for an individual farm is influenced by (informs every aspect of) human behaviour: “….a plan a number of factors over which the farmer has no direct is any hierarchical process in the organism that can control, e.g. climate, soil fertility, government policies, control the order in which a sequence of operations is state of knowledge about agricultural techniques and to be performed”. the value of inputs and outputs. Miller et al. (1960) concluded that each action is Functional planning is essential for the realization the result of a complex preliminary process, which and achievement of the goals set. A good plan should they called a TOTE (Test Operate Test Exit) unit. This provide an understanding of the situation and how means that each action is preceded by an assessment it can be changed and thus assist farmers to see the of the situation and a visualization of the action to problems, analyse them and be able to make sound be undertaken (test); next the action is carried out decisions when choosing between alternative uses of (operated); its results are evaluated (test); then the their resources. While farm management planning helps sequence is concluded (exit); and a new one begun … farmers to choose the type and quantity of commodities to produce, advice from crop and livestock production (ii) Planning as a rational choice specialists is required to help them decide how to (a) This confines planning to matters of deliberate produce them in an efficient way. When an enterprise choice. A rational choice is one that meets certain requires buildings or other structures, the rural building standards of logic. specialist will suggest alternative designs for the efficient (b) In this case, planning becomes “a process for use of resources. The best plan for the whole farm determining appropriate future actions through operation is normally the result of interaction between a sequence of choices” (Davidoff and Reiner, the various farm planning disciplines. 1962). Similarly, engineers can produce standard designs (c) There is, however, a difference between that are suitable for a large number of farms in an area, rationality and planning as processes. Whereas either as they are or with small modifications. However, both attempt to achieve a preferred and – through the number of case studies and designs must be sufficient deliberate choice – comprehensive approach and to allow all farmers to be given advice reflecting their link to action, planning can be distinguished by individual situation, and which they are likely to adopt. its emphasis on the future orientation of any decision. What is planning? An overview (d) The weakness of this view of planning is its The term ‘planning’ is a very general one. Its various almost sole focus on choice, with only a vague definitions cover a wide range but do not provide link to action (if a group makes plans but does 6 Rural structures in the tropics: design and development not commit to implementing them, is this still (v) Planning is what planners do planning?). This definition describes the contribution of planners, Such planning aims to apply the methods of rational as technical experts, to public policy-making. This choice for determining the best set of future actions includes: to address novel problems in complex contexts. Defining the problem, stating the terms in which Furthermore, it is attended by the power and intention problems are solvable and comparing the importance of to allocate and commit often scarce social and economic the (always) conflicting values inherent in any solution. resources (and to act as necessary) to implement the Although this definition has the merit of being simple chosen strategy. and obvious, the reality is that planners are not merely people who plan. (iii) Planning as a control of future action This definition embodies what could be seen as the FORMS OF PLAnnInG antithesis of the narrowness of the above definition. It implies that planning does not exist when the process Regional planning does not include implementation. What is a region? “…Planning may be seen as the ability to control Essentially, it is a tract of land which, by virtue the future consequences of present action. The more of geographical, political, economic, administrative, consequences of controls, the more one has succeeded historical and other factors, or a combination of these, in planning. The purpose is to make the future different is distinguishable as a unit, a separate entity. This unit from what it would have been without this intervention” may be: (Wildavsky, 1973). (i) geographical, e.g. lake district; “… The problem is no longer how to make a (ii) social/political, e.g. a state in Nigeria; decision more rational but how to improve the quality (iii) single-function area, e.g. coalfield; of the action” (Friedmann, 1966). (iv) a farming region, e.g. wheat fields; (v) a river catchment area, e.g. Congo River Basin; This view of planning is equally flawed. For instance, is (vi) a metropolitan area, e.g. Johannesburg. it planning when someone pays the water bill? (Because this influences the future actions of the water company A clear delineation of regional units for land- and commits resources to continued supply). use planning is still lacking. For instance, there is a If so, then the definition of planning becomes so dilemma concerning whether to adopt administrative or diluted that it may set standards so high that they geographical/ecological units (e.g. cross-border resource become impossible to meet. management for Lake Victoria, Mara/Amboseli, etc.). Regional planning seeks not to achieve any specific (iv) Planning as a spatial kind of problem-solving objective (though specific regional strategies do have Whereas the above definitions were process-oriented their various objectives) but to regulate the relationship (addressing the ‘what’ and ‘how’ aspects of planning), between human and environmental factors. this definition is more situational (addressing the (i) Interregional – concerned with activity between specific realm in which planning activity occurs). central government and the regions and between One opinion defines planning as problem-solving one region and another; aimed at very particular kinds of problems referred (ii) Intraregional – between a region and the to as ‘wicked’. A wicked problem has no definitive localities it contains. formulation, no clear rules, no true or false answers (can only be better or worse) and no clear test for the solution. urban planning Each problem is unique but, at the same time, a symptom Urban planning is the physical planning of concentrated of another deeper, more extensive malady (disease, human settlements designated as urban areas. It is a illness). Worse still, unlike the scientific experimenter, the special case of planning that indicates that a certain problem-solving planner cannot afford mistakes. degree of detail is required of the planner. Henry Hightower goes beyond wicked problems in Urban planning requires the designation of an urban defining planning, accommodating the planner’s need to planning region with a base resident population not less question values, institutions and given decision rules. He than that stipulated in the policy document to indicate isolates the planning aspect, which uses rough, imprecise an official town or urban area. On a larger scale, it data, in contrast to the exact data used in science and becomes city planning. engineering, and planning has an action orientation. Urban plans are represented in the same way as The weakness of this approach is that it is too physical plans but they normally include more detail, inclusive; solving wicked problems is not restricted to including: planners but could also be applied to entrepreneurs, • infrastructure network administrators and politicians. • spatial organizational structures Chapter 2 – Planning farm and rural structures 7 • detailed action area plans Note that, in many developing countries, rural plans • density distribution are often non-existent or very limited because all that exists • zoning regulations for the areas is often a ‘top-down’ regional plan that only recognizes • location of functions in the urban system including the rural areas as components in the larger regional plans, population, industry, commerce, institutions, rather than as key actors in plan preparation. This model recreational facilities, utilities, natural resources, has often contributed to stagnating rural regions because environmental action plans and other essential ‘top-down’ systems often lead to poor identification of information thought to be important for the the most pressing needs at rural level. Later approaches, future growth of the urban region. such as Participatory Rural Appraisal (PRA), Action Research (AR) and Participatory Approval (PA), are Urban plans are essential tools for town management, ways and means through which development actors which need constant updating because of the complexity have tried to make local levels active in achieving their of urban regions. planning priorities. Rural planning Infrastructure planning A rural region, like an urban region, is another category Physical planning involves the distribution of goals, of region for planning purposes. In developing countries objects, functions and activities in space. The content in particular, rural areas tend to be home to as much as of physical planning continues to change, yet the 80 percent of the country’s population, and therefore approach has been fairly consistent. Physical planning urban planning becomes secondary to rural planning. can be regarded as the nuts and bolts of the way the Rural planning is carried out in the national interest built environment is conceived. One of the components to improve living conditions, match agricultural of physical planning includes infrastructure planning. production to demand and conserve natural resources. The historic origins in many a region relate to a Many factors in the national or regional plans may somewhat different tradition – that of municipal directly influence the choice of production on farms and civil engineering and public works. Today it is and thus the requirements for buildings. not unknown for these aspects to remain separately The aims of planning strategies in rural areas are institutionalized in terms of recruitment, organization based on political decisions. These may include: and statutory mandates. 1. Provision of support services such as extension Infrastructure planning involves planning for the education, market development, processing and provision of roads, water services, energy, health and credit. education facilities and other utilities that are necessary 2. Development of infrastructure such as roads, for the effective functioning of communities. Their electricity and water supplies. provision contributes greatly to rural transformation 3. Self-help activities to develop community and improved standards of living for the population. facilities. Transport planning, in particular, interacts closely with 4. Increased non-farm employment opportunities. land-use planning. Transport planning covers a range of geographic levels from the region to the street Rural plans try to define the best strategy for rural intersection or multimodal node, and also deals with areas in order to mobilize their resources to produce the the various modes of transportation – from air travel assets required for development in the regions. As rural to bicycle routes – either separately or in combination. regions are generally large, it is necessary to delimit The two are interconnected in that land use generates subregions (i.e. through administrative boundaries), on travel demand, and access boosts the development which the plan will focus. potential of land. Transport planners follow much the A rural plan therefore lays down rural region same generic process as land-use planners. specifics: An improved road network may, for example, make • Land-use systems and activities (at policy level). new urban markets accessible, thus making it feasible • Identification and definition of resource utilization for farmers to go into vegetable or milk production. policies. This in turn may require housing for animals and stores • Linkages between the specific rural region and for produce and feed. It would therefore be advisable other regions. to investigate any plans for rural development in an • Local initiatives for administration and area during the planning stages at an individual farm, management of the region. or to implement an extension campaign promoting • Strategic environmental management for the improved building designs in that area. Government region. policy is often an important factor in determining long- • Population management activities of the region. term market trends and thus the profitability of market • It is also important for the rural plan to show how production, and it is therefore of special importance the political structure of the region integrates with when planning for production operations involving the larger regional political system. buildings. 8 Rural structures in the tropics: design and development Environmental planning base. The export base is made up of those goods and The broad objective of the planning process is to services that the community exports to other towns or promote the welfare of citizens through the creation regions in order to bring in money. This will enable the and maintenance of a better, healthier, more efficient community to grow. Secondary base businesses serve and more attractive living environment. Economic the local community. If the size of the community is forces in a free-market economy are not a reliable small, the size of the local community may not grow guide for directing urban activities towards the desired much. healthier life because they tend to maximize profits or individual wellbeing at the expense of societal Feasibility wellbeing. There are three golden rules in formulating a project: Moreover, human development activities, especially (i) Ensure that all the factors necessary for its in low-technology areas, have tended to exploit rather success are taken into account from the outset. than generate resources. Where exploitation continues (ii) Carry out careful preinvestment studies. unchecked, depletion will follow. (iii) Build in flexibility. Environmental planning has become a necessary component of planning at all levels, to act as a check When the scope of the project has been determined, on market forces and to press for more health-oriented five main aspects must be taken into account: planning, more consideration for human social (a) Technical feasibility: Have all the alternatives institutions, more awareness of resource conservation been considered? Is there a need for the project and more efficient utilization systems. Environmental at all? For example, could better dry-farming planning covers a wide range of concerns, but essentially techniques and moisture conservation increase has the following main objectives: output just as much as irrigation? Are the • To minimize threats to human health and life by proposed methods, design and equipment the organizing activities in such a way as to reduce the best for the purpose? Are the cost estimates spatial concentration of pollutants in our water by realistic and can the successive phases of the limiting dangerous and hazardous areas. project be carried out in the time allowed? • To preserve resources for future use, e.g. (b) Economic viability: Does the chosen technical minimizing soil erosion and deforestation. solution offer the highest economic and social • To achieve recreational goals such as preserving returns of all the technically and financially certain areas in their natural state. feasible alternatives? • To minimize damage to the environment for its (c) Financial: Are the necessary funds available? own sake rather than for humanity’s sake, e.g. by Will the project be able to meet its financial preserving the habitat of a rare species that has no obligations when it is in operation? For known or readily foreseeable use to us. example, will the farmer have sufficient income to cover repayments and interest on a loan? Environmental planning has previously been (d) Administration: Will the administrative included in planning, but recently greater efforts have structure proposed for the project and its staff been made in this field because of impending major be adequate to keep the project on schedule and threats to the human population. manage it efficiently? Will interdepartmental rivalries be an obstacle and, if so, can the Economic planning and feasibility proposed coordination machinery ensure an All countries carry out economic plans to forecast how organized flow of decisions and the assignment the economy will manage the scarce resources available of responsibilities within the chain of command? to the population. Such plans may be yearly, two-yearly (e) Commercial: What are the arrangements for or five-yearly. Most nations have five-year plans. buying materials for the project? Where will Smaller regions of a country may also have economic they come from? How will they be funded? plans for much the same reason as the country, but on a How will the output of the project be sold? much smaller scale and in greater detail. Economic plans are largely statistical, indicating Economic planning of the farm operation sectors, financial expenditure and revenue and forecasts Most textbooks on agricultural economics describe for the subsequent plan periods. They are largely methods of economic planning for commercial farms policy-oriented. Economic plans are also carried out in developed western countries, but very few deal with by smaller bodies, such as local authorities. In this methods relevant to African agriculture, which is, and will case the plan will comprise an inventory of how the for the foreseeable future be, dominated by smallholder community earns a living and where it is heading in farmers. Although the principles of economic theory terms of resource stability. may be relevant when reviewing African small-scale Most community economic plans are divided into farms, their applications will undoubtedly differ from two segments: the export base and the secondary those used when reviewing large commercial farms. Chapter 2 – Planning farm and rural structures 9 Traditional applications assume, for example, that use where the gains and losses are a mixture of money crops and livestock can be analysed separately, that and non-money elements and to take into consideration the concept of farm size can be unequivocally defined, farmers’ personal beliefs so that the resulting plans that the farmer makes all the decisions concerning farm reflect their individual goals and value system. There operations, and that increasing cash income is the major are usually a variety of reasons for reviewing the objective. However, in most cases African agriculture economic planning for the entire farming operation. is traditional and based on communal land ownership. The plan will establish the resources available, as In quite a number of cases this includes a multifamily well as the limitations and restrictions that apply to the situation in which two or more wives each have their construction of a proposed building. A comprehensive own plots but also participate in joint enterprises and economic plan for a farm, whether an actual farm or a are subordinate to the husband’s general decisions. This case-study farm, may include the following steps: situation would make an approach to local community 1. Establishment of individual farmers’ objectives, groups more relevant than emphasizing individual farms. priorities and constraints for their farm operation. A multiple cropping system or a livestock-feed The objectives should preferably be quantified so crop system may serve to reduce risk and result in that it can be determined whether they are being, a more uniform supply of food and cash, as well as or can be, achieved. family labour demand and, although the yields of the 2. Analysis of financial resources, i.e. the farmer’s individual enterprises may be low, it may provide an assets as well as the cost and possibility of acceptable overall result. obtaining loans. Money is the commonly used  – and often the 3. Listing of all available resources for the farming most convenient  – medium of exchange in economic enterprises, quantifying them and describing calculation. However, other units may occasionally be their qualities, e.g. quantity and quality of land, more relevant when small farms, with limited cash flow water resources, tools and machines; roster of and strong non-monetary relations between production labour including a description of training and operations and the household, are analysed. Subsistence skills; existing buildings and evaluation of their farmers may, for example, value the security of having serviceability; and the farmer’s management skills. their own maize production, so much so that they will 4. Description of all factors in the physical, produce enough for the household even if an alternative economic and administrative environment that enterprise using the land and labour would generate directly influence the farming enterprises, but more than enough cash to buy the maize at the market. over which the farmer has no direct influence, The principles of economic theory are valid whatever e.g. laws and regulations, rural infrastructure, appropriate medium of exchange is used to specify the market for produce, availability of supplies, quantities, e.g. units of labour used to produce units of prices and market trends. grain or meat. The difficulty or challenge, depending on 5. Individual analysis of each type of farm enterprise, the perspective, is to find a suitable alternative unit to whether crop or animal production, to determine Fertilizer plan Plant husbandry plan Feeding plan 1 season 5 years 1 season Soil conservation Animal husbandry plan plan 20 years Labour utilization plan 2 years Animal breeding plan Land area plan 5 years Investment plan 50 years Equipment & Building plan Mechanization plan 30 years 10 years Figure 2.1 Schedule of a sub-plan in a farming enterprise 10 Rural structures in the tropics: design and development its allowance of total capital. Note that where of each individual enterprise may not necessarily mean multiple cropping is practiced, the mix of various that the total farming enterprise is optimized. crops grown together is considered to be one If farmers already operate their farm according to enterprise. a sound economic plan, a less ambitious approach, 6. Determining the optimum mix of enterprises that involving analysis of only the enterprise requiring a new satisfies the farmer’s objectives and makes the or remodelled building and an investment appraisal, best use of resources. may suffice. A number of investment appraisal methods have been advocated for use in agriculture to give The resulting plan will be an expression of the a rough indication of the merits of an investment. farmer’s intentions for the future development of the However, smallholders generally hesitate to risk cash for family farm. The plan will contain several interrelated investment in fertilizer, pesticides and feed concentrate, subplans as shown in Figure 2.1. as well as improved buildings and machinery, until Note that the subplans in the Figure 2.1 may interact enough food for the household is produced, a market in many more ways than have been illustrated. Many of with a cash economy is readily available and farmers these interrelationships are of great importance when are confident of their own technical, agricultural and trying to maximize the result of the total production at the economic skills. Money therefore, may not always be farm, whether or not the product is sold. Optimization the most relevant unit to use in the calculations. BOx 2.1 Building process in kenya The establishments that undertake planning and building in Kenya range from households to large state and non-state actors. There are numerous laws that govern the building process. The laws that govern building in rural and urban Kenya include Local Government Act Cap 265, Physical Planning Act Cap 286, EMCA Act of 1999, Public Health Act Cap 242, Architect and Quantity Surveyors Registration Act, Cap 525 and Engineers Registration Act Cap 530. These laws provide the basis on which planning and building can be carried out in a systematic way. They provide for registration and professional development of key staff in the sector. Furthermore, the laws provide a basis for undertaking sustainable developments. The key characteristics of planning and building in rural and urban Kenya are: 1. It is a process involving many stakeholders, principally regulators, developers, professionals and contractors. 2. Stakeholders are clustered and regulated by different legal and regulatory regimes. 3. It employs many labourers, especially in urban areas where it is the leading employer. Planning and building process Step 1 The developer (a household or corporation) identifies the project and the land on which the building will be constructed. Step 2 The developer identifies a team of consultants (architects, quantity surveyors, surveyors, planners, environmental impact assessment experts, etc.) who manage the planning and building process. In rural areas, the master builder or ‘fundi’ is mostly responsible for management of the process. Step 3 The design team carries out site investigations to determine the suitability of the site and the feasibility of the project. Step 4 If the project is judged feasible, the design team applies to a local authority for planning approval. Step 5 If planning approval is obtained, the design team prepares the design and submits it to a local authority for development approval. Step 6 Upon approval, the design team appoints a contractor to build the project. Step 7 Upon successful completion, the developer applies to a local authority for an occupancy certificate and registers the property with the Ministry of Lands. These steps describe a process that is lengthy and involves several professionals, especially in urban areas. In rural Kenya, not all these steps are undertaken. Chapter 2 – Planning farm and rural structures 11 An APPROACH TO BuILdInG PLAnnInG the area. Where the design is developed for a specific Once the building requirements have been established farm or farming enterprise, priority should be given in the economic planning, it will be the task of the to gathering as much information as possible from farm-building engineer to work out the functional that farm or about that enterprise. All information and structural designs and deal with the farmstead should be critically evaluated prior to its acceptance plan. While there are laws, regulations and guidelines as background material for the design of the proposed enacted by the central or local governments that govern building or for a standard drawing. the building and construction industry, most are only When developing an economic plan, the farm- applicable to areas that have been designated as urban building engineer should obtain as much of the above (townships, municipalities and cities). (see box 2.1 for information as possible, in addition to data relating to Kenya). Rural areas are governed by County, District the following factors: or Rural Councils with limited capacity to enforce such 1. A comprehensive master plan of the farmstead. laws and regulations. 2. For storage structures, data concerning the The planning process always starts with a list expected acreage and yield of the crop to be dried of available resources and restrictions and other and stored, the length of the storage period, i.e. background material. The major outline for the design is the amount of produce to be sold or consumed at then sketched. The final design is developed by working the time of harvest. from rough sketches towards increasingly detailed plans 3. For animal housing, the quantity and quality of the different parts of the building. Often, however, of animals currently owned and the possibility when some internal units such as farrowing pens have and time scale for increasing and improving the been designed and the required number established, the herd through a breeding programme should be dimensions of the final building will be influenced by considered. the pen size and number. The farmer will often impose 4. Availability of building materials and construction restrictions on the design before the planning process skills at the farm or in the rural area concerned. begins. These should be critically evaluated and their 5. Laws and regulations applicable to the proposed effectiveness examined before they are accepted as building and the enforcement agencies involved. part of the final design. It will be useful to discuss the extent of the proposed building and enterprise with an Calculations agricultural economist if the plan has not been based on The standardized economic calculations used to an overall economic plan. determine the gross margin in a farm operation are Standard solutions, promoted using demonstration often limited in scope and therefore a more detailed structures and extension campaigns, will be the most examination of the enterprise housed in the building important means of introducing improved building may be of use. Knowing the expected production designs to small-scale farmers in rural areas for the volume, additional data are calculated using the foreseeable future. However, improved standard designs background information. will be widely accepted by farmers only if they are In the case of a building to be used for storage, based on a thorough understanding of the agricultural the expected volume of the crop to be stored is practices and human value systems prevalent in the determined, as well as the required handling capacity. local farming community and are developed to utilize In a multipurpose store where several different locally available building materials and skills. commodities are held, a schedule of the volumes New ideas, materials and construction methods and storage periods will be useful to determine the should be developed and introduced to complement maximum storage requirement. the strengths of indigenous methods. Local builders will be valuable sources of information regarding Analysing the activities indigenous building methods and effective channels Activity analysis is a tool used for planning production through which innovation can be introduced. Close in large, complex plants such as factories, large-scale cooperation between builders and farmers will help the grain stores and animal-production buildings, but it local community to deal with its own problems and to can also be a useful instrument in smaller projects, evolve solutions from indigenous methods and local particularly for the inexperienced farm-building resources that will have a good chance of becoming engineer. accepted. Most production operations can be carried out in several ways involving various degrees of Background information mechanization. By listing all conceivable methods in An economic plan for the farming operation will a comparable way, the most feasible method from a provide much of the background information required technical and economic standpoint can be chosen. This by the farm-building engineer. As this is often will ensure good care of produce and animals, as well missing, such information will have to be obtained by as effective use of labour and machinery. Uniformity interviewing farmers and by studying similar farms in in handling improves efficiency, e.g. produce delivered 12 Rural structures in the tropics: design and development Product & quantity Quantity supporting services analysis & time information Product & route Activity relationship of information different operations Flow of materials Basic flow diagram Rural Farmstead development plan plans Modified flow diagram 1st approach Planning considerations Constructional considerations Practical considerations Alternative flow diagrams & layout plans Overlays for evaluation & selection Final layout Figure 2.2 Layout diagram of the planning procedure in bags to a store should be kept in bags within the similarly analysed. Note that the analysis of handling store, particularly if it is to be delivered from the store operations for feed produced at the farm should include in bags. harvesting and transport from the field because these In animal housing projects, the handling operations operations may determine the most appropriate storage for feed, animals, animal produce and manure are and handling methods inside the building. Chapter 2 – Planning farm and rural structures 13 road milk out convenience and work efficiency when the building co is being designed. The communication schedule is not ncentrates always accounted for separately, but instead may be in dairy included in the schedule of activities. barn milk Following the principle of working from the major h collection parlour outline of the project towards the details, the next step ay yard cows for is to place the proposed building on the farmstead. culling, co ws disease, Efficient communication within the farmstead is of cows cow calfing great importance in creating functional and harmonious s straw exercise holding pen operations. The schedule of functions serves as a grain yard cows for checklist when transportation is analysed. The room cows for cow disease, heifers grazing calfing schedule provides information on the size of the fields building and the structural concept likely to be used. h calving eife boxes r A standard design can obviously not be shaped calves to fit a specific farmstead. Nevertheless, the group calves of farms for which the design is developed may have calves calf house common features, which allows the designer to make field recommendations concerning the location of the new building. Some structures have special requirements Figure 2.3 Example of a material flow diagram for a concerning where they can be constructed on the dairy unit farmstead. A maize drying crib, for example, must be exposed to wind. Where the plan includes the addition of a new When all handling operations have been analysed, building to an existing farmstead, alternative locations the result is summarized in a schedule of activities. for the proposed building are sketched on the master Labour efficiency is often an essential factor in plan or, better still, on transparent paper covering small farm development. If farmers have a reasonable the master plan, and the communication routes are standard of living, cultural norms and social pressures indicated by arrows between the buildings, the fields may limit their willingness to invest in labour for a and the access road. relatively low return, while labour-efficient methods Considering all the planning factors and allowing for a reasonable return on the labour invested requirements, one of the proposed building locations is may increase their willingness to produce a surplus. likely to have more advantages and fewer disadvantages than other alternatives. The transport routes to and Room schedule from the building are then further studied and noted for This is a brief description of all rooms and spaces use when the interior of the building is being planned. required for work, storage, communication, servicing Farmers will often have firm opinions about the of technical installations, etc. As variations in yield location of the building from the start of the planning and other production factors are to be expected, an process. Their opinion should be critically analysed, allowance is added to the spaces and the volumes. It but naturally it should be given considerable weight would be uneconomical, however, to allow for the most when the site is finally chosen. extreme variations, particularly if a commodity to be stored is readily marketable and can be bought back at Functional design of the building a reasonable price later. Sketching alternative plan views of the building is The total space requirement is then obtained by mainly a matter of combining and coordinating the simple addition. Also, partial sums indicate how the requirements that have been analysed in earlier steps. production operations can be divided into several Some general guidelines are as follows: houses. 1. Concentrate functions and spaces that are naturally connected to each other, but keep dirty Communication schedule activities separate from clean ones. This describes the requirement and frequency of 2. Communication lines should be as straight and communication between the various rooms and spaces simple as possible within the building and, to within the building and between the building and reduce the number of openings, they should be other structures at the farmstead. A schedule for coordinated with those outside, as shown in the movements between the farmstead, the fields and the farmstead plan. market is also essential. It may also include quantities 3. Avoid unused spaces and long communication to be transported. Based on this information, the corridors. rooms between which there is frequent movement of 4. Provide for simple and efficient work. Imagine goods and services can be placed close together for that you are working in the building. pl ier s su p supp liers 14 Rural structures in the tropics: design and development 5. Use as few handling methods as possible and other structures. A carefully developed plan should choose methods that are known to be reliable, provide a location for buildings and facilities that allows flexible and simple. adequate space for convenient and efficient operation 6. Provide a good environment for labourers and of all activities, while at the same time protecting the animals or produce. environment from such undesirable effects as odours, 7. Provide for future expansion. dust, noise, flies and heavy traffic. A wide range of 8. Keep the plan as simple as possible within the factors, described in the ‘Communication Schedule’ limits of production requirements. section, should be considered when planning the location of buildings and services at the farmstead. Finalization of sketching Although the immediate objective of these plans After a number of sketches have been produced, they may be the inclusion of a new building in an existing are carefully analysed to select the one that best reflects farmstead, provision should be made for future the farmer’s objectives. However, because a farmer’s expansion and the replacement of buildings. In this objectives are usually complex and difficult to elicit, way a poorly laid out farmstead can be improved over it is common to use more readily evaluated criteria the long term. such as total construction cost or cash expenditure. The selected building plan is then drawn to the correct Zone planning scale, sections and elevations are sketched and, where Zone planning can be a useful tool, but it is most applicable, the building is positioned on the master effective when planning a new farmstead. The farmstead plan. In many cases, the results of earlier steps in the is divided into zones 10 metres to 30 metres wide by planning process, such as the activity schedule or room concentric circles, as shown in Figure 2.4. schedule, may have to be reviewed and adjusted as the work progresses. . Prior to being widely promoted, standard designs N Prevailing wind are often tested at a few typical farms. The construction direction phase and a period of use will often give rise to useful experience that may result in improvements to the 4 design. Only if the designer is prepared to modify the design continuously as needed to adapt it to changing 3 agricultural practices will it have a good chance of being 5 successful in the long run. 2 A ‘one of a kind design’ intended for a specific farm can obviously not be tested in practice prior to 1 its construction. Therefore the sketch, including a cost IV III II I estimate, must be presented and carefully explained ROAD to farmers so that they understand the plan and feel confident that they can run an efficient and profitable 1 Family living area including the dwelling production system in the building. Notwithstanding 2 Implement and machinery storage, farm workshop this, the farmer is likely to have objections and 3 Grain and feed storage suggestions for alterations, which must be considered 4 Livestock buildings and worked into the final sketches. As an understanding 5 Farm access road and courtyard of the operation and a positive attitude by all concerned Figure 2.4 Zone planning in four zones are basic requirements for efficient production, farm labourers and members of the farmer’s family who will be working in the building should also be given an Zone 1 at the centre of the farmstead is for family opportunity to review the sketches. living, and should be protected from odour, dust, flies, etc. Clean, dry and quiet activities, such as implement Final design sheds and small storage structures, can be placed in When all sketches (farmstead plan, functional plan and Zone 2. Larger grain stores, feed stores and small animal structural concept) have been corrected, coordinated units are placed in Zone 3, whereas large-scale animal and approved by the farmer, the final building production is placed in Zone 4 and beyond. documents are prepared. The advantage of zone planning is that it provides space for present farm operations, future expansion and FARMSTEAd PLAnnInG a good living environment. However, in many African The farmstead forms the nucleus of the farm operation cultures the livestock has traditionally been placed at where a wide range of farming activities are undertaken. the centre of the farmstead. Thus the zone concept runs It normally includes the dwelling, animal shelters, counter to tradition and may not be desirable. storage structures, equipment shed, workshop and Chapter 2 – Planning farm and rural structures 15 Farmstead planning factors construction fail to qualify for a grading of one-hour Good drainage, both surface and subsurface, provides fire resistance, which in many countries is the lowest a dry farm courtyard and a stable foundation for grade recognized. In contrast, most masonry walls have buildings. A gentle slope across the site facilitates good fire-resistance ratings. drainage, but a pronounced slope may make it difficult Timber framing can be improved with the use of fire- to site larger structures without undertaking extensive retardant treatments or fire-resistant coverings such as earthmoving work. Adequate space should be provided gypsum plaster or plasterboard. Steel columns can be to allow for manoeuvring vehicles around the buildings protected with plaster or concrete coatings, while steel and for the future expansion of farm operations. roof trusses are best protected with suspended ceilings Air movement is essential for cross-ventilation, but of gypsum plaster or plasterboard. excessive wind can damage buildings. As wind will carry odours and noise, livestock buildings should be placed Classification of fire hazards downwind from the family living area and neighbouring Some types of activities and installations in farm homes. Undesirable winds can be diverted and reduced buildings constitute special fire hazards. Wherever by hedges and trees or fences with open construction. practical they should be isolated in a room of fireproof Solar radiation may adversely affect the environment construction or in a separate building away from other within buildings. An orientation close to an east-west buildings. A list of special fire hazards includes: axis is generally recommended in the tropics. 1. Flammable, highly combustible or explosive An adequate supply of clean water is essential on materials in excess of very small quantities, e.g. any farm. When planning buildings for expanded liquid and gas fuel, ammonium nitrate fertilizer, livestock production, the volume of the water supply hay and bedding. must be assessed. Where applicable, the supply pipe 2. Hot-air grain drying and dust from grain handling in a good building layout will be as short as possible. may be explosive in high concentrations. Similarly, the length of utility supply lines (e.g. electric, 3. Furnaces and heating equipment; poultry gas) should be kept to a minimum. brooder; fireplaces. The safety of people and animals from fire and 4. Farm workshop (especially welding) and garage accident hazards should form part of the planning for vehicles. considerations. Children, especially, must be protected 5. Electrical installations; continuously running from the many dangers at a farmstead. It is often mechanical equipment. desirable to arrange for some privacy in the family living area by screening off the garden, outdoor meeting/ In addition, lightning, children playing with fire, resting places, veranda and play area. smoking and lanterns are potential sources of fire Measures should be taken for security against outbreaks. Thatched roofs are highly combustible and theft and vandalism. This includes an arrangement of prone to violent fires. buildings where the farmyard and the access driveway can be observed at all times, especially from the house. Fire separation A neat and attractive farmstead is desirable and much Fire spreads mainly by windborne embers and by can be achieved toward this end, at low cost, if the radiation. Buildings can be designed to resist these appearance is considered in the planning, and effective conditions by observing the following recommendations: landscaping is utilized. 1. Adequate separation of buildings by a minimum of 6 metres to 8 metres, but preferably 15 metres SAFETy And FIRE PROTECTIOn to 20 metres, particularly where buildings are Measures to prevent fire outbreaks and to limit their large or contain special fire hazards. A minimum effect must be included in the design of buildings. distance may be specified in the building code. Fire prevention measures include the separation of 2. Construction using fire-resistant facing and buildings to prevent fire from spreading and to permit roofing materials. firefighting, and a farm or community pond as a source 3. Avoidance of roof openings and low roof slopes, of water for extinguishing fires. which can be more easily ignited by embers. 4. Use of fire-resistant walls that divide a large Fire resistance in materials and construction building into smaller fire compartments. To The ability of a building to resist fire varies widely be effective, such walls must go all the way up depending upon the construction materials and the through the building to the roof and any openings manner in which they are used. Fire resistance is graded in the walls must be sealed by a fireproof door. according to the period of time that a construction element is able to withstand standardized test conditions Evacuation and fire extinguishers of temperature and loading. In the event of a fire outbreak, all personnel should be Bare metal frameworks and light timber framing able to evacuate a building within a few minutes, and exhibit a low order of fire resistance and both types of animals within 10 to 15 minutes. Equipment, alleys and 16 Rural structures in the tropics: design and development doors should be designed to facilitate evacuation. Smoke • Programme: the strategy to be followed and major and panic will delay evacuation during a real fire, so actions to be taken in order to achieve or exceed evacuation during a fire drill must be much faster. objectives. In animal buildings, exit doors leading to a clear • Schedule: a plan showing when individual or passage, preferably a collecting yard, should have a group activities or tasks will be started and/or minimum width of 1.5 metres for cattle and 1  metre completed. for small animals so that two animals can pass at the • Budget: planned expenditures required to achieve same time. Buildings with a floor area exceeding 200 m² or exceed objectives. should have at least two exit doors as widely separated • Forecast: a projection of what will happen by a as possible. The travel distance to the nearest exit door certain time. should not exceed 15 metres in any part of the building. • Organization: design of the number and kinds of Fire extinguishers of the correct type should be positions, along with corresponding duties and available in all buildings, in particular where there responsibilities, required to achieve or exceed are fire-hazardous activities or materials. Water is objectives. commonly used for firefighting, but sand or sandy • Procedure: a detailed method for carrying out a soils are effective for some types of fire. Dry powder policy. or foam extinguishers are best for petrol, diesel, oil and • Standard: a level of individual or group electrical fires. Regardless of type, fire extinguishers performance defined as adequate or acceptable. require periodic inspection to ensure that they operate properly in an emergency. Project evaluation and techniques Project evaluation is a management tool. It is a time- Bushfire bound exercise that attempts to assess systematically The dry season or any period of prolonged drought and objectively the relevance, performance and success brings with it a constant fire hazard. Fanned by strong of ongoing and completed projects. Evaluation is winds and intensified by heatwave conditions, a large undertaken selectively to answer specific questions to bushfire is generally uncontrollable. guide decision-makers and/or project managers and to Firebreaks are an essential feature of rural fire provide information on whether underlying theories protection and should be completed before the fire and assumptions used in project development were season starts. It is desirable to completely surround the valid, what worked and what did not work, and why. homestead with major firebreaks at least 10 metres wide. Evaluation commonly aims to determine the relevance, Breaks can be prepared by ploughing, mowing, grazing, efficiency, effectiveness, impact and sustainability of a green cropping or, with great caution, by burning, and project. may include any watercourse, road or other normal The main objectives of project evaluation are: break that can be extended in width or length. (i) To inform decisions on operations, policy, or Shelter belts or even large trees are useful in strategy related to ongoing or future project deflecting wind-borne burning debris. For further interventions. protection, all flammable rubbish and long, dry grass (ii) To demonstrate accountability to decision- should be removed from the surroundings of the makers. buildings and any openings, such as windows, doors (iii) Improved decision-making and accountability and ventilators, covered with insect screens to prevent are expected to lead to better results and more wind-borne embers from entering the building and efficient use of resources. starting a fire Other objectives of project evaluation include: PROJECT PLAnnInG And (i) To enable corporate learning and contribute EVALuATIOn TECHnIquES to the body of knowledge on what works and what does not work, and why. Project planning (ii) To verify/improve project quality and Project planning is customarily defined as strategic, management. tactical, or operational. Strategic planning is generally (iii) To identify successful strategies for extension/ for five years or more; tactical can be for one to five expansion/replication. years and operational normally covers six months to (iv) To modify unsuccessful strategies. one year. (v) To measure effects/benefits of projects and Project planning means determining what needs project interventions. to be done, by whom, and by when, in order to (vi) To give stakeholders the opportunity to have a fulfil assigned responsibilities. There are nine major say in project output and quality. components of the project planning phase: (vii) To justify/validate projects to donors, partners • Objective: a goal, target, or quota to be achieved and other constituencies. by a certain time. Chapter 2 – Planning farm and rural structures 17 Evaluation is often construed as part of a larger The main questions in an evaluation should address: managerial or administrative process. Sometimes this (a) Effectiveness: Is the project or programme is referred to as the planning-evaluation cycle. The achieving satisfactory progress toward its stated distinctions between planning and evaluation are not objectives? The objectives describe specifically always clear; this cycle is described in many different what the project is intended to accomplish. ways, with various phases claimed by both planners Accomplishments on this level are sometimes and evaluators. Usually, the first stage of such a cycle is referred to as project outputs (what was done), the planning phase. and are assumed to be linked to provision of Project evaluation involves a needs assessment, inputs (human, financial and material resources which entails assessing the use of methodologies contributed to achieve the objectives). that help in conceptualization and detailing and the (b) Efficiency: Are the effects being achieved at application of skills to help assess alternatives and make an acceptable cost, compared with alternative the best choice. approaches to accomplishing the same objectives? The project may achieve its objectives at lower Methodology cost or achieve more at the same cost. This involves The evaluation phase also involves a sequence of considering institutional, technical and other stages that typically includes: the formulation of arrangements as well as financial management. the major objectives, goals and hypotheses of the What is the cost-effectiveness of the project? programme or technology; the conceptualization and (c) Relevance: Are the project objectives still operationalization of the major components of the relevant? What is the value of the project in evaluation – the programme, participants, setting and relation to other priority needs and efforts? Is measures; the design of the evaluation, detailing how the problem addressed still a major problem? these components will be coordinated; the analysis of Are the project activities relevant to the national the information, both qualitative and quantitative; and strategy and plausibly linked to attainment of the utilization of the evaluation results. Different means the intended effects? of evaluation include: (d) Impact: What are the results of the project? What • Self-evaluation: This involves an organization are the social, economic, technical, environmental or project holding up a mirror to itself and and other effects on individuals, communities, assessing how it is doing, as a way of learning and and institutions? Impacts can be immediate or improving practices. long-term, intended or unintended, positive or • Participatory evaluation: Participatory evaluation negative, macro (sector) or micro (household). provides for active involvement in the evaluation (e) Sustainability: Is the activity likely to continue process of those with a stake in the programme: after donor funding, or after a special effort, providers, partners, customers (beneficiaries) and such as a campaign, ends? Two key aspects any other interested parties. of sustainability for social development Participation typically takes place throughout programmes are social-institutional and all phases of the evaluation: planning and design; economic (for economic development projects, gathering and analysing the data; identifying environmental sustainability is a third the evaluation findings, conclusions, and consideration). Do the beneficiaries accept the recommendations; disseminating results; and programme, and is the host institution developing preparing an action plan to improve programme the capacity and motivation to administer it? Do performance. they ‘own’ the programme? Can the activity • Rapid participatory appraisal/assessment: become partially self-sustaining financially? Originally used in rural areas, the same methodology can, in fact, be applied in most EnVIROnMEnTAL MAnAGEMEnT communities. It is semistructured and carried out The environment consists of the land, air and water by an interdisciplinary team over a short time. on the planet Earth. It encompasses all living and non- • External evaluation: This is an evaluation conducted living things occurring naturally on Earth or any region by a carefully chosen outsider or outside team. thereof. The built environment on Earth comprises the areas and components that are strongly influenced by Project evaluation involves: humans. A geographical area is regarded as a natural (i) Looking at what the project or the organization environment if the human impact on it is kept below a intended to achieve. What difference did it want certain limited level. to make? What impact did it want to make? The construction and operation of rural structures (ii) Assessing its progress towards what it wanted and infrastructure have the potential to introduce to achieve and its impact targets. pollution into the environment. Pollution is the (iii) Looking at the strategy of the project. introduction of contaminating substances into the (iv) Looking at how it worked. environment that lead to its degradation. 18 Rural structures in the tropics: design and development Environmental management is management in the village, including livestock keeping, chicken of the interaction of modern human societies with, rearing and small-scale processing of products and and their impact upon, the environment. The aim of animal feeds. There are also buildings scattered all over management is to limit environmental pollution and the homestead. degradation. In line with all management functions, effective management tools, standards and systems are 1. Illustrate how you would go about reorganizing required. These include the environmental management the existing farmstead in preparation for future standards, systems and protocols that that have been expansion. set up to reduce the environmental impact, measured 2. With the aid of sketches, present a plan and against objective criteria. elevations of a simple rural farm building There are various international and national standards for environmental management. The ISO 14001 standard is the most widely used standard for environmental risk management and is closely aligned to the European Eco-Management and Audit Scheme (EMAS). As a common auditing standard, the ISO 19011 standard explains how to combine this with quality management. In the tropics, various statutory agencies exist to enforce environmental standards. These include the National Environment Management Council (NEMC) of Tanzania, the Bangladesh Environment Conservation Act (BECA), the Environmental Management Authority (EMA) of Trinidad and Tobago, the Environmental Protection Agency (EPA) of Guyana, the Philippines Department of Environment and Natural Resources, the Federal Environmental Protection Agency of Nigeria. In most countries, there is a legal requirement to conduct an Environmental Impact Assessment (EIA) before any construction project is given a license to proceed. The EIA is an assessment of the possible impact – positive or negative – that a proposed project may have on the environment, together consisting of the natural, social and economic aspects. The purpose of the assessment is to ensure that decision-makers consider the ensuing environmental impacts to decide whether or not to proceed with the project. The International Association for Impact Assessment (IAIA) defines an environmental impact assessment as “the process of identifying, predicting, evaluating and mitigating the biophysical, social, and other relevant effects of development proposals prior to major decisions being taken and commitments made.” For ongoing enterprises an annual Environmental Audit is a legal requirement in many countries. Environmental audits are intended to quantify environmental performance and environmental position. In this way they perform a function analogous to financial audits. An environmental audit report ideally contains a statement of environmental performance and environmental position, and may also aim to define what needs to be done to sustain or improve on these performance and position indicators. WORkInG PROJECT You have visited a village in a rural part of your country. A wide range of farming activities take place Chapter 2 – Planning farm and rural structures 19 FuRTHER REAdInG Davidoff, P. & Reiner, T.A. 1962. A choice theory of planning. Journal of American Institute of Planners (AIP), pp. 103-115. FAO. 1980. Farm management research for small farmer development, by J.I. Dillon & B.J. Hardaker. FAO Agricultural Services Bulletin No 41. Rome. FAO. 1984. Agricultural extension systems in some African and Asian countries, by P. Von Blanckenburg. FAO Economic and Social Development Paper No 46. Rome. FAO. 1984. Agricultural extension: a reference manual, by B.E. Swanson (ed.). 2nd edition. Rome. Government of Guyana. 2010. Environmental Protection Agency (EPA) (available at http:// epaguyana.org). Government of Nigeria. 1992. Federal Environmental Protection Agency (available at http://www.nigeria- law.org). Government of Philippines. 2010. Department of Environmental and Natural Resources (available at http://www.denr.gov.ph). Government of Trinidad and Tobago. 2011. Environmental Management Authority (EMA) (available at http://www.ema.co.tt/cms). Hall, P. 1974. The containment of urban England. The Geographical Journal. Vol. 140. No 3, October, pp. 386–408. Blackwell Publishers. Harwood, R.R. 1979. Small farm development: understanding and improving farming systems in the humid tropics. Boulder, Westview Press Inc. Midwest Plan Service. 1974. Farmstead planning handbook, Ames, Iowa, Midwest Plan Services. MidWest Plan Service (MWPS-2). 2005. Farmstead Planning Handbook: Guidelines for Planning and Expanding Agricultural Facilities and Operations. USA, Ames. Miller, G.A., Galanter, E. & Pribran, K. H. 1960. Plans and the structure of behaviour. New York. Holt, Rinehart and Winston. Noton, N.H. 1982. Farm Buildings. Reading, College of Estate Management. Ruthernberg, H. 1980. Farming Systems in the Tropics. 3rd edition. Oxford University Press. Wildavsky, Aaron B. 1973. If planning is everything, maybe it’s nothing. Policy Sciences. Vol. 4. Summer, pp 127–153. 21 Chapter 3 Graphical techniques InTROduCTIOn • The drawings are clean, neat and highly presentable. Graphics are essential for planning buildings, completing • The drawings can be subdivided into smaller engineering designs, estimating quantities of materials parts that can be reused or worked on by several and relative costs and, lastly, communicating to the people. builder all the information that the designer has • Updating drawings is much faster than with hand- formulated. drawn plans that would have to be redrawn. Computing, drafting, typing and printing • Drawings can be presented in different formats, technologies have changed dramatically since the early thereby facilitating transfer from one system to 1980s. Slide rules have been replaced by calculators and another. computers. Drawing tables, pencils, pens, T-squares • Several integrated tools are used to check drawings and erasers have been replaced by computers. Various for errors. computer hardware systems have been developed to • It is possible to work with real world units – the process high-quality graphics at very high speeds and to CADD system performs scaling automatically to keep project costs to a minimum. These technologies are fit any size of paper. known as computer-aided design and drafting (CADD). Computer-aided design and drafting (CAdd) CADD is an electronic tool for preparing quick and accurate drawings with the aid of a computer instead of the traditional tools (pencils, ink, rulers and paper). Unlike the traditional methods of preparing drawings on a drawing board, CADD enables high-precision drawings to be created on a computer. CADD software has generally replaced the traditional drawing board in drafting offices. In the 1990s, CADD was used only for specific high-precision Figure 3.1 A modern drafting office engineering applications. This was a result of the high price of CADD software, which made it accessible to only a few professionals. However, as prices have It should also be noted that current CADD systems fallen there has been a significant increase in the use are moving away from traditional drawing-oriented of CADD software and it is now widely used by most solutions towards fully featured architectural solutions professionals. (i.e. building information processing), which can be CADD software can be used to produce two- used not only to design a project, but also to manage the dimensional (2D) drawings directly or to build a three- enormous quantities of information (such as materials, dimensional (3D) model of a project, from which the prices or utilization) that go into an architectural software can extract 2D drawings that will be printed project. on paper. Some CADD software also includes modules for rendering realistic images. CAdd hardware and software Much more can be achieved using CADD than The main components of a CADD system are the with the traditional drawing board. Some of the major hardware and the software. The CADD hardware capabilities are: presentations, flexibility in editing, refers to the electronic and electromechanical parts units and accuracy levels, storage and access for of the system. The hardware include: system unit, drawings, sharing CADD drawings, project reporting, central processing unit, memory, hard disk, floppy disk, engineering analysis, computer-aided manufacturing CD-ROM, external storage devices, monitor, printers (CAM), design and add-on programmes. and plotters, keyboard, digitizer and mouse. Using CADD to produce a building drawing has the The CADD software refers to the instructions following advantages: that tell the hardware via the operating system how to perform specific tasks. A CADD programme contains 22 Rural structures in the tropics: design and development hundreds of functions to perform specific drawing be postponed to another time. Also, modifications can tasks such as drawing objects, editing objects, data be made to the files for use in another project. Such a management, data storage and data output. file can be renamed leaving the original file intact. The CADD programme usually implements a user The files dealt with in a given drawing office can interface to enable the user and the computer to run into hundreds. Thus, it is important that proper file communicate efficiently. Most CADD applications management be put in place. This is made easy by the provide a graphical user interface (GUI) for this computer as it allow files to be stored in directories and purpose. Using the GUI, the user may communicate subdirectories. Files organized in this manner are easier with the computer via menu bar, command line, tool to trace when required. buttons, dialogue boxes and the model space. These can be used to directly or indirectly call functions CAdd design applications implemented in the drawing module, which supplies The CADD system, as the acronym suggests, should the user with tools to: have a design component inbuild. But this is not the • draw lines case most of the times. Many CADD programmes • select line types; have only the drafting component even if they bare the • draw flexible curves; name CADD. However, mild design can be carried out • draw arcs and circles; with these systems. A CADD programme can only be • draw ellipses and elliptical arcs; called a design programme, if it has capabilites to solve • add text to drawings; problems and perform analyses. • manipulate text styles; Where a CADD system has design capabilities, • add dimensions to drawings; the programme is usually based on a number of • set dimension styles; principles that will vary from product to product. The • add hatch patterns to drawings; product can be based on performing calculations; it • draw symbols; may employ comparison and logic; use a database or • draw arrows. another form of artificial intelligence or combination of everything. For a given single task, several functions are executed The following are some examples of design behind the scenes that then results into the final programmes: drawing, which is basically a grouping of individual 1. Calculation programmes: These are extremely components (lines, arcs, circles, ellipses, polylines, text, effective in solving complex mathematical dimensions, pointers, symbols, borders and patterns). problems. Performing the same tasks with the traditional drawing 2. Intelligent CAD systems: These are based on board would involve use of several instruments, which logic and comparison and have a number of is time consuming and inaccurate. With a CADD applications in product design, mechanical system, all these are automated and can be performed design, spatial planning, etc. efficiently. 3. Knowledge-based CAD systems: These are also The edit module in CADD programmes provides known as expert systems. They make use of great flexibility in changing drawings. If the editing information gathered from previous projects (or functions of CADD were not available, then it would parameters defined by the programmer) and use probably take the same time to complete a drawing as it for new design proposals. it would on a drawing board. However, with editing capabilities inbuilt, CADD becomes a dynamic tool PROJECTIOnS that results in significant time savings. Changes that Projections are often useful in presenting a proposed may look extremely difficult on a drawing board can building to someone who is not familiar with a be easily accomplished with CADD. For example, presentation in the form of plans, sections and elevation even for major changes redrawing is not necessary drawings. Isometric or oblique projections are useful because diagrams can be manipulated in a number of for presenting a pictorial, although slightly distorted, ways to rearrange existing pieces of the drawing to fit view of a structure. The axonometric projection is the new shape. The basic editing capabilities include best suited to showing the interior of rooms with erasing, moving, rotating, mirroring, scaling, copying their furniture, equipment or machinery. The two- and changing the appearance of drawing objects. point perspective, which is a little more complicated Drawings created in CADD can be stored in the to construct on a drawing board, can be generated computer hard drive as memory blocks called files. The easily using CADD and gives a true pictorial view of user can name the files as desired, though the operating a building as it will appear when standing at about the system may impose some restrictions on the use of same level as the building and at some distance. specific characters and symbols. This is important as All types of projection can be constructed to scale, it enables the files to be accessed when required in the but they become really useful to the building designer future. For example, if some work on a specific file is to once the technique is so familiar that most of the details Chapter 3 – Graphical techniques 23 in the drawing, and eventually even the major contours Axonometric projection of the picture, can be drawn freehand. In an axonometric projection, the plan view of the building is drawn with its side inclined from the Isometric projection horizontal at any angle. Usually 30°, 45° or 60° is With isometric projection, horizontal lines of both chosen because these are the angles of a set square. All the front view and the side view of the building are vertical lines of the building remain vertical and are drawn 30° from the horizontal using dimensions to drawn to the scale of the plan view. scale. Vertical lines remain vertical and the same scale is used. Simple 2D functions can be used and lines drawn at specific angles to complete an isometric drawing. Polar coordinates are particularly helpful for measuring distances along an angle. Oblique projection An oblique projection starts with a front view of the building. The horizontal lines in the adjacent side are then drawn at an angle, usually 30° or 45°, from the horizontal. The dimensions on the adjacent side are made equal to 0.8 of the full size if 30° is used, or 0.5 if 45° is used. 30° 60° 30° 30° Figure 3.2a Isometric Figure 3.3 Axonometric projection PERSPECTIVE The different technical terms used in perspective drawing can be explained by imagining that you are standing in front of a window looking out at a building 45° from an angle where the two sides of the building are visible. If you then trace on the window pane what is seen through the glass, this gives the outline of the Figure 3.2b Oblique projections building. This results in a perspective drawing of the 24 Rural structures in the tropics: design and development building and, if the glass could be removed and laid THREE-dIMEnSIOn dRAWInG And on the drafting table, the drawing would look like a MOdELLInG In CAdd perspective drawing made on paper. The isometric, oblique and perspective views of objects The station point is the viewing point, supposedly can be easily, accurately and efficiently drawn in occupied by the eye of the observer. The viewing point CADD. CADD also provides a great deal of flexibility is also determined by the eye level, usually assumed in terms of editing and display compared to the to be 1.7 metres above ground level. Looking across drawing board. a large body of water or a plain, the sky and water/ Two methods can be used to draw 3D object. ground appear to meet in the distance, on the horizon The drawings can be done using 2D functions or 3D line. This must always be considered to be present, even functions. The 2D approach enables the designer to when hidden by intervening objects. The horizon line draw 3D objects in the traditional drawing board style is at eye level. in which drawing tools such as lines, arcs and ready to When standing and looking down a straight road, use 2D objects can be joined together to come up with the edges of the road appear to meet at a point the desired drawing. The 2D approach is quick way to called the vanishing point, which is on the horizon draw simple isometric and oblique views. However, 3D line and therefore also at eye level. Similarly, the objects drawn in this manner are static just as they are parallel horizontal lines of a building appear to meet at in traditional drawing board. vanishing points, one for each visual side. The inclusion 3D modules within a CADD system The outline of the building is brought to the greatly enhances it 3D drawing capabilities including window by your vision of the building, along the vision ability to perform 3D modelling and the ability to derive rays. The picture is traced on the window pane, which the 3D models from their 2D drawing views. The 3D is called the picture plane. capabilities enable the designer to create 3D models that As the technique with a window pane obviously are virtually realistic as the actual objects and can even cannot be used for a proposed, but still non-existent be made better by rendering programmes. The models building, the perspective has to be constructed from developed can be rotated on the screen, displaying views available documentation. A perspective drawing of a from different angles. This is advantageous when the building can be constructed using the plan view or, if designer need to view the model at different angles so as several buildings are to be included, the site plan may to make necessary adjustment and during presentations be more suitable. In addition you need elevations of to the clients, who are then able to view the end product all visual sides of the building(s), i.e. in the case of one in a virtual world and suggest changes if desired. The 3D building the front elevation and one end elevation. models developed can be wire-frame, surface or solid Side view 3D model 5 000 172.25 1 034 1 034 1 034 1 034 172.25 Front view Plan view Figure 3.4 Three-dimensional modelling 2 300 1 955.50 750 Chapter 3 – Graphical techniques 25 models. Figure 3.4 shows a 2D drawing together with its The following are the most important considerations 3D model developed within a CADD system. for plotting: • selecting a scale for drawings; PRInTInG And PLOTTInG PROCESS • composing a drawing layout; CADD drawings are printed using a printer or a • selecting text and dimension heights; plotter. The printing process is as simple as selecting • choosing pen colours and line weights. the print or plot function from the menu. This action sends data from the computer to a printer or plotter, Selecting a scale for drawings which produces the final drawing. The drawings are When working on a drawing board, a specific scale can neat, clean and – depending on the quality of the printer be used to draw diagrams. For example, when a plan of – highly accurate. a building or a township has to be drawn, the size of the diagrams can be reduced to 1/100 or 1/1000 of the actual size, i.e. using a scale of 1:100 or 1:1000. When a diagram of a small machine part has to be made, it is drawn many times larger than its actual size. CADD uses the same principle to scale drawings but takes a different approach. Standard paper sizes used for plotting As building drawings include many details, they should be large enough to be accurately executed and easily read. The standard formats from the A-series should be used for all drawings for a building. However, several detailed drawings may be put on one sheet. The A-series includes the following sizes: A0 841 × 1189 mm A1 594 × 841 mm A2 420 × 594 mm Figure 3.5 A plotter in use A3 297 × 420 mm A4 210 × 297 mm A number of parameters can be specified to control If the building plans tend to be very long, one of the the size and quality of a plot. A drawing can be plotted following alternative sizes may be useful: to any size by applying an appropriate scale factor. Line thicknesses and colours can be specified for different A10 594 × 1189 mm drawing objects. A number of other adjustments can A20 420 × 1189 mm also be made, including rotating a plot, printing only A21 420 × 841 mm selected areas of a drawing, or using specific fonts for A31 297 × 841 mm text and dimensions. A32 297 × 594 mm 8 8 54 10 20 REV. NOT. REVISION REFER TO. SIGN. DATE. Planning & designing Company. Project name or client name. Drawing contents & titles. DRAWN BY: DESIGN BY: CHECKED BY: SCALE. PLACE. DATE. SIGNATURE. JOB No. DRAWING No. REV. 26 26 26 44 44 12 78 100 Figure 3.6 Title box with revision table 50 12 8 30 6 26 Rural structures in the tropics: design and development If possible, only one format should be used for Glass window all drawings in a project, or alternatively all drawings should be of equal height. The formats A0, A10 and A20 are difficult to handle and should therefore be avoided. It is better to use a smaller scale or divide the Right hinged door figure into more drawings. with threshold CADD provides a number of special functions to compose a drawing layout. Diagrams can be arranged on a sheet as required and any scale factor can be applied. The drawing can then be plotted on the best Fire resisting fitting standard paper size. door Title box All drawings must have a title box, as shown in Figure 3.6. Double doors Note that the lines indicating the dimension limits do not touch the figure. Architectural symbols These are graphical representations of different features Left hinged door that appear on blueprint plans or elevation drawings without threshold of buildings. The graphics themselves can vary in appearance from one plan to another, but can usually be distinguished fairly easily by anyone with a basic understanding of their meaning. Sliding door Swing door Staircases - Arrows indicate movement up A GL A FLOOR PLAN SECTION ‘A-A’ Indication of section - Arrows show direction of view Figure 3.7 Architectural symbols Chapter 3 – Graphical techniques 27 Ceiling switch Wash hand basin Cord operated ceiling switch Two way switch Sink and drier One way switch One way, two gang switch Push switch Shower Electric bell Ceiling lighting point (bulb) Shower tray D Ceiling lighting point with drop cord Ceiling lighting point (bulb) in section Bath tub Wall lighting point (bulb) Fluorescent light one tube Two tube fluorescent light Socket outlet Two gang socket outlet 2 Switched socket outlet Water closet Fan In take and main control Tap hole Clock point Waste hole Outdoor lighting point Valve E Emergency light fighting Stop valve Fuse SV Earthing H Hydrant point for One phase power outlet with earthing fire protection Three phase power outlet socket with earthing P Pump M Electric motor Pressure tank Electric cooker Cold water pipe Hot water pipe G Gas cooker SYMBOLS FOR SANITATION MH Manhole Floor drain Soil vent pipe SVT GT Gulley trap Drain pipe Figure 3.8 Symbols for installations in buildings 28 Rural structures in the tropics: design and development Rough wood Finished wood Gravel or stone Conc. block Black in elevation concrete Common brick Insulation or Sheet metal Sheetmetal Earth loose packing & all metals in elevation Rock Sand Figure 3.9 Symbols for materials dOCuMEnTATIOn FOR A BuILdInG PROJECT Foundation plan A building project normally requires several types of Scale 1:200, 1:100 or 1:50 drawing that will be discussed in sequence in this section. • Earthwork for foundation; In small- and medium-sized projects, two or three • drainage; drawings may be combined into one, whereas in large • footings and foundation. projects each title listed may require several drawings. It is not advisable to include so much information in one Plan view drawing that interpretation becomes difficult. Scale 1:200, 1:100 or 1:50 • Outer walls; Site plan • load-bearing walls; Scale 1:1000, 1:500 or 1:200 • partitions; The location of the building in relation to its • main openings in walls and partitions (doors and surroundings, including: windows); • existing buildings, roads, footpaths and gravelled • door siting; or paved areas; • stairs in outline; • the topography of the site with both existing and • fixed equipment, cupboards and furniture; finished levels; • sanitary fittings; • plantings, fences, walls, gates, etc.; • major dimensions and positions of rooms, • north point and prevailing wind direction; openings and wall breaks; • the extent of earthworks including cutting, filling • section and detail indications; and retaining walls. • room names; • grid and column references (where applicable); Plan of external service runs • in multistorey buildings a plan is required for Scale 1:500, or 1:200 each floor. The layout of external service runs including: • electricity and telephone; Section • well or other source of water; Scale 1:100 or 1:50 • drainage (run-off rainwater, groundwater); • Structural system for the building; • drainage (wastewater, urine, manure); • major dimensions of heights, levels and roof slopes; • sanitation (septic tank, infiltration). • annotations on materials for walls, ceiling, roof and floor; External service runs are often included in the site plan • foundation (if not in a separate foundation plan). or the foundation plan. Chapter 3 – Graphical techniques 29 Elevation Plan of water and sanitary installations Scale 1:200, 1:100 or 1:50 Scale 1:200, 1:100 or 1:50 • Doors; • Pump, pressure tank, storage tank; • windows; • water heater; • miscellaneous external components; • water pipe locations; • shading and hatching for the texture of facing • tapping points, valves and control equipment; surfaces (optional); • wastewater pipe location; • dimensions of all projections from the building, • wastewater drains and sanitary installations; including roof overhangs. • annotations, dimensions, levels and slopes. details List of drawings Scale 1:20, 1:10, 1:5, 1:2 or 1:1 Where there are several drawings for a building project, The information that builders need for each element of the loss or omission of a single drawing can be avoided the building they are to construct may be classified as by listing all of them on an A4 sheet. Information on the follows: latest revisions ensures that all drawings are up to date. • What has to be installed or erected, including information about its nature and the physical Technical specifications dimensions. The technical specifications should set out quality • Where it is to be placed, requiring both graphical standards for materials and workmanship for the and dimensional information regarding its building elements that have been described in the location. drawings. Where general specifications are available • How it is to be placed or fixed in relation to they are commonly referred to and only variations are adjacent elements. specified in the technical specifications. However, in drawings for small- and medium-sized The designer must include all details necessary for the farm building projects, there is a tendency to include builder to complete all elements of the building. When directly on the drawings much of the information standard practice, general specifications or building normally given in the specifications. codes are not followed, it is particularly important As a basic rule, information should be given only to include complete detail drawings, annotations and once, either in the specifications, or on the drawing. specifications. Otherwise there is a risk that one occurrence will be Where prefabricated elements are used, for example forgotten in a revision and thus cause confusion. windows, a specification rather than a detail drawing is adequate. This allows the builder to choose the least Functional and management instructions expensive alternative that meets the specification. Frequently information has to be transferred to the Where machinery and equipment require special person using a structure to enable him or her to utilize foundations, supports, openings and cavities, the it in the most efficient way, or the way intended by the required detail drawings will, in most cases, be supplied designer. In a pig house, for example, different types of by the manufacturer. pen are intended for pigs of different ages. Alleys and Often there is no need to produce detail drawings door swings may have been designed to facilitate the specifically for each project. An established drawing handling of pigs during transfer between pens. In a grain office will have detail drawings covering the most store, the walls may have been designed to resist the frequent requirements, which may be affixed to current pressure from grains stored in bulk to a specified depth. projects. Bill of quantities Plan of electrical installations The bill of quantities contains a list of all building Scale 1:200, 1:100 or 1:50 materials required and is necessary to make a detailed • Incoming power supply and all wire locations; cost estimate and a delivery plan. It cannot be produced, • main switch, fuses and meter; however, until the detailed working drawings and • location of machinery and switches; specifications have been completed. Bills of quantities • location of lighting points and switches, both are further discussed in Chapter 9 of this text. internal and external; • sockets; Cost estimate • annotations and dimensions. The client will require a cost estimate to determine whether or not the building should be constructed. Clients need to know whether the proposed design is within their financial means and/or whether the returns on the intended use of the building justify the investment. 30 Rural structures in the tropics: design and development Month n. Activities 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 Prepare working drawings 2 Prepare bills of quantities 3 Tendering 4 Site clearing & excavation 5 Building operations 6 Delivery of feed and animals 7 Commencing farrowing 8 Commencing sale of pigs Figure 3.10 Time schedule Time schedule cardboard or are computer-generated using 3D A simple progress chart, as shown in Figure 3.10, graphics, and there is usually little attempt to will considerably facilitate the planning of building show details, although furnishings and equipment operations and subsequent activities. may be indicated. Windows and door openings Farmers may obtain information concerning when are shown with dark-coloured areas or left open. they and any farm labourers will be involved in Contours are shown only if they are of importance construction operations, when animals and feed should for the building layout. be delivered, when a breeding programme should be • Fully developed models may be used in extension started, or the latest starting date for the construction campaigns, for public exhibition, etc. These models of a grain store to be completed before harvest. This show details to scale and represent as accurately as is the type of information needed to enable the returns possible the actual materials and colours. Part of on the investment to be realized as early as possible. the roof is left out or made removable in models A contractor will require a more detailed chart for the aiming to show the interior of a building and, with actual construction operations to ensure the economical current CADD software, it is even possible to use of labour, materials and equipment. make a virtual tour of the building. MOdEL BuILdInGS Even people with a good basic education will need considerable experience to be able to envisage fully a building from a set of drawings. The rural building engineer will therefore soon learn that the average rural dweller not only finds it very difficult to understand simple plan view and section drawings, but may even find it hard to interpret fully rendered perspectives. However, the fact that a model, unlike drawings, is three-dimensional and thus can be viewed from all sides brings more realism to the presentation and usually results in better communication and transfer of ideas. There are three types of model in common use for the presentation of rural building projects: • Three-dimensional maps or site plans are used to present development plans for large areas or the Figure 3.11 Computer-generated 3d model addition of a new building on an old site with existing structures. These models have contours to show the topography, while structures are rendered in simple block form with cardboard or solid Physical model wood, usually with no attempt to show detail. Where the model is to be physically built, a sturdy base • Basic study models are used to examine the for the model, made of either plywood or particle board, relationships and forms of rooms and spaces not only facilitates handling but also helps to protect in proposed buildings. They are often built of the model. For models to be displayed in public, it is Chapter 3 – Graphical techniques 31 advisable to have well-finished borders, preferably in materials, and thin grass glued to the cardboard can be hardwood and, although expensive, an acrylic plastic used to represent thatch. (plexiglass) cover. During transport, a plywood box The strength and rigidity of models can be increased without a bottom, fixed to the base of the model with by bracing the walls with square pieces of cardboard screws, will provide sufficient protection if handled in positions where they will not be seen in the finished with care. Otherwise, where appropriate, a full virtual model. Bracing is particularly important in models tour of a building project can be shown to the public that are going to be painted, as paint tends to warp directly from a computer using an electronic projector. cardboard and sheet wood if applied over large areas. The size of the model is determined by the scale Regardless of the material being represented, colours to which it is made and the size of the actual project. should be subdued and have a matt, not glossy, finish. While detail is easier to include in a model made to a Distemper or water colour is best for use on cardboard large scale, too much detail may distract from the main and unsealed wood, but care must be taken to remove outlines and essential features. If the model is too large excess glue, as this will seal the surface and cause the it will be more costly and difficult to transport. Basic colour to peel off. study models are often made to a scale of 1:50 or 1:100 A photograph of the model may be used in cases to allow for coordination with the drawings, while fully where it is not feasible to transport the model, or when developed models of small structures may be made to photos need to be included in information material a scale of 1:20 or even larger. Whatever scale is used but the actual building has not yet been completed. for the model, it is desirable to include some familiar Models often appear more realistic when photographed, objects, such as people or cars, to the same scale as the particularly in black and white because of the better model to give the observer an idea of the size of the contrast, but adequate lighting from a direction that actual structure. produces a plausible pattern of sun and shade on the The construction of contours and elevations requires building should be used. Outdoor photography allows access to a map or a site plan with contour lines to the sky or terrain to be incorporated as a background in the same scale as that used in the model. One way of the photograph of the model. showing contours is to build up a model with layers of cardboard or styrofoam sheets of a thickness equal to Computer-generated models the scale of the real difference in height between contour These models can be built directly using 3D CADD lines. Employing one piece of cardboard for each software or from the 2D drawings of the building. contour line, trace the line onto the cardboard using Once the model is complete, the addition of features carbon paper, cut out the contour, place it on the model to the model from the material library can achieve a and secure it with glue. The contours can either be left as realistic model picture. The quality of the model can they are, giving sharp, distinct lines, or be smoothed to a be further improved by the use of rendering software. more natural slope using sandpaper or filler. If the CADD software allows, the developed model For more elaborate models the landscaping may can be used to simulate different conditions that may be represented by painting. Trees and bushes can be be experienced in the real building. For example, the made from pieces of sponge or steel wool on twigs or building may be oriented in different directions and the toothpicks. Coloured sawdust can be used for grass and effect of environmental factors such as wind and solar fine sand for gravel. If available, model railroad supplies radiation can be studied to achieve the optimum design and other hobby materials can be useful. conditions. Although the same materials employed in the actual building, or close simulations, are used for the most REVIEW quESTIOnS elaborate models, cardboard (or for models made to 1. What are the advantages of CADD over manual a large scale, plywood) is usually easier to work with drafting? and can be finished by painting to represent most types 2. Outline some of the capabilities of a CADD of material. Cardboard or plywood of the right scale system. thickness for use as walls is often unavailable, but it 3. Describe the components of a CADD system. will make no difference as long as the overall scale and 4. How can a designer benefit from the 3D dimensions of the building are maintained. capabilities of a CADD system? Round wooden posts commonly used in farm 5. What are the advantages of developing a buildings for post-and-beam or pole construction computer-generated model over a physical model can be conveniently made from twigs or hardwood of a building project? sticks. Any finish on the walls to represent openings or materials should be applied before the model is assembled. Neat, clean-cut lines are easier to achieve in this way. While a plain cardboard roof is adequate for most purposes, corrugated paper painted in a suitable colour may be used to represent corrugated roofing 32 Rural structures in the tropics: design and development FuRTHER REAdInG Bellis, H.F. & Schmidt, W.A. 1971. Architectural drafting. New York, McGraw-Hill Book Co. Duggal, V. 2000. CADD primer: a general guide to computer-aided design and drafting. Mailmax Pub (also available at http://www.caddprimer.com). Giesecke F.E., Mitchell, A.T., Spencer, H.C., Hill, I.L., Dygdon, J.T. & Novak, J.E. 2008. Technical Drawing. 13th edition. Prenice Hall. Jefferis, A., Medson, D.A., & Madsen, D.P. 2010. Architectural drafting and design. 6th edition. Kicklighter, C.E. & Kicklighter, J.C. 2008. Architectural: residential drawing and design. 10th edition. Goodheart-Willcox. McBean, G., Kaggwa, N. & Bugembe, J. 1980. Illustrations for development. Nairobi, Afrolit Society. Osbourn, D. & Greeno, R. 2007. Introduction to building. 4th edition. London, Pearson Education, Prentice Hall. Shumar, T.M. & Madsen, D.A. 2009. Autocad and its applications – basics 2010. 17th edition. Goodheart – Willcox. Styles, K. 1982. Working drawings handbook. London, Architectural Press. Taylor, R. 1971. Model building for architects and engineers. New York, McGraw-Hill Book Co. Winden,V.J., de Keijzer, M., Pforte, W. & Hohnerlein, F. Rural building - drawing book. Maastricht, Netherlands, Stickting Kongregatie F.I.C. 33 Chapter 4 Geospatial techniques InTROduCTIOn In large and complicated projects, it is necessary to Geospatial technology is an integration of various engage the services of a geospatial expert, also known technologies in the mapping, visualization and recording as a surveyor. In small buildings or infrastructure of phenomena in the Earth system and space. Down projects, and other professional, such as an engineer, through history humankind has been attempting to may perform the geospatial tasks. fully understand and document the Earth, and this has At different times in history, a variety of tools have driven innovation in geospatial science to the current been developed and used for land measurement, plan state of the art. production and setting out buildings. These range Geospatial technology encompasses the following from pacing methods to hand-held instruments. The specialist areas: various methods are discussed in the section ‘Survey of 1. Engineering survey: This involves the preparation a building site’. of maps and plans for planning and designing structures, as well as ensuring that they are SuRVEy OF A BuILdInG SITE constructed in accordance with the required A simple survey of a building site provides accurate dimensions and tolerances. information needed to locate a building in relation 2. Geographic Information Systems: This involves to other structures or natural features. Data from the collecting and manipulating geographic survey are then used for drawing a map of the site, information and presenting it in the required including contours and drainage lines if needed. Once form. located, the building foundation must be squared and 3. Cartography: This is the accurate and leveled. This section covers the various procedures precise production of maps or plans and the involved. representation of the information in two or three dimensions. distances 4. Photogrammetry: This involves obtaining Steel tapes or surveyor’s chains are used for measuring information from photographic images in order distances when stations are far apart and the tape or to produce a plan of an area. chain must be dragged repeatedly. Linen or fiberglass 5. Hydrographic survey: This involves measuring tapes are more suitable for measuring shorter distances and mapping the Earth’s surface that is covered such as offsets when making a chain survey or laying by water. out a foundation. To obtain accurate results, a chaining crew must first practice tensioning the chain or tape so In the development of rural structures, the that the tension will be equal for each measurement. engineering survey is most important because it allows: Range poles are 2 to 3 metres metal or wooden poles 1. Investigation of land using manual or computer- painted with red and white stripes, and are used for based measuring instruments and geographical sighting along the line to be measured. knowledge to work out the best position for Land arrows come in sets of 10 and are set out by constructing buildings, bridges, tunnels, water the lead person in a chaining crew and picked up by the channels, fences and roads. following person. The number picked up provides a 2. Production of plans that form the basis for the check on the number of lengths chained. design of rural structures. A field book is used for drawing sketches and 3. Setting out a site so that a structure is built in the recording measurements. correct position and to the correct size. When measuring for maps or site plans, horizontal 4. Monitoring the construction process to make distances are required. Thus, when chaining on sloping sure that the structure remains in the right land, stepping will be necessary. This procedure allows position, and recording the final position of the the tape or chain to be kept level, as checked with a structure. hand or line level, while the point on the ground under 5. Provision of control points by which the future the high end of the tape is located with a plumb-bob, as movement of structures, such as roads, water shown in Figure 4.1. dams, channels and bridges, can be monitored. 34 Rural structures in the tropics: design and development Plumb-bob The distance measured in each step depends on whether a Heavy Chain or a Light Tape is used A B C Figure 4.1 Stepping on sloping ground Angles There are several types of tripod-mounted levels Figure 4.2 illustrates this procedure, as well as the available, some of which are equipped with horizontal method of swinging an arc to erect a perpendicular. rings allowing them to be used for measuring or setting Two simple instruments for setting out right angles out horizontal angles. Theodolites are designed to are the cross-stave and the optical square (Figure 4.3). measure or set out both horizontal and vertical angles. Either can be mounted at eye level on a range rod at the Although these surveying instruments provide the most corner where the angle is to be set out. The instrument accurate means of measuring angles, they are expensive is then turned carefully until one line of the right angle and rather delicate. Fortunately much of the surveying can be sighted. The second line can then be swung of rural building sites involves only distances, 90° slightly until it can also be sighted. angles and contours that can be measured or set out with fairly simple equipment. One simple yet accurate means of setting out the 90° corners of a building foundation makes use of the Pythagorean Theorem, or the 3–4–5 Rule (or any multiple of the same). Starting at the corner of the foundation site, a line is stretched representing one side of the foundation. A distance of 4 metres (m) along the line is marked. Then another line is stretched from the corner at approximately 90°, and 3 m is measured along this line. When using the tape between the 4-m and the 3-m marks, the second line is swung slightly until exactly 5 m is measured between the marks. The first two lines then form a 90° angle. Figure 4.3a Cross-stave Q 5 m 3 m Chain A 4 m P Line Figure 4.2a The 3–4–5 Rule P AQ = QB 90° Chain A Q B Line Figure 4.2b Erecting a perpendicular Figure 4.3b Optical square Chapter 4 – Geospatial techniques 35 Vertical alignment Hand levels and Abney levels are both hand-held A surveyor’s plumbline consists of a sturdy cord, a instruments incorporating a spirit bubble tube and a distance bar and a conically shaped plumb-bob with split-image mirror. When they are held to the eye and a hardened steel point. It is used for positioning the bubble centred, the viewer is looking at a point surveying instruments or when stepping with a tape exactly at eye level. They are useful for keeping a chain or chain. It may also be used to check the vertical or tape horizontal when stepping, and for doing simple alignment of foundations, walls and posts. A simple contouring. The accuracy of work with either of these plumbline for these jobs can be made from string and a levels may be improved somewhat by placing the level stone (see Figure 4.4). on a rod of known length, still keeping the instrument approximately at eye level. As they have either a low- power scope or no telescope, they are only suitable for distances of up to approximately 30 metres. Distance bar For levelling the lines used in laying out a foundation, a builder’s water level is a simple, inexpensive device that provides a satisfactory degree of accuracy. It consists of a length of rubber or plastic tubing, at each end of which there is a transparent sight-tube of Plumb bob glass or plastic. It works well over a distance of about 30 m and is particularly useful for transferring levels around corners, from outside a building to inside, or around obstacles where the two levelling points are not intervisible. It is also a useful tool for obtaining the Figure 4.4 Plumb bobs slope in pipe runs. See Figure 4.6 for the method of use. Leveling Builders Just as in the case of angle measurement, there is a wide level variety of surveying instruments used for leveling. Most Water level are designed for accuracy and are rather expensive. ‘B’ Glass tubes ‘A’ Although built for use in the field or on a building site, as with any precision instrument they require careful Hose handling and regular attention to ensure good service. Fortunately, there are several rather simple devices that may be used for leveling foundations, running 1 Set corner profiles at one corner as at right contours or aiding in step-chaining. 2 Place hose as shown Builder’s levels are made of wood, plastic or 3 Fill with water until water level is at top of corner profile A aluminium and are available in several lengths, 1 metre 4 Mark water level at opposite end B and set profile to mark being a convenient size. The bubble tubes are graded for sensitivity to suit the work. Most are now made of Figure 4.6 Setting out corner profiles plastic and filled with fluorescent liquid – an aid in poor light. see Figure 4.5. Line levels are designed to hang on a tightly stretched Chain surveying line. Both of these types are useful in foundation In a chain survey, the area to be surveyed is enclosed by construction work. one or more triangles, the sides of which are measured and recorded. Then the perpendicular distance from the side of a triangle to each point of detail, such as trees, buildings or boundaries, is measured. From this information, a detailed plan of the site can be drawn to scale. A proposed structure may then be superimposed on the plan and its location transferred to the actual land site. The following step-by-step procedure is used in a chain survey: 1. Make a preliminary survey by walking around the site, deciding where to put stations and Alluminium line and surface level where the main survey lines should be arranged. Stations should be selected so that they are Figure 4.5 Builder’s level and line level intervisible and the lines laid out so that obstacles 36 Rural structures in the tropics: design and development are avoided. Make a sketch of the site in the The contour lines may then be indicated by fieldbook (Figure 4.7a). interpolation. Contour points are plotted on each 2. Set the range poles, chain the triangle sides and line between each pair of spot levels in the grid, record the distances. assuming the ground has a fairly constant slope. 3. Measure the perpendicular offsets from the chain A smooth curve is then drawn to link up points lines to the details of the site. This will be easier of the same height. Note that contour lines may to do if the chain lines have been arranged so that not cross, but they may approach closely at points offsets can be kept as short as possible. Record where the gradient of the ground surface is steep. the measurements in the field book (Figure 4.7b). Each page should record offsets along one To produce the final map or site plan, cover the chain line. Entries start from the bottom of the preliminary drawing with tracing paper and draw the page and details are entered to the left or right final plan, omitting the survey lines, offset lines and grid. of the centre column where distances along the chain line are noted. Not all details are measured by perpendicular offsets. Sometimes it is more D C accurate and convenient to use pairs of inclined offsets which, together with a portion of the chain, form acute-angled triangles. Note the top Well corner of the house in Figure 4.7b. Gum 4. If contour lines need to be included on the map House or site plan, the next step will be to measure Hedge levels with a levelling instrument and a staff. Proposed The grid method is most commonly used for barn Gum construction projects, provided the ground does B not slope too steeply. The grid is pegged out on the site in the position considered most suitable, and levels are taken at points where lines intersect. Road Sides of squares may be 5 m to 30 m, according Gum A to the degree of accuracy required. If the area is reasonably small, staff readings may be recorded near to each point on a sketch or drawing similar Figure 4.7a Field book sketch of the site with stations to that shown in Figure 4.7c. Alternatively, and main survey lines staff readings may be recorded in a field book. Each point has a reference letter and number. If all points on the site are within range of the TO C 33 65 levelling instrument and, providing the staff at each point can be seen through the telescope, the 8∙5 60 instrument should preferably be set up near the 13∙5 TO B 65 middle of the site so that all readings can be taken from one position. The first staff reading is made on an ordnance benchmark (OBM), if one is available in the near vicinity, or alternatively on a site 10∙0 5∙0 50 57 17∙ ø 4∙5 5 datum, which may be assumed to be at a reduced ø 3∙9 5∙7 level of 10  m, or any other convenient height. 9∙0 4∙0 40 It is normal practice to leave a number of 37∙5 16∙1 selected and carefully driven pegs in position 12∙0 33∙5 on the site to assist in the work of setting 9∙5 4∙5 30 6∙15 out when development work commences. 28∙3 14∙5 From the spot levels obtained by this grid method, the contours can be drawn, the volume 12∙5 7∙5 20 of earth to be excavated can be calculated and the average level of the grid can be determined. 13∙8 13∙0 5. Make a map or site plan. Start by making a scale 16∙0 11∙0 10 7∙25 6∙0 drawing showing the main surveying lines. Then 7 TO C 49 plot the offsets to buildings and other features in the ° TO B 47 same order as they were recorded in the field book. If contour lines are to be included, start by Figure 4.7b Field book recordings of offsets along drawing the grid to the scale of the drawing. chain line A–d H PROPOSED OUSE BARN 9∙75 14∙5 Chapter 4 – Geospatial techniques 37 D Setting out Setting out line line 2 90° 8∙3 7∙2 ∙ 9∙7 9∙6 9∙2 000 0 6 8∙0 2∙0 0 8∙5 8∙7 Corner post 10∙1 10∙0 9∙5 8∙6 5 9∙0 9∙5 Figure 4.8 Builder's square 10∙0 9∙9 9∙8 10∙6 10∙6 10∙2 4 10.5 Having obtained the direction of all lines, measured 10∙5 11∙2 10∙4 10∙2 all distances and driven pegs and nails at the points, 3 11∙1 the accuracy of the setting-out may be checked by 11∙0 measuring the overall horizontal distances in both 10∙6 10∙7 10∙3 10∙6 2 11∙0 10∙5 directions. Pairs of lines should be exactly equal. 10∙0 Check again the accuracy of the setting-out by 9∙5 9∙0 9∙1 measuring the diagonals of the rectangle. For buildings 9∙8 9∙6 9∙2 1 8∙9 with sides between 5 m and 20 m long, the length of the diagonals A and B in Figure 4.9 should not differ by more than 0.5 percent. If adjustments are necessary A B C D E subsequent to this check, it is advisable to keep the two longest parallel sides fixed and to make the required A Site datum (8·0 m) adjustments on the short sides. Line A-D Finally, check the drawing against the setting-out to ensure that lines and corners are in their correct Figure 4.7c Site plan made up to scale from field book positions and that dimensions are correct. recordings Carpenters square may be used in laying Setting out the building work 1 Put up profiles at corners out approximate corners Before a decision about the final site of a building can 2 Outside edges of building be made, a number of factors need to be taken into account. Consideration must be given to local authority Plumb line and planning regulations, to functional requirements, A orientation, view, prevailing wind, noise, shelter, water B supply, access, slope of ground, privacy and the type of soil on which to build. Orientation can be important – perhaps the best position for comfort is an east–west alignment. This When length A equals arrangement eliminates much glare by confining the length B the corners are square when opposite sides sun’s rays to the end walls. It also allows cross- are equal ventilation – crucial when humidity is high. To set out a building there needs to be a base line Figure 4.9 Corner profiles and checking for accuracy (one side of the building) and a fixed point on the line, usually one corner of the building. At this point, as at all other corners, a peg is first driven and then a nail is When the setting-out and checking have been driven into the top of the peg to mark the exact position completed, timber profiles are erected. Profiles consist of the corner. of horizontal rails supported by vertical pegs set up clear The distance from one peg to the next is carefully of the excavation. Inside and outside faces of the wall and measured with a steel tape, and the peg and nail firmly the width of the foundation are marked on the horizontal driven. Depending on the size and nature of the rail by means of fine nails or saw cuts. Strings are later building, the correct position of all other lines and pegs stretched between these nails or saw cuts on opposite in relation to the base line and to each other may be rails to guide the workers during trench excavation and obtained by means of: footing and foundation wall construction. • a levelling instrument fitted with a horizontal circle; Ideally, profiles should be set up for all corners and • a cross-stave or optical square; internal walls. The profile shown at A in Figure 4.10 • a flexible tape, using the 3–4–5 method; should be located at A1, if the foundation area is to be • a builder’s square (see Figure 4.8). excavated. 38 Rural structures in the tropics: design and development Excavation depth control When any building work is to be done, it is usually necessary to excavate at least a foundation trench. In many cases, if concrete is to be used, some excavation is required in order to make the floor finish at the level A1 A required. In addition, it may be necessary to finish a surface such as a roadway or ditch bottom to an even gradient. In all these cases it is necessary to control the depth of the excavation to ensure that the correct amount of soil is removed. Sight rails Sight rails are made either across the line of an excavation, such as a trench, as shown in Figure 4.11, Figure 4.10 Plan of walls and profiles or alongside an area such as a roadway or floor. If the Sight rail Traveller or boning rod Sight rail Figure 4.11 Sight rails and traveller for boning Sight line Ground level Formation line Figure 4.12 Section between two sight rails on a gradient Chapter 4 – Geospatial techniques 39 Sight line Traveller length Ground level 1∙5 m 0∙7 m Minimum depth 0∙5 m Formation line Figure 4.13 Section showing level excavation excavation is to be level, then the tops of the crosspieces Often, however, the land has a considerable slope must all be at the same height. If there is a gradient and must be levelled before construction can begin. to the excavation, however, the tops of the sight rails Sometimes the soil will need to be removed from the should be set at heights so that they fall on the same site but, in many cases, soil removed from the building gradient (see Figure 4.12). site can be used for fill in an adjacent area. It is rather On a small building site it may be possible to use a more difficult to estimate how much to ‘cut’ to ensure long straight-edge with a spirit level to ensure that the that the soil removed just equals the ‘fill’ required to sight rails are level. However, with longer excavations give a level site. Several approaches are explained in or where a gradient is required, it may be necessary to surveying books, but a graphical method using the use a tape and level to achieve the appropriate fall from information from the site contour map should be one sight rail to another. satisfactory for rural building construction. A scale drawing of the building foundation is made Traveller and the contours superimposed on it (Figure 4.14a). A A traveller, also known as a ‘boning rod’, is T-shaped line is drawn through the centre of the building plan and normally wooden. The overall length is the same as and a section constructed using the values obtained the distance from the sight rail down to the excavation from the intersections of the contour lines and the depth required, as shown in Figure 4.11. It can be section line (Figure 4.14b). an advantage, therefore, to set up the sight rails at a known height above the excavation. For example, a level excavation will normally be specified as having a minimum depth. If a trench is required with a Contours superimposed on minimum depth of, say, 0.5 m and the ground rises building plan along the length of the trench by 0.7 m, then the first M profile must be set high enough for the second to be 11∙0 above the ground, and a traveller of 1.5 m may be used. The first profile will then be 1 m above the ground. See 10∙5 Figure 4.13. 12 As the excavation progresses, the depth can be checked by looking across from the top of one profile 10∙0 6 to another. As long as the traveller crosspiece can be 9∙5 seen, the excavation is not deep enough and should be continued until the crosspiece is just invisible. N Volume of earth to be removed The labour and expense involved in moving soil can be substantial. Careful planning and volume estimation Figure 4.14a Contours for establishing a cut and fill line will minimize the amount that needs to be moved. When the land is essentially level, the volume to be removed from an excavation can be estimated by A horizontal line is then drawn that is estimated to multiplying the cross-section area of the excavation by produce equal areas for cut and fill. The elevation of the the length. line indicates an optimum elevation for the building. The approximate volume to be moved is given by the equation: 40 Rural structures in the tropics: design and development V = ½ hbw These technologies are important in the positioning, = ½ × 0.6 × 6 × 6 mapping and design of rural structures and infrastructure. = 10.8 m3 Remote sensing where Remote sensing involves the detection and measurement h= height above line of radiation/reflectance of different wavelengths b = base of cut area reflected or transmitted from distant bodies. It is the w = width of cut area science and art of identifying, observing and measuring objects without coming into direct contact with them. 11∙5 SECTION M-N Global Positioning System (GPS) Elevation- 11∙0 The GPS is a system of 24 satellites owned and managed by metres cut the United States Air Force. The satellites orbit the Earth 10∙5 fill 10∙0 continuously and transmit radio signals that are tracked 9∙5 by GPS receivers on the Earth’s surface to compute their 0 1 2 3 4 5 6 7 8 9 10 11 12 three-dimensional position in an Earth-fixed coordinate frame/system. With the three-dimensional coordinates computed, the position of the receiver is defined uniquely in the three-dimensional coordinate frame. 2∙5 UNEVEN SLOPE 10m WIDE 2∙0 The GPS infrastructure is composed of three 1∙5 segments (see Figure 4.15): 1∙0 1. Space segment: This comprises a constellation of 0∙5 24 satellites that broadcast electromagnetic signals 0∙0 to the GPS receivers on Earth. They also receive 0 1 2 3 4 5 6 7 8 9 10 commands from the ground control stations. 2. Control segment: This monitors the space Figure 4.14b–c Estimating cut and fill segment and sends commands to the satellites. It computes the satellite orbit data and uploads them to satellites for transmission to receivers. It also If the slope is not as uniform as illustrated in Figure monitors the satellite clocks for synchronization 4.14b, the slope line must be averaged as shown in and general satellite health. Figure 4.14c. In this example the volume to be moved 3. User segment: This comprises satellite receivers is estimated to be 45.6 m3. sited on the Earth surface, including the air and the sea. The receivers record and interpret the V = ½ hbw electromagnetic signals broadcast by the satellites = ½ × 1.9 × 4.8 × 10 and compute the position to varying degrees of = 45.6 m3 accuracy depending on the type of the receiver and the physical conditions. When excavated, the volume of firm soil will increase by approximately 20 percent. If this soil is used for fill, it must either be allowed to settle for some time or be compacted to reduce it to the original volume before GPS SV any construction work can begin. In addition, African soils are generally prone to settlement and erosion. Problems may be experienced in wet areas at the edge of the fill if it is not adequately stabilized with vegetation or a retaining wall. Therefore the ‘cut and fill’ technique Data to SV should be avoided and, if used, a reinforced concrete Data from SV footing may be required. Data from SV MOdERn GEOSPATIAL TECHnOLOGIES Modern scientific advances in information technology have resulted in the development of the following areas of geospatial science: 1. Remote sensing 2. Global Positioning System (GPS) Control station Receiver 3. Geographic Information Systems (GIS) 4. Digital mapping Figure 4.15 GPS segments Chapter 4 – Geospatial techniques 41 The computation of three-dimensional coordinates of points in the Earth space allows the mapping of H S features in a given reference system. The mapping accuracy varies according to the type of the GPS receiver used. Hand held receivers are less accurate as compared to geodetic receivers. R r Principle of GPS positioning The GPS can be explained by a simple resection process where ranges/distances/vectors are measured from a user’s GPS receiver to the orbiting satellites 20 200 km GR above the Earth surface. Earth Surface Consider a satellite S, at a single epoch (instant of ρ GPS time system) being tracked by a GPS receiver GR, on the Earth surface (Figure 4.16a). The Geocentre, Geocentre as shown, is the centre of the Earth with coordinates X,Y,Z (0,0,0). Figure 4.16a One satellite tracking The space co-ordinates of the satellite, relative to the earth centre can be determined from the ephemeris (orbit data) broadcast. The vector r, the geocentric co-ordinates of the satellite are therefore known. The HS vector/range R, from the receiver to satellite is normally 1 H S2 H measured by the receiver from the signals from the S satellite. This is achieved by multiplying the GPS signal 3 R3 travel time from the satellite to the receiver and the R2 r R r 2 velocity of the GPS signal. 1 1 The vector r, is the geocentric co-ordinate of the receiver at ground station whose values are unknown. r The unknowns in this case are the three Cartesian 3 coordinates (X, Y, Z). The solution of the three GR co-ordinates requires at least three equations. Given that every measured vector to a satellite produces one Earth Surface ρ equation and there are three unknowns, observations to at least three satellites as shown in Figure 4.16b are required. Geocentre Using the three vector/ranges equations, the solution is computed as indicated by the following equation: Figure 4.16b Three satellite tracking Rs = (X − X 2 Ys Y 2 s r) + ( − r) + (Zs − Z )2 r r HS1 H S2 H where Rs r is the range between the satellite and the S R 3 receiver, (Xr, Yr, Zr) are the coordinates of the receiver R 3 2 r and (Xs, Ys, Zs) are the coordinates of the satellite. 2 R When the system is using code pseudo-ranges for 1 r1 r 3 range measurements, additional unknown, the GPS R4 S4 time exists, increasing the total number of unknowns to four. Thus measurements to at least four satellites GR r are necessary (see Figure 4.16c). With carrier phase 4 measurement, an additional unknown, the ambiguity ρ Earth Surface number, is introduced. Here again, the solution will require at least four satellites. Whichever the case, any GPS measurements therefore requires observations to Geocentre at least four satellites for observation positions to be determined. In this case the position of the receiver is Figure 4.16c Four satellite tracking obtained as X, Y, Z, t, where t is the time. Figure 4.16 GPS positioning principles H 42 Rural structures in the tropics: design and development Geographic Information Systems (GIS) digital mapping This is the merging of cartography and database Digital mapping is the production of maps in electronic technology. It refers to a set of systems that captures, or computer-compatible formats. In digital mapping, stores, analyses, manages and presents data that are the geographic location of terrain points in a given linked to location. Technically, a GIS is a system reference system is stored in an electronic medium in that includes mapping software combined with the form of letters or numbers (i.e. X, Y, Z or latitude, an application for remote sensing, land surveying, longitude and heights). Digital maps are compiled from aerial photography, mathematics, photogrammetry, aerial photographs and/or satellite imagery. geography and software tools. Apart from supporting Photogrammetry is the art, science and technology map drawing on demand, GIS is able to support of obtaining reliable information about physical objects decision-making because of its ability to match the and the environment through a process of recording, spatial characteristics of the data (such as position and measuring and interpreting photographic images the topology of lines and polygons, i.e. how points are and patterns of recorded radiant energy and other connected to each other) with other textual data. This phenomena. auxiliary data about points, lines and polygons on a Photographs are still the principal source of map forms part of the attribute data (see Figure 4.17). information, and included within the domain of photogrammetry are two distinct processes: (1) metric photogrammetry and (2) interpretive photogrammetry. Spatial data Attribute data Metric photogrammetry involves making precise Natural resources layers measurements from photographs to determine the Vegetation Agriculture relative location of points. This enables angles, distances, Topography areas, elevations, sizes and shapes to be determined. The most common application of metric photogrammetry Cadastral layers Owner Value is the preparation of planimetric or topographic maps from aerial photographs. The principle of metric photogrammetry is to Statistical layers Population distributions conduct measurements on a pair of photographs Sociological data (stereo-pair) with an overlap area that appears in both photographs (see Figure 4.18). Through mechanical Transportation layers Street networks or computational means, the position and orientation Traffic flows of the two cameras are determined in a model, which includes the photographed terrain in its three- Infrastructure layers dimensional (scaled) visualization. The horizontal Water mains Telephone (planimetric) and vertical (height) coordinates can be scaled off this virtual model (viewed stereoscopically through special optics). Figure 4.17 The GIS concept incorporating spatial data layers and attribute data Left eye Right eye Right photo In a GIS database, each layer of a map may have a set of global attribute data specifying, for example, o o2 1 the origin of the data set, when it was last updated, o1 o o2 1 the purported accuracy of the data and how it was o2 captured. This is sometimes referred to as ‘metadata’. h However, attribute data for specific feature points in Φ0 a map (or map layer) usually take the form of a text table. The definition of the text table normally differs A for different features. For example, a tree may have Φ attribute data linked to it concerning species, height, 0 girth, etc., while the attribute data linked to a road B segment will have different entries, such as width, date of construction or surface quality. The ability to access this additional data (hidden in attribute data tables) simply by pointing to the map feature on the computer screen makes GIS a powerful query tool. Figure 4.18 Stereoscopic reconstruction of terrain objects using overlapping aerial photos Chapter 4 – Geospatial techniques 43 Some of the most significant advances in FuRTHER REAdInG photogrammetry have been to transform the process Allan, A.L. 2007. Principles of geospatial surveying, of making maps using labour-intensive, analogue London,Whittle Publishing. photogrammetric techniques into modern, automatic Anderson, J.M. & Mikhail, E.M. 1998. Surveying digital photogrammetric procedures, where the theory and practice. Boston, WCB McGraw-Hill. photographic image is stored in a computer rather than Chang, Kang-tsung. 2008. Introduction to geospatial in the form of a plate or negative. Although airborne information systems, 4th edition. Toronto, McGraw cameras are still used to make the initial image, in the Hill Higher Education. near future high resolution satellite images will be used Clancy, J. 1991. Site surveying and levelling. 2nd edition. increasingly for small-scale maps. These images have a A butterworth – Heinemann title. resolution finer than 1 metre. Collett, J. & Boyd, J. 1977. Eight simple surveying levels. Agricultural equipment and tools for farmers REVIEW quESTIOnS designed for local construction, No. 42. London, 1. Name five or more necessary types of surveying Intermediate Technology Publications Ltd. equipment that a rural farm or rural area surveyor Cooper, M.A.R. 1987. Control surveying in civil must own or possess, preferably stating the engineering. Collins, London. importance of each alongside its name. Irvine, W. & Maclennan, F. 2005. Surveying for 2. When setting out farm structures, which factors construction, 5th edition. McGraw Hill Higher should be considered when deciding which Education. equipment to acquire for use in the particular Kaplan, E.D. 1996. Understanding GPS principles and set-up? applications, Artech house publishers. 3. In which instances in rural farm surveying Kavanagh, B.F. 2003. Surveying principles and does older equipment still prevail over modern applications. New Jersey, Prentice Hall. surveying equipment? Olliver, J.G. & Clendinning, J. 1979. Principles of 4. Compare instances in rural surveying where surveying. Volume 1. Plane Surveying. 4th edition. modern surveying equipment would be preferred New York, Van Nostrand Reinhold Co. over older equipment? Schoffield, W. 2001. Engineering surveying. 5th edition. 5. Modern GPS technology has revolutionized the Oxford, Butterworth. surveying industry. What positive changes has it Scott, G.A. 1973. Construction surveying. London, brought to the process of managing rural farms? Longman Group Ltd. 6. What are the main challenges facing rural Wellenhof-Hofmann, B., Lichtenegger, H. & Collins, surveyors and field technicians with regard to J. 2001. GPS: Theory and Practice, 5th edition, adaptation of modern surveying technology? Springer Wien. New York 7. Briefly describe three modern geospatial Wolf, P.R. & Ghilani, C.D. 2002. Elementary surveying; techniques. an introduction to geomatics. 10th edition. New 8. Briefly discuss four uses of modern geospatial Jersey, Prentice Hall. techniques. 45 Chapter 5 Construction materials InTROduCTIOn Hardwoods versus softwoods A wide range of building materials is available for the Wood cut from deciduous trees (which drop their construction of rural buildings and structures. The leaves sometime during the year) is considered to be proper selection of materials to be used in a particular hardwood, while that cut from coniferous (needle- building or structure can influence the original cost, bearing) trees is considered to be softwood. However, maintenance, ease of cleaning, durability and, of course, this classification does not accurately reflect whether appearance. the wood itself is soft or hard. In this book, hardwood Several factors need to be considered when choosing will be used to classify wood with hard characteristics. the materials for a construction job, including: 1. Type and function of the building or structure Wood characteristics and the specific characteristics required of the Strength in wood is its ability to resist breaking when materials used, i.e. great strength, water resistance, it is used in beams and columns. Not only is strength wear resistance, attractive appearance, etc. related to the species, but also to moisture content 2. Economic aspects of the building/structure in (MC) and defects. Strength is also quite closely related terms of original investment and annual cost of to density. maintenance. Hardness is the resistance to denting and wear. While 3. Availability of materials in the area. hardwoods are more difficult to work, they are required 4. Availability of the skilled labour required to for tools, tool handles, flooring and other applications install some types of material. subject to wear, or where a high polish is desired. 5. Quality and durability of different types of Woods that are stiff resist deflection or bending material. when loaded. Stiff woods are not necessarily very 6. Transportation costs. strong. They may resist bending up to a point and then 7. Selection of materials with compatible properties, break suddenly. dimensions and means of installation. Tough woods will deflect considerably before 8. Cultural acceptability or personal preference. breaking. Even after fracturing, the fibres tend to hang together and resist separation. Tough woods are WOOd resistant to shock loading. Wood is a commonly used construction material in Warping is the twisting, bending or bowing many parts of the world because of its reasonable cost, distortions shown by some woods. The method of ease of working, attractive appearance and adequate sawing and curing affects the amount of warping, but life if protected from moisture and insects. However, some species are much more prone to warping than forests are a valuable natural resource that must be others. conserved, particularly in areas with marginal rainfall. Nail-holding resistance for hardwoods is greater As good a material as wood may be, there are regions than for softer woods. However, woods that are so hard where other materials should be considered first, simply that they tend to split when nailed, lose much of their on a conservation basis. holding ability. Preboring to 75 percent of the nail size Wood for building is available from many different avoids splitting. species with widely varying characteristics. Some The workability, such as sawing, shaping and species are used in the form of small poles for light nailing, is better for soft, low-density woods than for construction, while other species are allowed to mature hardwoods, but usually they cannot be given a high so that timber (lumber in many countries) may be sawn polish. from the large logs. The species that produce small, Natural-decay resistance is particularly important inexpensive poles in rather short growing periods often in the warm, humid regions of east and southeast grow in the fringes of agricultural land and can be used Africa. A wide range of resistance is shown by different without danger to the ecology of the region. species. However, for all species, the heartwood (the The various species of wood have a number of darker centre area of the tree) is more resistant than physical characteristics that will be discussed in relation the sapwood (the lighter outer area of the tree). In to their use in building construction. addition to selection for natural-decay resistance, wood 46 Rural structures in the tropics: design and development preservatives should be considered where contact with insects, there are great variations among the species. the ground is likely. The density of the timber is no guide to its resistance to Paint-holding ability differs between woods and, as termite damage, as some of the lighter timbers are more a general rule, this should be considered when selecting immune than heavier varieties. materials. Weathering is the disintegration of wood caused by alternate shrinkage and swelling as a result of rain, defects in wood rapid changes in temperature, humidity and the action Defects to watch for when selecting timber are: of sunlight. Painting, when properly carried out, does Brittle heart, found near the centre of many tropical much to prevent weathering. The paint must be of trees, makes the wood break with a brittle fracture. exterior quality, however, and applied according to the Wide growth rings indicate rapid growth resulting maker’s instructions. in thin-walled fibres with consequent loss of density and strength. POLES And TIMBER Fissures include checks, splits, shakes and resin pockets. Knots are the part of a branch that has become Wooden poles enclosed in a growing tree. Dead knots are often loose In farm buildings and rural structures, wood is often and therefore reduce the effective area that can take used in the form in which it has grown, i.e. round poles. tensile stress. Knots can also deflect the fibres, thereby In some areas where enough trees are grown on the reducing strength in tension. farm or in local forests, wooden poles can be obtained Decay, which results from moisture levels between at very low cost. These poles have many uses in small 21 percent and 25 percent in the presence of air, reduces building construction, such as columns for the load- the strength of the wood and spoils its appearance. bearing structure, rafters, trusses and purlins. Sticks Insect damage caused by borers or termites. and thin poles are often used as wall material or as a The fungi that feed on wood can be divided into framework in mud walls. three main categories: staining fungi, moulds and Where straight poles are selected for construction, decay fungi. Most fungi thrive under moist conditions. it is as easy to work with round timber as with sawn Staining fungi live mainly on the sapwood but they may timber. However, somewhat crooked poles can also be penetrate deeply into the wood and spoil the timber’s used if they are turned and twisted and put into positions attractive appearance. Moulds do not penetrate below in which the effects of the bends are unimportant. the surface and do not seem to affect the strength of Round timber can generally be considered stronger the wood, but they look unsightly. Decay fungi eat the than sawn timber of the same section area because the cell walls of the wood. This causes the tree to lose its fibres in round timber are intact. The pole is normally strength and often reduces it to a crumbling, rotting tapered and therefore the smallest section, the top end, mass. These decay fungi never attack timber that is must be used in the calculation of compressive and seasoned to a moisture content of less than 20 percent tensile strength. and kept well ventilated and dry. A great number of species can be considered when The main species of borer that attack tropical woods selecting poles for construction, but only a limited are the pinhole borer and the Lyctus, or powderpost number are available on the commercial market. Some beetle. The pinhole borer attacks newly felled logs and species are more suitable for silviculture (growing on sometimes standing trees. The attack can occur within farms) and silvipasture (growing on pastures) than hours of felling. The beetles do not normally continue others, but must always be selected to suit local climatic to operate in seasoned timber. The powderpost beetle and soil conditions. Generally there are several species attacks seasoned tropical hardwoods – particularly suitable for each location that are fast and straight- those that contain starch on which the larvae feed. growing, and produce strong and durable timber. In Timber is sometimes sprayed in the yard to protect it addition to building poles or timber, some species will until it is transported. produce fodder for animals, fruit, fuelwood, etc. Termites are normally of two kinds: the dry wood Many species of eucalyptus, from which gum poles are types that are able to fly and the subterranean type. obtained, are very fast and straight-growing hardwoods. Termites usually operate under cover and it is only However, they warp and split easily. Dimensions suitable after the first signs of damage appear that the full extent for building construction are obtained by harvesting the is realized. Flying termites usually enter the end-grain still immature trees. Gum poles provide a strong and of untreated timber and build up a colony from inside, durable material if chemically treated. finally devouring all the interior wood and leaving In high-altitude areas, several species of acacia only a thin skin behind. Some subterranean termites, produce good building poles. Acacia melanoxylon white ants, operate from a central colony and travel in (Australian blackwood) is very resistant to attack by search of food. Their nests or hills sometimes achieve termites, but grows more slowly than eucalyptus. In great size and house millions of ants. While no timber low- to medium-altitude areas with sandy soils and low is completely immune to attack from ants or other rainfall, Casuarina produces straight and durable poles. Chapter 5 – Construction materials 47 Unprocessed round wood material can be joined by being nailed or tied with string or wire. A special connector has been developed to join round wood in trusses where several members may have to be connected at each point. Sawing timber The rate at which a tree grows varies with the season. The resulting growth rings of alternate high and low density form the grain in the sawn timber (lumber). The method of sawing has a considerable effect on the appearance, resistance to warping, shrinking, paint- holding ability and wear resistance of the final piece. There are several methods of sawing a log into boards and planks, giving different relationships between the growth rings and the surface,  i.e. more or less parallel to the surface in plain sawn timber and at right angles Figure 5.1 Pole connectors in radial sawn timber. Radially sawn boards shrink less, are less liable to cup and twist and are easier to season. Unfortunately, Cedar posts for fencing are obtained by splitting cutting methods that produce a high proportion of large logs. The posts are durable and resistant to rot and quarter-sawn timber are wasteful and therefore only attack by termites. They are also suitable for wall posts used to produce material for high-class joinery work in the construction of buildings. (see Figures 5.2 and 5.3). In coastal areas, mangrove poles are widely used for posts in walls and trusses in roofs. Boxed heart A B C B A Through and through sawn Billet sawn Quarter cut Common method More than 45° Commercial quarter cut True Quarter cut quarter cut Figure 5.2 Methods of sawing timber 48 Rural structures in the tropics: design and development Offcuts Air seasoning A tree is tapered and cylindrical, whereas boards and Timber should be protected from rain and from the planks are rectangular. This results in the outer pieces ground. It should therefore be stacked so that air with tapered edges and less than full dimensions can circulate freely around all surfaces, reducing the throughout the length. Such pieces, called offcuts, can risk of twisting and cupping, as well as minimizing sometimes be obtained at low cost and used for rough attacks by fungi and insects. In favourable conditions, building. thin softwoods can be air-seasoned in weeks but in unfavourable conditions some hardwoods require a Seasoning of timber year or more. The strength, stiffness and dimensional stability of wood are related to its moisture content. Hence, if wood is dried (seasoned) before use, not only can Air currents higher strength values be used in a design, but a more durable structure will result. In developing countries, most timber is not seasoned and it is sold in what is called its ‘green’ state. Timber must be stacked, supported and sometimes restrained so as to minimize distortion during seasoning. If drying is too rapid, the outer parts, in particular the unprotected ends, shrink before the interior does, and this leads to surface checking and splitting, as well as the possible extension of ring and heart shakes. Some Figure 5.4 Air-drying of timber timber species are more difficult to season satisfactorily than others. Artificial seasoning Artificial seasoning can be either moderate or rapid, depending on the temperature of the air injected into LOW the chamber where the timber is piled, and on the rate at which the air is circulated and extracted from BEST the chamber. This method is expensive and can only be applied on small quantities of timber. Timber can GOOD be artificially seasoned from the green condition, but MEDIUM often hot-air seasoning is used only at a later stage, after most of the moisture has been removed by air seasoning. Smoke seasoning is a moderate process and involves Cupping of plain sawn placing the timber over a bonfire. It can take a month boards caused by greater or two, depending on the size and type of wood shortening of longer growth rings drying being seasoned. This method is considered to be both a seasoning and a treatment method for timber. Presumably it protects the timber against pest attacks and increases durability. However, it is not very reliable and can lead to splitting of the timber because of the Plain (slash) lack of control over the heat from the bonfire. sawn Care of seasoned timber Radial sawn Timber should be protected from moisture on the building site. Close piling and covering with tarpaulins delays the absorption of atmospheric moisture, particularly in the interior of the pile. Ovalling Grades and sizes for timber Diamonding Grades Grades are established by various government agencies. Even within a single country, more than one grading Figure 5.3 Effects of cupping and shrinkage of different system may be in use. While the grade may not be methods of sawing important for small construction jobs, in large projects Chapter 5 – Construction materials 49 where materials are bought by specification, it is resist. Wood, like other materials, has safe fibre stress important to indicate the required grade. values given in N/mm² that have been determined by Grades that provide specific information in structural destructive testing to obtain first an ultimate stress, and design are most useful. The grade standard established then, by the use of various correction and safety factors, by the Kenya Bureau of Standards, shown in Table 5.1, the safe fibre stress to be used for designing a structure. is a good example. Table 5.2 lists basic working-stress values for several types of loadings in five strength groups. Table  5.3 divides some representative species into the strength TABLE 5.1 groups used in Table 5.2. Timber grades and application Grade Applications F Furniture, high-class joinery TABLE 5.3 Some representative timbers grouped according to GJ General joinery strength and density S-75 Structural grade, having a value of 75% of basic stress Group Latin name Common name S-50 Structural grade, having a value of 50% of basic stress 1 Pinus radiata (12 years) Young pine C A general construction grade for non-stressed Polyscias kikuyuensis Mutati construction 2 Cordia abyssinica Muringa L A low grade for low quality work Pinus patula (17 years)* Pine Pinus radiata (17 years) Pine Cupressus lusitanica** East African cypress It is the S-75 and S-50 grades that are significant in 3 Podocarpus Podo/musengera building construction, as will be seen in later sections. Juniperus procera African pencil cedar/mutarakwa Octea usambarensis East African camphorwood/muzaiti Sizes Acacia melanoxylon Australian blackwood Timber in eastern and southeastern Africa is available Grevillia robusta Grevillea/silky oak in a number of Système Internationale (SI) metric sizes, Vitex keniensis* Vitex/muhuru/meru oak but not all are available in all localities. The dimension Pterocarpus angolensis Muninga indicates actual size as sawn. Smoothing will reduce the Khay anthot heca African mahogany timber to less than dimension size. Eucalyptus regnans Australian mountain ash 4 Cassipourea malosana Pillarwood/musaisi Dombeya goezenii Mueko Timber measurement for trade Eucalyptus saligna Saligna gum/Sydney blue gum Even though timber is normally sold by length (running Premna maxima* metre or foot), the price may be calculated per cubic Afzelia quanensis Afzelia metre when sold in large quantities. Basic lengths are 5 Olea hochstetteri East African olive/musharagi between 1.8  metres and 6.3  metres, although pieces * One group lower in compression perpendicular to grain longer than about 5.1  metres are scarce and costly. ** One group lower in joint shear Timber normally comes in running lengths, that is to say, not sorted by length. There are dozens more tree species found in eastern and southern Africa, many of which are used only Strength of wood in very local areas. In order to obtain approximate Building materials of any type that are under load are working-stress data for these indigenous species, their said to be subjected to fibre stress. The safe fibre stress densities may be used to place them in the proper for a material is the load that the material will safely group in Table 5.2. If the density is not known, a good TABLE 5.2 Guide to basic working-stress values and modulus of elasticity for timber Maximum Maximum compression Maximum shearing Strength Strength density density bending strength Modulus strength strength group rating green 12% MC and tension of elasticity Parallel Perpendicular parallel to grain to grain to grain Beams Joints   kg/m3 kg/m3 N/mm2 kN/mm2 N/mm2 N/mm2 N/mm2 N/mm2 1 Weak < 520 < 400 10 4.0 2.5 0.6 1.0 0.4 2 Fairly strong 521–650 401–500 15 6.0 10.0 1.2 1.3 1.6 3 Strong 651–830 501–640 20 7.5 13.0 2.0 1.9 2.4 4 Very strong 831–1 040 641–800 30 9.0 20.0 3.2 2.4 3.5 5 Exceptionally strong > 1 041 > 801 50 10.5 29.0 5.0 3.2 4.1 50 Rural structures in the tropics: design and development approximation can be found quite easily. A bucket, a where Px is the mechanical property at a given graduated cylinder (millilitres) and an accurate scale moisture content, for example, tensile strength at for weighing a sample of the wood will be needed. The 8%  MC; P12 is the property at 12%  MC; Pg is the procedure is as follows: property value in green condition; Mx is the MC at 1. Weigh the sample. which property is desired and Mp is the moisture content 2. Place the bucket on a level surface and fill to the at the intersection of a horizontal line representing the rim with water. strength of greenwood and an inclined line representing 3. Carefully submerge the sample and then remove. the logarithm of the strength/MC relationship for dry 4. Refill the bucket from the graduated cylinder, wood. It is usually taken to be 25% MC. noting the amount of water needed to refill the Owing to the effect of moisture, mechanical bucket. properties are determined in green condition (above 5. Density = weight / volume = kg/m³ fibre saturation) or in air-dry conditions (12% MC). 6. Place the species in the appropriate group using This makes it possible to have comparable results. the appropriate density column for a green or dry Correction factors are used to adjust moisture content sample (see Table 5.2, column 3 or 4). to these two standard values. Table 5.2 lists basic working-stress values. For design 3. Density purposes, these should be adjusted for a number of Density is a measure of the wood substance contained different variables, including: grade, moisture content, in a given volume. The substance of which wood duration of load, exposure and use of the structure. is comMpose X −d1 2has a specific gravity of about 1.5, yet Factors that affect timber strength include: −  Pwood Px = P  12 e MP −12    floats on water. This would indicate that wood 12 Pcon tains numerous cell cavities and pores. As the 1. Sloping grain  sgtrength of timber is a function of the wood material As timber is a material with maximum mechanical present, density is a good indicator of strength, and the properties in the direction of the grain, any load not relationship is given by Equation 5.3: applied in this direction will be resisted by lower strength and stiffness characteristics. The effect of S gn a sloping grain on the strength of beams must be = (5.3) considered in the design. For example, lowering the S ' g' grain slope from 1 in 20 to 1 in 8 reduces the strength of timber by over 50 percent. The reduction in strength where S and S’ are values of strength corresponding to resulting from the sloping grain (deviation) may be densities g and g’ and n is an index with a value in the calculated from the relationship. range 1.25 to 2.50. 4. Temperature PQ N= (5.1) The influence of temperature can be analyzed at two Psinθ + Qcosθ levels: where N is the strength at angle θ from the fibre direction; (i) Reversible effects P is the strength parallel to the grain (θ = 0°) and Q is the In general, the mechanical properties of wood strength perpendicular to the grain (θ = 90°). decrease when heated and increase when cooled. At constant MC and below 150 ºC, the relationship 2. Moisture between mechanical properties and temperature When moisture decreases below the fibre saturation is approximately linear. At temperatures below point, it begins to affect the mechanical properties of 100  ºC, the immediate effect is essentially wood. A decrease in moisture content increases the reversible, i.e. the property will return to the value strength of wood because the cell walls become more at the original temperature if the change is rapid. compact. Cell walls are compacted because, with the loss of moisture, the mass of wood substance contained (ii) Irreversible effects in a certain volume increases. This occurs at high temperatures. This permanent Given any mechanical property at standard values of effect results in degradation of the wood moisture content, it is possible to predict the values of substance, which results in the loss of weight that property at any moisture content using Equation 5.2: and strength. However, wood will not often reach the daily extremes in temperature of the MX −12  air around it in ordinary construction. Long- −  term effects should therefore be based on the P    P = P  12 e  MP −12  x 12 P  (5.2) accumulated temperature experience of critical  g  structural parts. S gn = S ' g' Chapter 5 – Construction materials 51 10 m.c. in 12% shear // g ra 5 shear // grain green 200 400 600 800 1 000 kg/m3 - 12% m.c. 50 45 40 35 30 Bending st re ngth gre en Compre ssi on // gra in12% 25 20 Compressi on // grain green 15 10 5 200 400 600 800 1 000 kg/m3 - 12% m.c 1 2 3 4 5 STRENGTH GROUPS Figure 5.5 Basic working stresses for timber Strength( N/mm2 ) ( N/mm2 ) Bending strength 12% 52 Rural structures in the tropics: design and development (iii) Time under load oil produced by the distillation of coal tar and, while it Static strength tests are typically conducted at a has many of the properties required of a preservative, rate of loading to attain maximum load in about it increases flammability, is subject to evaporation, and 5  minutes. Higher-strength values are obtained creosoted wood cannot be painted. It should not be for wood loaded at more rapid rates, and lower used on interiors where the characteristic smell would values are obtained at slower rates. For example, be objectionable. Unfortunately, creosote has been the load required to produce failure in a wood found to be a carcinogen and must be used with caution. member in 1 second is approximately 10 percent Coal tar is not as effective a preservative as the higher than that obtained in a standard strength creosote produced from it. Tar is less poisonous, does test. not penetrate the timber because of its viscosity, is blacker than creosote and is unsuitable for interior Grades wood work. As an example, the Kenya Forest Department Unleachable metallic salts are based mostly on recommends that the following grades should be used: copper salts. A combination of copper/chrome/ arsenate is used. The copper and arsenical salts are toxic Grade 1 75% of basic working-stress value preservatives that are rendered non-leaching (cannot be Grade 2 50% of basic working-stress value washed out) by the chrome salt, which acts as a fixing Grade 3 35% of basic working-stress value agent. The timber is impregnated using a ‘vacuum- Grade 4 15% of basic working-stress value pressure’ process. Preservation by metallic salt is being used increasingly because the treated surfaces are Moisture odourless and can be painted or glued. Table values need to be reduced when timber is installed Water-soluble preservatives are not satisfactory for green and will remain wet and uncured continuously. exterior use as they are liable to be washed out of the Use Figure 5.5 to find a suitable stress value for green timber by rain. wood corresponding to the dry value in Table 5.2. By contrast, they are very suitable for interior work as they are comparatively odourless and colourless and Exposure the timber can be painted. Timbers exposed to severe weather and decay hazards Used engine oil can often be obtained free of charge, should be designed using a 25  percent stress-value at least in small quantities. The oil contains many decrease, particularly for columns and for bearing points. residual products from combustion and some of them act as preservatives, but it is not nearly as effective as TIMBER PRESERVATIOn commercial preservatives. It can be thinned with diesel The main structural softwood timbers of eastern and fuel for better penetration. The combination of 40 litres southeastern Africa are not naturally durable. If used of used engine oil and 1  litre of Dieldrin is a viable in conditions subject to fungal, insect or termite attack, alternative in rural construction. they will fail after some time. To avoid this, the timber used in permanent structures should be treated with a Methods of wood preservation preservative. There are two categories of timber preservation Effective preservation depends on the preservative methods: and how it is applied. An effective preservative should be poisonous to fungi and insects, permanent, able Non-pressure methods to penetrate sufficiently, cheap and readily available. These are applicable for both green and dried timber It should not corrode metal fastenings, nor should (less than 30 percent MC) and include: the timber be rendered more flammable by its use. It 1. Soaking (steeping), used for small quantities of is sometimes desirable to have a preservative-treated timber. surface that can be painted. 2. Hot and cold soaking: the tank with the If a structure is correctly designed and built, and preservative and timber is heated to nearly the moisture content of its timber does not exceed boiling point for 1-2 hours and allowed to cool. 20  percent, then a preservative treatment is generally During the heating period, the air in the cells unnecessary for protection against fungal attack. expands and some is expelled. When cooling, a However, where the above conditions are not present, partial vacuum develops in the cells and liquid is there will be a risk of fungal decay, and proper absorbed. preservation is recommended. 3. Steam and cold quenching. 4. Superficial methods such as painting and Wood preservatives spraying. Creosote is an effective general-purpose preservative that is cheap and widely used for exterior work and, To make non-pressure methods more effective, storage to a lesser degree, indoors. It is a black to brownish in a closed environment is recommended. Chapter 5 – Construction materials 53 Pressure methods of wood, in order to fill the wood cells with air The treatment in pressure processes is carried out in prior to preservative injection. Pressurization steel cylinders, or ‘retorts’. Most units conform to size times vary with wood species. For some limits of 2–3  metres in diameter and up to 46  metres species, only a few minutes of pressurization or more in length, and are built to withstand working are required, while more resistant species may pressures of up to 1 720 kPa. The wood is loaded on require pressure periods of 30-60  minutes. Air special tram cars and moved into the retort, which is pressures used typically range from 172  kPa then closed and filled with the preservative. to 690  kPa, depending on the net preservative Pressure forces preservatives into the wood until retention desired and the resistance of the wood. the desired amount has been absorbed. Three processes After the initial pressurization period, – full-cell, modified full-cell and empty-cell – are preservative is pumped into the cylinder. As commonly used. These processes are distinguished by the preservative enters the treatment cylinder, the sequence in which vacuum and pressure are applied the air escapes into an equalizing, or Rueping, to the retort. The terms ‘empty’ and ‘full’ refer to the tank at a rate that maintains the pressure within level of preservative retained in the wood cells. the cylinder. When the treatment cylinder is The full-cell process achieves a high level of filled with preservative, the pressure is raised preservative retention in the wood cells, but less above the initial air pressure and maintained penetration than the empty-cell process. On the other until the wood will take no more preservative, hand, the empty-cell process achieves relatively deep or until enough has been absorbed to leave the penetration with less preservative retention than the desired preservative retention level after the full-cell process. final vacuum. After the pressure period, the preservative is removed from the cylinder and 1. Full-cell process surplus preservative is removed from the wood The Bethel full-cell process is generally used with with a final vacuum. This final vacuum may water-based preservatives, especially for timber that is recover 20–60  percent of the gross amount of difficult to treat and also requires high retention. The preservative injected. The retort then is unloaded, full-cell process steps are listed below: and the treated wood stored. • The wood is sealed in the treatment cylinder and an initial vacuum is applied for approximately (ii) Lowry process 30 minutes to remove as much air as possible from The Lowry process is an empty-cell process the wood and from the cylinder. without the initial air pressure. Preservative is • The preservative, either heated or at ambient pumped into the treatment cylinder without temperature depending on the system, enters the either an initial air pressurization or vacuum, cylinder without breaking the vacuum. trapping the air that is already in the wood. • After the cylinder is filled, the cylinder is pressurized After the cylinder is filled with the preservative, until no more preservative will enter the wood, or pressure is applied and the remainder of the until the desired preservative retention is obtained. process is identical to the Rueping process. The • At the end of the pressure period, the pressure is advantage of the Lowry process is that full-cell released and the preservative is removed from the equipment can be used without the accessories cylinder. required for the Rueping process, such as an air • A final vacuum may be applied to remove excess compressor, an extra tank for the preservative, or preservative that would otherwise drip from the a pump to force the preservative into the cylinder wood. against the air pressure. However, both processes are used widely and successfully. 2. Empty-cell process The empty-cell process results in deep penetration MAnuFACTuREd BuILdInG BOARdS of the preservative with a relatively low net preservative There are a number of building boards made from retention level. If oil preservatives are used, the empty- wood veneers or the waste products of the timber cell process will most probably be used, provided it will industry that are convenient and economical materials yield the desired retention level. The Rueping process to use in building construction. In general, they offer and the Lowry process are the two most commonly excellent bracing for the building frame, together with used empty-cell processes. Both use compressed air to labour savings because they are available in large sizes drive out a portion of the preservative absorbed during requiring a minimum of fitting. the pressure period. Some manufactured boards are designed with specific characteristics, such as fire resistance, ease (i) Rueping process of cleaning, high insulation value or resistance to In the Rueping process, compressed air is forced weathering. into the treatment cylinder containing the charge 54 Rural structures in the tropics: design and development TABLE 5.4 Plywood Safe spans for plywood panels parallel to the grain of Plywood is produced by gluing together three to seven the plys veneers that have been peeled from logs. The grain Load of each successive veneer is angled at  90° from the previous one, resulting in a board that has considerable 167 Pa 4 790 Pa strength and rigidity in all directions. Waterproof glue Thickness (170 kg/m²) (490 kg/m²) is most commonly used, giving a product that is highly 9 mm 400 mm - resistant to moisture. Waterproof glue panels should 12 mm 600 mm - always be chosen for farm buildings. As the wood itself 15 mm 770 mm 300 mm is not waterproof, the panels are still subject to swelling 19 mm 925 mm 400 mm and shrinkage from moisture changes. Grades of plywood Plywood is generally given four to five grades, based Other manufactured boards on the appearance of the surface veneers. Each panel Blockboards and laminboards are made of strips of has a double-letter grade to indicate the grade of wood between 8 mm and 25 mm wide, glued together the face of the panel and the back of the panel. and covered with one or more veneers on each side. At The top-grade surface is generally free enough from least one pair of corresponding veneers will have the defects to be finished naturally, while the second-best grain at right angles to the grain of the core. If the finish grade is good for painting. Lower grades are used for grain is to run parallel with the core, there must be at structural applications where appearance is of little least two veneers per side. importance. Theoretically, between 10 and 15 different The same 12  panel sizes listed for plywood are grade combinations are possible. In practice, only a few also listed for blockboard. However, the thicknesses tend to be available from timber merchants. are greater, ranging from 15  mm to 50  mm, in 5  mm increments. The same appearance, grades and types Sizes of plywood panel of glue listed for plywood also apply to blockboards. As an example of a standard used in the region, the Blockboard panels are often used for doors. Kenya Bureau of Standards provides a standard with Particleboards are formed by pressing chips or 12 panel sizes and 9 different thicknesses. Combining flakes of wood between pairs of heated platens so that grades, panel sizes and thicknesses, there are numerous the particles lie in random fashion with their longer permutations, but only a few will be manufactured. dimensions parallel to the surface of the board. The The most common panel size is 2 400 by 1 200 mm, in chips are bonded with thermosetting synthetic resins. thicknesses of 9 mm, 12 mm, 15 mm and 19 mm. Depending on the size of the particles, these boards are variously known as particleboard, chipboard or Plywood for structural members waferboard. Strength and rigidity generally increase Plywood panels are made from many different with density, but that alone is not a measure of quality, species of wood and have a wide range of strengths as moisture resistance varies considerably and most and stiffnesses. Specific strength characteristics for particleboards should not be used in moist locations. plywood can be provided by either the manufacturer Softboards are made from uncompressed woodchips or a trade association that publishes grade standards or sugarcane fibres mixed with water and glue or to which manufacturers adhere. In general, plywood resins, giving a density of less than 350 kg/m³. They are panels should equal or exceed the strengths shown in inexpensive and can be used for wall or ceiling surfaces Table 5.4. that are not subject to high-moisture conditions. THREE PLY MULTI PLY Figure 5.6 Plywood Chapter 5 – Construction materials 55 Sawdust is a by-product from sawmills. It is a good BLOCKBOARD natural insulating material, and also a good bedding material for use in animal housing. Wicker made from shrubs, bushes and trees is used either directly, for fencing or wall cladding, or sealed by smearing on mud, plaster, etc. OTHER ORGAnIC MATERIALS Core strips Bamboo Bamboo is a perennial grass with over 550  species, Figure 5.7a Blockboard found in tropical, subtropical and temperate zones. It contains a large percentage of fibre, which has high- tensile, bending and straining capacity. However, bamboos have some shortcomings that CHIPBOARD limit their application. The low durability of bamboo is one of its most serious defects, along with its flammability and tendency to split easily. This usually prevents the use of nails. Cutting a notch or a mortise in a bamboo drastically reduces its ultimate strength. The remedy is the use of nodes as places of support and joints, and the use of lashing materials (strings) in place of nails. Dry Fine chips bamboo is extremely susceptible to fire, but it can be covered or treated with a fire-retardant material. Coarse chips The strength properties of bamboo vary widely with species, growing conditions, position within the culm, Figure 5.7b Particleboard seasoning and moisture content. Generally bamboo is as strong as timber in compression and very much stronger in tension. However, bamboo is weak in shear, with only Softboards have little resistance to rupture and must about 8 percent of compressive strength, whereas timber be supported frequently (300–400 mm) when installed. normally has 20–30 percent. It is used mainly in building The 2 400 mm by 1 200 mm size is most common in construction, for wall poles, frames, roof construction, thicknesses of 6.4 mm to 25 mm. roofing and water pipes and, after splitting, to form Mediumboards, with a density ranging from 350 kg/ flattened boards or woven wall, floor and ceiling panels. m³ to 800  kg/m³, are used for panelling, in particular New stalks of bamboo are formed annually in clumps those with a density at the higher end of the range. growing out of the spreading roots. The individual The most common size is 2 400 mm by 1 200 mm, and bamboo shoots complete their growth within a period of thicknesses range from 6.4 mm to 19.0 mm. 4–6 months in the first growing season. A strengthening Hardboards are made of wood fibres compressed to process takes place during the subsequent 2–3 years, and more than 800 kg/m³. They are usually smooth on one the culm reaches maturity after the fifth or sixth year, surface and textured on the other. The most common or even later depending on the species. It must be cut size is 2  400  mm by 1  200  mm size in thicknesses of before blooming because it looses its resistance and dies 3–12.7 mm. An oil-treated grade labelled ‘tempered’ has after blooming. Some bamboos grow to 35  metres in good resistance to moisture. height, while others are no more than shrubs. Diameters vary from 10 mm to 300 mm. OTHER WOOd PROduCTS Bamboo without proper seasoning and preservative Woodwool slabs consist of long wood shavings, mixed treatment will rot and be attacked by insects, particularly with cement, and formed into slabs 25–100 mm thick if used in moist locations, such as in earth foundations. with a high proportion of thermal insulating voids. Although combustible, they are not easily ignited and Bamboo joints provide good sound absorption. As nailing causes splitting and notching, drastically Shingles are cut from clear rot-free timber logs. reducing the strength of a bamboo culm, lashes are They are made about 2  mm thick at the top end and generally used as binding elements for framing. They 10 mm thick at the bottom, and usually about 400 mm may be split from the bamboo itself, or made from long. Some woods need treatment with preservatives vines, reeds or the bark of certain trees. Soft galvanized before being used as roofing shingles, whereas others wire is also used for binding. Bamboo can be kept from will last 10–15 years without treatment. splitting when bending by boiling or steaming it first, then bending it while hot. 56 Rural structures in the tropics: design and development Binding To fit vertical member Wire or vine binding DOUBLE BUTT BENT JOINT Tongue bent tight across member and tied Figure 5.9b Split the culm the rest of the way by driving a hardwood cross along the cuts Connection of bamboo to round pins Figure 5.8 Lashing bamboo joints Several methods can be used for splitting bamboo culms. The edges of the strips can be razor sharp and should be handled carefully (see Figure 5.9). Figure 5.9c use a knife to split the harder outer strip from the soft, pithy inner strip, which is usually discarded Bamboo preservation Immediately after cutting, the freshly cut lower end of the culm should be dusted with insecticide. The bamboo is then air seasoned for 4–8 weeks, depending on the ambient humidity. Bamboo should be stacked Figure 5.9a Make four cuts in the upper end of the well off the ground so that air can circulate freely. When culm with a splitting knife the culms have dried as much as conditions permit, Chapter 5 – Construction materials 57 they should be trimmed and all cut surfaces should be Sisal stems dusted with insecticide immediately. The seasoning is Before dying, at 7–12  years of age, the sisal plant finished in a well ventilated shelter where the culms are forms a pole shoot to carry the flowers. The pole may protected from rain and dew. reach a height of 6  metres or more and has a fibrous If the bamboo is to be stored for a long time, circumference, which makes it tough, but the inner stacks and storage shelves should be treated with an parts are quite soft. Sisal poles have limited structural insecticide every 6  months. Bamboo that has already strength and durability, but are sometimes used for wall been attacked by insects, fungus or rot should never be cladding in semi-open structures, such as maize cribs. used for construction. Culms that have fissures, cracks The poles can be split and are joined in the same way or cuts in the surface should also be rejected. as bamboo. natural fibres Sisal fibre Natural fibres have been used for building since ancient Sisal fibre is one of the strongest natural fibres. It has times. Fibrous materials can be used by themselves as traditionally been used as reinforcement in gypsum roofing material or for walls and mats. Natural fibres plaster sheets. Sisal fibres have the ability to withstand can also be combined with hydraulic-setting binders to degradation from bacteriological attack better than make various types of roofing board, wall board, block other organic fibres, but are attacked by the alkalinity and shingle. Animal hair is often used for reinforcing of cement. However, research has been carried out to plaster. make sisal fibre, like other natural fibre composites, into a reliable cement reinforcement for long-term use Thatch in exposed situations. Refer to the section on fibre- Thatch, whether made of grass, reeds, palm or banana reinforced concrete. leaves, is susceptible to decay caused by fungi and insects, and to destruction by fire. Preservative treatment is Coir waste desirable but expensive. A treatment combining copper Coir is a by-product of coconuts. The husk is used sulphate, sodium chromate and acetic acid reduces for making coir mats, cushions and as fuel. It can be attack by rot and may considerably increase the life mixed with cement, glue or resins, either to produce span of a thatched roof (see Chapter 8). low-density boards with good insulating and sound- absorption properties, or to be compressed to make Grass building boards. It is also used as reinforcement in The use of thatched roofs is common in many cement for making roofing sheets. countries, and suitable grass can be found almost everywhere. When well laid and maintained, it can last Elephant grass for 10–20 years or longer. Elephant grass is a tall plant similar to bamboo, but A good-quality thatching grass must be fibrous and with the difference that the stem is not hollow. The tough, with a minimum length of 1 metre. It should fibres of the grass can be used to partly or wholly also have thin stems without hollows, a low content of replace the asbestos in net and corrugated roofing easily digestible nutrients and the ability to withstand sheets. However, the sheets are more brittle and have repeated wetting without decaying. a slightly lower strength than asbestos-cement sheets. An annual treatment with a mixture of the following chemicals will improve the fire-resistance of a thatched Straw roof, and also give some protection against decay: 14 kg Baled straw, if supported by a framework of wooden ammonium sulphate, 7 kg ammonium carbonate, 3.5 kg poles, can be used to construct temporary walls. Straw borax, 3.5 kg boric acid, 7 kg alum and 200 kg water. has also been used as raw material for manufactured building boards. Straw and split bamboo can be cement- Reeds plastered to permanent structures, such as vaults and Reeds must be dry before use as a building material, and domes, at low cost. can be impregnated or sprayed with copper-chrome preservatives to prevent rotting. Ammonium phosphate nATuRAL STOnE PROduCTS and ammonium sulphate are used to protect the reeds Natural stones are strong in compression and are against fire (see Chapter 8). generally extremely durable, although deterioration Reeds can be woven into mats for use as wall may result from the action of soluble salts, wetting and or ceiling panels, shade roofs, etc. The mats can be drying, or thermal movement. According to the manner plastered easily. In tropical areas, thatch from untreated of their geological formation, all stones used in building reeds may last only 1 year but, if well laid, treated and fall into one of three classes: igneous, sedimentary or maintained, it can last 5–10 years. metamorphic. Igneous rocks are mostly very hard and difficult to cut to size and shape. However, they are very durable. 58 Rural structures in the tropics: design and development Sedimentary rocks, such as sandstone and limestone, subsoil, and should not be confused with the geological are used extensively for building. They are not difficult or agricultural definition of soil, which includes the to work, yet are quite durable. Coral stone is found weathered organic material in topsoil. Topsoil is in coastal areas, where chips or small stones are used generally removed before any engineering works are in mud walls. Coral stone is also cut into blocks and, carried out, or before soil is excavated for use as a although not very strong, can be used in foundations building material. Mud is the mixture of one or more and walls in multistorey houses. types of soil with water. Metamorphic stones consist of older stones that have There are several ways in which soil may be classified: been subjected to intense heat and pressure, causing by geological origin, by mineral content (chemical structural change. Thus clay becomes slate, limestone composition), by particle size or by consistency (mainly becomes marble and sandstone becomes quartzite. Slate related to its moisture content). develops cleavage planes during formation. Roofing slates are split along these planes. They make very Particle size durable roof surfaces, but require strong frames because Soils are grouped and named according to their particle of their weight. size, as shown in Table 5.5. At the building site, the stones can be dressed to obtain a smooth surface. Often only the side(s) that will Grading be visible are dressed. The soil materials in Table 5.6 seldom occur separately, Stones may also be used in the forms and sizes in and this necessitates a further classification according which they naturally occur, and be embedded in mortar to the percentage of each contained in the soil. This is for foundation and wall construction. Stones are also shown in the soil classification triangle, which shows, crushed and sorted for size and use. Small crushed for example, that a sandy clay loam is defined as soil stones are used in making concrete. Large sizes are used that contains 50–80 percent sand, 0–30 percent silt and as hardcore for filling purposes. 20–30 percent clay. Only a few mixes can be used successfully for EARTH AS A BuILdInG MATERIAL building construction in the state in which they are Earth is one of the oldest materials used for building found. However, many mixes can be improved to make construction in rural areas. The advantages of earth as a good building material by correcting the mix and/or building material are: adding stabilizers. 1. It is resistant to fire. The clay fraction is of major importance in earth 2. It is cheaper than most alternative wall materials, construction because it binds the larger particles and is readily available at most building sites. together. However, soils with more than 30 percent clay 3. It has a very high thermal capacity, which enables tend to have very high shrinkage/swelling ratios which, it to keep the inside of a building cool when it is together with their tendency to absorb moisture, may hot outside and vice versa. result in major cracks in the end product. High-clay 4. It absorbs noise well. soils require very high proportions of stabilizer or a 5. It is easy to work using simple tools and skills. combination of stabilizers. Some soils produce unpredictable results, caused by These qualities encourage and facilitate self-help and undesirable chemical reactions with the stabilizer. Black community participation in house building. cotton soil, a very dark coloured clay, is an example of Despite its good qualities, earth has the following such a soil. Generally speaking, soils that are good for drawbacks as a building material: building construction purposes are characterized by 1. It has low resistance to water penetration, good grading, i.e. they contain a mix of different-sized resulting in crumbling and structural failure. particles similar to the ratios in Table  5.6, where all 2. It has a very high shrinkage/swelling ratio, voids between larger particles are filled by smaller ones. resulting in major structural cracks when exposed Depending on use, the maximum size of coarse particles to changing weather conditions. should be 4–20 mm. 3. It has low resistance to abrasion, and requires Laterite soils, which are widely distributed frequent repairs and maintenance when used in throughout the tropical and subtropical regions, building construction. generally give very good results, especially if stabilized with cement or lime. Laterite soils are best described However, there are several ways to overcome most as highly weathered tropical soils containing varying of these weaknesses that make earth a suitable building proportions of iron and aluminium oxides, which are material for many purposes. present in the form of clay minerals, usually together with large amounts of quartz. Their colours range from Soil classification ochre, through red, brown and violet to black. The Soil and earth are synonymous when used in relation darker the soil, the harder, heavier and more resistant it to building construction. The term ‘soil’ refers to is to moisture. Some laterites harden on exposure to air. Chapter 5 – Construction materials 59 TABLE 5.5 Classification of soil particles Material Size of particles Means of field identification Gravel 60–2 mm Coarse pieces of rock, which are round, flat or angular. Sand 2–0.06 mm Sand breaks down completely when dry; the particles are visible to the naked eye and gritty to the touch. Silt 0.06–0.002 mm Particles are not visible to the naked eye, but slightly gritty to the touch. Moist lumps can be moulded but not rolled into threads. Dry lumps are fairly easy to powder. Clay Smaller than 0.002 mm Smooth and greasy to the touch. Holds together when dry and is sticky when moist. Organic Up to several Spongy or stringy appearance. The organic matter is fibrous, rotten or partially rotten, centimetres several centimetres deep, with an odour of wet, decaying wood. Gravel, sand and silt are sometimes subdivided into coarse, medium and fine fractions. SAND % CLAY % Clay Clay loam Sandy clay Silty clay Sandy clay loam Silty clay loam Sand Silt 0 10 20 30 40 50 60 70 80 90 100 Sandy loam Loam Silty loam SILT % Figure 5.10 Soil-classification triangle TABLE 5.6 Soil gradings suitable for construction Clay Silt Clay& Silt Sand Gravel Sand & Gravel Cobble Organic Matter Soluble salts use (%) (%) (%) (%) (%) (%) (%) (%) (%) Rammed- earth walls 5–20 10–30 15–35 35–80 0–30 50–80 0–10 0–03 0–1.0 Pressed- soil blocks 5–25 15–35 20–40 40–80 0–20 60–80 - 0–03 0–1.0 Mud bricks (adobe) 10–30 10–40 20–50 50–80 - 50–80 - 0–0.3 0–1.0 Ideal, general- purpose mix 15 20 35 60 5 65 - 0 0 If the soil at hand is not suitable, it may be improved by adding clay or sand. The best soils for construction are sandy loam and sandy clay loam. Sandy clay gives fair results if stabilized. 100 90 80 70 60 50 40 30 20 10 0 100 9 0 8 0 70 60 50 40 30 20 10 0 60 Rural structures in the tropics: design and development Plasticity index As soils can vary widely within small areas, samples Clays vary greatly in their physical and chemical of the soil to be tested must be taken from exactly the characteristics. Although the extremely fine particles area where soil is going to be dug for the construction. make it very difficult to investigate their properties, Soil samples should be collected from several places some can be conveniently expressed in terms of distributed over the whole of the selected area. First plasticity using standard tests. remove the topsoil (any dark soil with roots and plants Depending on the amount of moisture it contains, in it), which is usually less than 60 cm. Then dig a pit a soil may be liquid, plastic, semisolid or solid. As a to a depth of 1.5 metres, and collect soil for the sample soil dries, the moisture content decreases, as does the at various depths between 0.8  metres and 1.5  metres. volume of the sample. With very high moisture content, The total volume required for a simple field test is the soil will flow under its own weight and is said to about a bucketful, whereas a complete laboratory test be liquid. At the liquid limit, the moisture content requires about 50 kg. Mix the sample thoroughly, dry has fallen to the extent that the soil ceases to flow and it in the sun, break up any lumps and pass it through a becomes plastic; it is continuously deformed when a 5–10 mm screen. force is applied, but retains its new shape when the In the laboratory, the classification by particle size force is removed. involves sieving the coarse-grained material (sand and A further reduction of the moisture content will gravel) and sedimentation for fine-grained material (silt eventually cause the soil to crumble under load and not and clay). The plasticity index is determined using the deform plastically. The moisture content at this point is Atterberg limit test. known as the ‘plastic limit’. The numerical difference Soil tests will only give an indication of the suitability between the moisture content at the liquid limit and at of the soil for construction purposes and the type the plastic limit is called the ‘plasticity index’. Both the and amount of stabilizer to be used. However, other liquid limit and the plasticity index are affected by the properties, such as workability and behaviour during amount of clay and the type of clay minerals present. compaction, may eliminate an otherwise suitable soil. A high liquid limit and plasticity index indicates a Soil tests should therefore be combined with tests on soil that has great affinity for water and will therefore the finished products, at least where high strength and be more susceptible to moisture movements, which can durability are required for the design and use. lead to cracks. For small projects, a simple sedimentation test combined with a bar shrinkage test normally gives Example 5.1 enough information about the proportions of various The following index properties were determined for particle sizes and the plastic properties of the soil. two soils X and Y. Simple sedimentation test Property X Y This test gives an impression of the grading of the soil Liquid limit 0.62 0.34 and allows the combined silt and clay content to be Plastic limit 0.26 0.19 calculated. Take a large, clear glass bottle or jar with a Determine the plasticity index of X and Y. flat bottom and fill it one-third full with soil from the sample. Add water until the bottle is two-thirds full. Solution Two teaspoons of salt may be added to dissolve the soil The plasticity index is the range of moisture content more rapidly. Close the bottle, shake it vigorously, and over which the soil remains plastic. The bigger this allow the contents to settle for 1  hour. Shake it again range, the greater the proportion of clay particles. and let it settle for at least 8 hours. The soil sample should now show a fairly distinct For soil X, plasticity index = liquid limit - plastic limit line, below which the individual particles can be seen = 0.62 - 0.26 = 0.36. with the naked eye. Measure the thickness of the For soil Y, plasticity index = 0.34 - 0.19 = 0.15 combined silt and clay layer above the line, and Soil X contains more clay particles. calculate it as a percentage of the total height of the soil sample. Soil-testing methods The test tends to give a lower figure than laboratory As indicated above, some soils are more suitable for tests, as a result of some silt and clay being trapped in building material than others. It is therefore essential the sand, and because some material remains suspended to have a means of identifying different types of in the water above the sample. soil. There are a number of methods, ranging from The main disadvantage with this test is that the silt laboratory tests to simple field tests. Laboratory soil and clay fractions cannot be determined separately. As tests are recommended for the production of buildings silt behaves differently from clay, this could result in on a large scale (i.e. several houses). mistaken conclusions about the soil’s suitability for stabilization and as a building material. Chapter 5 – Construction materials 61 Shrinkage ratio = ( Length of wet bar) − (Length of dried bar) × 100 (5.4) Length of wet bar 1/3 Water Water a Silt - Clay 1/3 Soil h Sand-Gravel sample Soil stabilization The main weakness of earth as a building material is its low resistance to water. While overhanging eaves Figure 5.11 Simple sedimentation and verandas help considerably, tropical rains of any intensity can damage unprotected walls. Due to the clay fraction, which is necessary for cohesion, walls Bar shrinkage test built of unstabilized soil will swell on taking up water, This test gives an indication of the plasticity index of and shrink on drying. This may lead to severe cracking the soil, because the shrinkage ratio of the soil when and difficulty in making protective renderings adhere dried in its plastic state is related to its plasticity index. to the wall. A wooden or metal box without a top and with a However, the quality as a building material of nearly square cross-section of 30–40 mm per side and a length any inorganic soil can be improved considerably by the of 500–600 mm, is filled with soil from the sample (see addition of a suitable amount of the correct stabilizer. Figure 5.12). Before filling, the soil should be mixed The aim of soil stabilization is to increase the soil’s with water to slightly more than the liquid limit. The resistance to destructive weather conditions, in one or consistency is right when a V-shaped groove cut in the more of the following ways: soil will close after about five taps on the box. Grease 1. By cementing the soil particles together, leading or oil the box, fill with the soil and compact it well, to increased strength and cohesion. paying special attention to filling the corners. Smooth 2. By reducing the movements (shrinkage and the surface by scraping off the excess soil. Place the box swelling) of the soil when its moisture content in the shade for 7 days. The drying can be hastened by varies according to weather conditions. placing the box at room temperature for 1 day, and then 3. By making the soil waterproof, or at least less in an oven at 110 °C until the soil is dry. permeable to moisture. If, after drying, the soil bar has more than three large cracks in addition to the end gaps the soil is not A great number of substances may be used for soil suitable. Measure the shrinkage ratio by pushing the stabilization. Owing to the many different kinds of soil dried sample to one end of the box and calculate the and types of stabilizer, there is no single solution for all length of the gap as a percentage of the length of the cases. It is up to the builder to make trial blocks with box. The soil is not suitable for stabilization if the various amounts and types of stabilizer. shrinkage ratio is more than 10  percent, i.e. a gap of Stabilizers in common use are: 60 mm in a 600 mm long box. The higher the shrinkage • sand or clay; ratio, the more stabilizer has to be used. The shrinkage • portland cement; ratio is calculated as follows: • lime; • bitumen; 40−50 WET SOIL BAR il b ar Wet so Dry so il b ar 500−600 Figure 5.12 Box for bar shrinkage test 40−50 62 Rural structures in the tropics: design and development • pozzolana (e.g. fly ash, rice husk ash, volcanic ash); Bitumen (or asphalt) emulsion and cutback are used • natural fibres (e.g. grass, straw, sisal, sawdust); mainly to improve the impermeability of the soil and • sodium silicate (water-glass); keep it from losing its strength when wet, but may • commercial soil stabilizers (for roads); cause some decrease in dry strength. They are only • resins; used with very sandy soils because it would be very • whey; difficult to mix them with clayey soils. In its natural • molasses; form, bitumen is too thick to be added to soil without • gypsum; heating, so it has to be thinned with other liquids to • cow dung. make it workable. The easiest way is to mix it with water to make an emulsion. After the emulsion has Many other substances may also be used for soil been added to the soil, the water will separate, leaving a stabilization, although their use is not well documented bitumen film on the soil grains. and test results are scarce. If the bitumen emulsion is fast-settling (i.e. the water Sand or clay is added to improve the grading of a separates too quickly before it is mixed into the soil), soil. Sand is added to soils that are too clayey, and clay the bitumen must instead be dissolved in kerosene or is added to soils that are too sandy. The strength and naphtha. This mix is called ‘cutback’ and should be cohesion of the sandy soil is increased, while moisture handled with care because it represents a fire hazard and movement of a clay soil is reduced. Improved grading explosion risk. After a soil has been treated with cutback, of the soil material does not stabilize the soil to a it must be spread out to allow the kerosene to evaporate. high degree, but will increase the effect, and reduce The bitumen content used is 2–4  percent, as any the required amount, of other stabilizers. The clay or more may seriously reduce the compressive strength clayey soil must be pulverized before mixing with the of the soil. sandy soil or sand. This may prove difficult in many A combination of lime and pozzolana makes a binder cases. that can be almost as good as portland cement. It is used Portland cement greatly improves the soil’s in the same way as a combination of lime and cement, compressive strength and imperviousness, and may but 2-4 parts of pozzolana are mixed to 1 part of lime, also reduce moisture movement, especially when and the curing time is longer than for ordinary cement. used with sandy soils. As a rough guide, sandy soils Natural fibres, used in a mixing ratio of about need 5–10 percent cement for stabilization, silty soils, 4 percent, greatly reduce moisture movement, but will 10–12.5  percent and clayey soils, 12.5–15  percent. make dry-soil blocks weaker and more permeable to Compaction when ramming or pressing blocks will water. greatly improve the result. Sodium silicate, or water-glass, is best used to coat The cement must be thoroughly mixed with dry the outside of soil blocks as a waterproofing agent. soil. This can be rather difficult, especially if the soil is clayey. As soon as water is added, the cement Cob starts reacting and the mix must therefore be used Cob is used extensively in tropical Africa, where immediately (1–2  hours). If the soil-cement hardens suitable soils are obtainable over wide areas. The best before moulding, it must be discarded. Soil-cement soil mix consists of gravel, sand, silt and clay in roughly blocks should be cured for at least 7 days under moist equal proportions. Sometimes chopped grass or straw or damp conditions. is added to reduce cracking. If the clay content is high, Non-hydraulic lime, or slaked lime, gives best results sand may be added. Laterite makes an excellent material when used with fine soils, i.e. silty and clayey soils. for cob walling. Lime decreases moisture movement and permeability, When a suitable soil has been found, the topsoil is by reaction with the clay, to form strong bonds removed and the subsoil dug up. Water is slowly added between the soil particles. The amount of lime used to the loose soil, which is then kneaded by treading varies between 4 percent and 14 percent. Lime breaks until the soil has a wet, plastic consistency. Natural down lumps and makes it easier to mix clayey soils. fibres are added for stabilization if required. Curing at high temperatures strengthens the cementing The wet cob is rolled into balls or lumps measuring molecules, which should be an advantage in the tropics. about 20 cm in diameter, which are then bedded on the The curing time is longer than for soil-cement. wall to form courses about 60 cm high. The outside of A combination of lime and cement is used when a the wall may be scraped smooth. In arid and semi-arid soil has too much clay for cement stabilization, or too climates, this type of wall may last for years if built on little clay for an extensive reaction with the lime. Lime a proper foundation and protected from rain by a roof will make the soil easier to work and the cement will overhang or veranda. increase the strength. Equal parts of lime and cement are used. Mixing the dry soil with lime first makes the Wattle and daub (mud and wattle) soil more workable. Blocks are cured for at least 7 days This method of building small houses is very common under moist conditions. in areas where bamboo or stalks (e.g. sisal) are available. Chapter 5 – Construction materials 63 It consists of a framework of split bamboo, stalks or Rammed earth wooden sticks, supported by wood or bamboo poles. This consists of ramming slightly damp soil between The soil, prepared as cob, is daubed on either side of stout formwork using heavy rammers. It makes fairly the laths, which act as reinforcement. Although most strong and durable walls and floors when it is made soil is suitable for this construction, if it is too clayey thick enough with properly prepared, stabilized soil. there may be excessive cracking. To minimize cracking, When used for walls, the soil may contain some stones are mixed with the soil, or laid in the wooden cobble, but the maximum size should be less than one- skeleton. When mudding the inside of a building, quarter of the thickness of the wall. When cement is the soil is often taken from the floor. Although this used for stabilization, it must be mixed with the dry soil increases the ceiling height, it greatly increases the by hand, or in a concrete mixer, until the dry mixture likelihood of flooding during the rainy season. has a uniform greyish colour. The amount of cement During drying, the weight of the soil is transferred required is approximately 5–7  percent for interior to the wooden structure, with the total weight of the walls, 7–10 percent for foundations and exterior walls, construction eventually resting on the poles. and 10–15 percent for floors. However, the amount of Wattle and daub construction generally has a short stabilizer required will vary with the composition of the lifespan because of soil erosion, and the uneven settling soil, the type of stabilizer and the use. For this reason, of poles and damage by fungi and termites. However, trial blocks should be made and tested to determine the the durability can be improved considerably (20– correct amount of stabilizer. 40  years) by using a proper foundation, raising the Water is sprinkled on the soil while it is being mixed. building off the ground, applying a surface treatment If the soil is sticky because of high clay content, hand and using termite-resistant or treated poles. mixing will be necessary. When the correct amount of water has been added, the soil will form a firm lump Clay/straw when squeezed in the hand, and just enough moisture The technique of building walls of clay/straw has been should appear on the surface to give a shiny appearance. highly developed in China, where grain storage bins of After the mixing has been completed, the soil should up to 8 m in diameter, 8.5 m in height and a 250-tonne be placed in the formwork immediately. The formwork holding capacity have been constructed with these can be either fixed or sliding, but must be stout. The materials. soil is placed in layers of about 10 cm, and each layer Any type of straw can be used, but it must be of is thoroughly compressed with a ram weighing 8–10 kg good quality. The clay should be of strong plasticity, before the next layer is placed. If water shows on the containing less than 5 percent sand. Some lime may be surface during ramming, the soil mix is too wet. added for stabilization if the sand content is a bit too If cement or pozzolana has been used for stabilization, high. the product should be cured for 1–2 weeks in a moist First, the straw bundles are produced. The straw condition before it is allowed to dry out. This can be is pruned level at the root ends and then divided into done either by keeping the product enclosed in the two halves, which are turned in opposite directions formwork, or by covering it with damp bags or grass and placed together so that they overlap by about that are watered daily. two-thirds of the length of the straw. The straw bundle is then spread out flat and soaked with clay mud. Adobe or sun-dried soil (mud) blocks Thorough covering of each straw is essential for the The best soil for adobe can be moulded easily, when final strength. The straw is then twisted together, and plastic, into an egg-size ball, and when it is allowed to any excess mud removed. The final clay/straw bundle dry in the sun it becomes hard, shows little deformity should be thick in the middle and tapered at both ends, and no more than very fine cracks. If wide cracks be of 80–100 cm in length and roughly 5 cm in diameter develop, the soil does not contain enough silt or sand, at the middle. The ideal proportion of straw and clay is and sand may be added as a stabilizer. 1:7 on a dry weight basis. The clay/straw bundles are placed on the wall either Preparing the soil straight and flat, or slightly twisted together. Walls for When a suitable soil has been found, all topsoil must be grain bins should have a thickness in centimetres equal removed. The soil is then loosened to a depth of l5 cm. to the internal diameter in metres +  12, i.e. a 6-metre If needed, water and sand are added and worked into diameter bin should have a wall thickness of at least the loose soil by treading it barefoot while turning the 18 cm. It is important to compact the wall thoroughly mass with a spade. during construction to ensure high density, strength Water is added slowly and the soil mixed and durability. The wall must be built in separate layers, thoroughly until all lumps are broken up and it usually about 20  cm, and be left to dry out to about becomes homogeneous and plastic. When it is the 50  percent moisture content before the next layer is right consistency for moulding, it is cast in a wooden added. mould made with one to three compartments and with dimensions as shown in Figure 5.13. 64 Rural structures in the tropics: design and development Before the mould is used for the first time it should Stabilized-soil blocks be thoroughly soaked in oil. Because of the shrinkage, When a suitable soil has been found, the topsoil should the finished blocks will be smaller than the moulds and, be removed and the subsoil dug out and spread out to depending on bonding, will give a wall thickness of dry in the sun for a few days. about 230 mm, 270 mm or 410 mm. Large particles and lumps must be removed before the soil is used, by breaking the larger lumps and passing all the soil through a 10 mm screen. If the proportion 75 of gravel in the soil is high, a finer screen, of 4.5–6 mm, should be used. The wire screen, usually measuring 300 about 1 metre square, is rocked in a horizontal position 1 150 300 by one person holding handles at one end, with the other end suspended in ropes from above. The amount 0 30 of loose, dry soil needed will normally be 1.4–1.7 times its volume in the compacted blocks. 75 Mixing 4 The amount of stabilizer to be used will depend on 50 the type of soil, the type of stabilizer and the building component being made. Tables 5.7 and 5.8 give a guide to the necessary minimum mixing ratio of soil-cement for blocks compacted in a mechanical press. For blocks compacted in a hydraulic press, the cement requirement 75 can be reduced considerably, whereas slightly more will be needed for hand-rammed blocks. The correct 250 proportion of stabilizer is determined by making test blocks with varying proportions of stabilizer, as 1 000 250 described later. 0 25 75 TABLE 5.7 Cement/soil ratio related to shrinkage ratio in the bar shrinkage test 300 Shrinkage Cement to soil ratio 0–2.5% 1:18 2.5–5% 1:16 Figure 5.13 Wooden moulds for making adobe blocks 5–75% 1:14 made of 100 × 25 mm sawn timber 7.5–10% 1:12 Moulding the blocks To prevent sticking, the mould must be soaked in water TABLE 5.8 before being placed on level ground, and filled with Cement/soil ratio related to the combined silt and clay mud. The mud is kneaded until all corners of the mould content in the simple sedimentation test are filled, and the excess is scraped off. The mould is Clay & silt Interior Exterior content walls walls Foundations Floor slab lifted and the blocks are left on the ground to dry. The mould is dipped in water each time before repeating 0–10% 1:16 1:16 1:16 1:8 the process. 10–25% 1:22 1:16 1:16 1:11 After drying for 3–4  days, the blocks will have 25–40% 1:22 1:11 1:11 1:11 hardened sufficiently to be handled, and are turned on edge to hasten drying. After a further 10 days, the blocks can be stacked loosely in a pile. Adobe blocks The importance of thoroughly mixing the dry soil should dry out as slowly as possible to avoid cracks, first with the stabilizer and then with the moisture, in with a total curing time of at least 1 month. two distinct steps, cannot be emphasized too strongly. The quality of the blocks depends largely on the The quantity of cement and dry soil is measured with workmanship, especially the thoroughness with which a measuring box, bucket or tin (never with a shovel), they are moulded. If the quality is good, only 1  in and put either on a clean, even and hard surface for hand 10  blocks should be lost from cracking, breakage or mixing, or into a drum-type mixer (concrete mixer). deformities. They are mixed until the dry mixture has a uniform Chapter 5 – Construction materials 65 TABLE 5.9 Batching for stabilized-soil blocks Approximate Requirement number of blocks per 50 kg cement Proportions cement of loose soil Size of blocks cement/soil content by per 50 kg by volume weight cement 290×140×50 290×140×90 290×140×120 290×140×140 290×215×140 1:22 5% 1 080 litres 366 203 152 130 85 1:18 6% 880 litres 301 167 125 107 70 1:16 7% 780 litres 268 149 111 95 62 1:14 8% 690 litres 235 131 98 84 54 1:12 9% 590 litres 203 113 84 72 47 1:11 10% 540 litres 187 104 78 66 43 1:10 11% 490 litres 170 94 71 61 39 1:9 12% 440 litres 154 85 64 55 36 1:8.5 13% 420 litres 146 81 61 52 34 1:8 15% 390 litres 138 76 57 49 32 greyish colour. Water is added, preferably through a Hinges sprinkler, while continuing the mixing. When the correct amount of water has been added, the soil, when squeezed into a ball, should retain its shape without soiling the hand. The ball should be capable of being pulled apart without disintegrating, but it should disintegrate when dropped from shoulder height on to a hard surface. Compaction by hand-ramming Moulds with one or more compartments can be made from either hardwood or steel. The mould should have hinges at one or two corners to enable it to be opened easily without spoiling the block. As mould has no bottom, it is preferable to place it on a pallet, rather Lock than directly on the ground, when moulding the block. The mould is treated as often as required with oil to make the block surface smooth and to prevent the block from sticking to the mould. The soil mixture should be placed in layers in the mould, and each layer thoroughly Figure 5.14 Mould for hand-rammed stabilized-soil compacted with a flat-bottomed ram weighing 4–5 kg. blocks made of 20 mm planed timber Each block may need as many as 80 good blows with the ram. The top of the block is levelled off, the block and mould are carried to the curing store where the mould is removed, then the whole process is repeated. Compaction with a mechanical press There are many mechanical block-making machines on the market, both motor-driven (enabling several blocks to be made at a time), and hand-operated. They all consist of a metal mould in which a moist soil mix is compressed. The moulding for a hand-operated press is carried out as follows: 1. The inside of the compaction chamber is cleaned and oiled, and a pallet is placed in the bottom, if required. 2. A measured amount of soil mix is poured into the compaction chamber and the soil is compacted into the corners by hand. Figure 5.15 Mechanical press for block making 66 Rural structures in the tropics: design and development 3. The lid is closed and the handle pulled down. The Comparison of masonry units amount of soil mix is correct if the handle can be made of various materials moved down to stop slightly above a horizontal There are many methods for making bricks and blocks, level. several of which are suitable for local production 4. The block is ejected and carried on the pallet to because they are labour-intensive but do not require the curing site, before returning the pallet to the especially skilled labour. press for reuse. The decision concerning which method of blockmaking or brickmaking to use depends on several Curing of blocks factors, such as: Soil-cement blocks should be placed on the ground, • the raw materials available; in the shade, as close together as possible, and be kept • the characteristics of the soil; damp (e.g. with wet grass). After 1–2 days, the blocks • raw material and production costs; can be carefully stacked and again kept damp for • the requisite standards of stability, compression 1–2 weeks. After this period, the blocks are allowed to strength, water resistance, etc. (3 N/mm² is often air dry for 2–3 weeks in a stacked pile before use. regarded as the minimum compressive strength for use in one-storey buildings); Testing of blocks • the existing facilities for the maintenance of In the laboratory, dry strength and wet strength are production tools and machines; determined by crushing two well-cured blocks in • the required productivity. a hydraulic press: the first in a dry state, and the second after having been soaked in water for 24 hours. BuRnT-CLAy BRICkS Durability is tested by spraying the blocks with Burnt-clay bricks have good resistance to moisture, water according to a standard procedure, and making insects and erosion, and create a good room environment. observations for any erosion or pitting. They are moderate in cost and have medium to high In order to find out how much stabilizer is required, compressive strength. the following simple weather-resistance test carried out Bricks can be made using sophisticated factory in the field may give a satisfactory answer. methods, simple labour-intensive methods, or a range of At least three different soil mixes with different intermediate mechanized technologies. Labour-intensive stabilizer-soil ratios are prepared, and at least three production methods are the most suitable for rural areas blocks are made from each of the different mixes. where the demand for bricks is limited. Handmade Mixing, compaction and curing must be done in the bricks will be of comparatively lower quality, especially same way as for the block production process. At the in terms of compressive strength, and will tend to have end of the curing period, three blocks are selected from irregular dimensions. However, they are economical and each set, immersed in a tank, pond or stream all night, require little capital investment or transportation costs. and dried in the sun all day. This wetting and drying Bricks made in this manner have been used in buildings process is repeated for 7 days. that have lasted for centuries. Their longevity depends The correct amount of stabilizer to use is the on the quality of the ingredients, the skill of the artisans smallest amount with which all three blocks in a set and the climate in which they are used. pass the test. While a few small holes are acceptable on the compaction surface, if many holes appear on Brickmaking all surfaces the blocks are too weak. If the blocks have Five main ingredients are required for brickmaking: passed the test and the dry block produces a metallic suitable clay and sand, water, fuel and manpower. The ring when tapped with a hammer, they will have clay must be easily available, plastic when mixed with satisfactory durability and hardness. small amounts of water, develop strength upon drying If the blocks fail the test, the reason may be any of and develop hard and durable strength when burned. the following: Suitable soils contain 25–50 percent clay and silt and • unsuitable soil; 50–75  percent coarser material, as determined by the • insufficient amount of stabilizer; simple sedimentation test. The soil must be well graded. • incorrect type of stabilizer; Another test consists of hand-rolling moistened soil • inadequately dried or lumpy soil; into a long cylinder 10 mm in diameter on a flat surface, • lumpy cement; and then picking it up by one end and letting it hang • insufficient mixing of the stabilizer; unsupported. • too much or too little water added; A soil is adequate for brickmaking if the piece of • not enough compaction; cylinder that breaks off is between 50 mm and 150 mm • incorrect curing. long. In the bar shrinkage test, using a mould 300 mm long and 50 mm wide and deep, a suitable soil should show no cracking, or only a little on the surface, and should shrink less than 7 percent, i.e. less than 20 mm. Chapter 5 – Construction materials 67 TABLE 5.10 Characteristics of masonry units Remarks Sun-dried bricks (adobe bricks) 1 1 1 1 1-3 4 3 1 3 3 1 Most easily produced locally; much improved with stabilizers; least stable and durable. Manually rammed stabilized-soil blocks 2 1-2 1-2 1-2 2-3 5 1 2 5 2 2 A little more effort; better quality and stability. Mechanically pressed stabilized-soil blocks 2 2 2 2 4 5 2 2 4 3 2 Stronger; more durable. Hydraulically pressed stabilized-soil blocks 3 3 3 3 4 5 3 3 3 3 2 Even stronger and more durable. Locally made burnt-clay bricks 3-4 2-3 3 3-4 1-3 4 2 4 4 3-4 4 Labour-intensive production. Bricks are generally of low quality. Factory-produced burnt-clay bricks 5 5 5 5 5 4 5 5 1-3 5 3 Commercial production is common. The plant requires large investment. Concrete blocks (sand and cement) 5 5 4 5 5 3 3-4 4 2-3 4 5 Long life; strong but heavy. Local production is generally more labour-intensive than commercial. Building stones 4 5 5 5 1-4 3 1 5 5 4 2 When locally available, a strong, durable and attractive material Scale 1-5 1 = lowest 5 = highest The clay is obtained by chipping it out of a clay The bricks should be left to dry for about 3  days bank and, where necessary, mixing it with sand to form in the place where they were made. They will then be a mixture that will not crack during drying. Water is strong enough to be stacked, as shown in Figure 5.17, for gradually added to make the clay plastic. When making bricks, the mould must be cleaned periodically with water. Before each brick is formed, the mould is sprinkled with sand. A lump, or ‘clot,’ TIE BRICK of clay that is only slightly larger than required for a brick is rolled into a wedge shape and then dipped in sand, before being thrown, point down, into the mould. When thrown correctly, the mould will be completely filled, and the excess clay is then shaved off the top with a bowcutter. The sand in the mould and on the clot helps to release the newly formed brick. A COMPLETED DRY STACK Figure 5.16 Mould for brick-forming Figure 5.17 Stacking pattern for brick-drying Cost Compressive strength Resistance to moisture Resistance to erosion uniformity of shape Room comfort Speed of production Skill required to make Labour requirements Ease of transport Energy requirements 68 Rural structures in the tropics: design and development at least 1 week of further drying. Clay tends to become criteria being strength, irregular dimensions, cracks lighter in colour when dry and, when sufficiently dried, and, sometimes, discoloration and stain. the brick should show no colour variation throughout the section area when it is broken in half. During drying, BIndERS the bricks should be protected from rain. When binders are mixed with sand, gravel and water, they make a strong and long-lasting mortar or concrete. Binders can be broadly classified as non-hydraulic FLUE OPENING or hydraulic. The hydraulic binders harden through a chemical reaction with water, making them impervious to water and therefore able to harden under water. BRICK WALL Portland cement, blast furnace cement (super sulphated), pozzolanas and high-alumina cement belong to the category of hydraulic binders. High-calcium limes (fat or pure limes) are non-hydraulic because they harden by reaction with the carbon dioxide in the air. However, if lime is produced from limestone containing clay, compounds similar to those in portland cement will be formed, i.e. hydraulic lime. 2∙ FIRE BOX GRID 5 m approximately 3∙0 m Lime Non-hydraulic lime refers to high-calcium limes that are produced by burning fairly pure limestone (essentially Figure 5.18 kiln for brick-firing calcium carbonate), in order to drive off the carbon dioxide, leaving calcium oxide or quicklime. The burning process requires a temperature of 900–1 100 °C. Kiln construction and brick-firing Quicklime must be handled with great care because it It is during firing that the bricks gain their strength. reacts with moisture on the skin and the heat produced In high temperatures, the alkalis in the clay, together may cause burns. When water is added to quicklime, with small amounts of oxides of iron and other metals, considerable heat is generated, expansion takes place, chemically bond with the alumina and silica in the clay breaking down the quicklime pieces into a fine powder, to form a dense and durable mass. and the resulting product is calcium hydroxide, also A kiln is a furnace or oven in which bricks are fired or called hydrated lime, or slaked lime. heat-treated to develop hardness. Where brickmaking is After drying, the powder is passed through a 3-mm carried out on a large scale, the firing operation is sieve, before being poured into bags for storage (in dry performed in a continuous-process kiln, referred to as a conditions) and distribution. ‘tunnel’ kiln. For brickmaking on a small scale, firing is a periodic operation where the bricks are placed in the kiln, the fire started and heat developed, and then, after Process Substance Chemical formula several days of firing, the fuel is cut off from the fire and Burning Limestone – quicklime CaC03 - CaO + C02 the entire kiln and its load are allowed to cool naturally. Slaking Quicklime – slaked lime CaO + H20 - The kiln is filled with well-dried bricks, stacked in Ca(OH)2 the same manner as during the drying process. The top Hardening Slaked lime – limestone Ca(OH)2 + CO2 - of the stack in the kiln is then sealed with mud. Some CaC03 + H20 openings are left for combustion gases to escape. Pieces of sheet metal are used to slide over the openings to control the rate at which the fire burns. Slaked lime is used mainly in building because it is Although a range of fuels can be used in a kiln, fat, i.e. it makes workable mortar and rendering and wood or charcoal are the most common. When the plaster mixes. Initially, a lime mortar becomes stiff by kiln is at the prime heat for firing, a cherry-red hue evaporation loss of water to absorptive materials such develops (corresponding to a temperature range of as bricks, but subsequent hardening depends on the 875–900  °C). This condition is maintained for about chemical reaction with carbon dioxide from the air 6  hours. Sufficient fuel must be available when the (carbonation) reforming the original calcium carbonate burning starts because the entire load of bricks could (limestone). be lost if the fires are allowed to die down during the Non-hydraulic lime is also produced from limestone operation. Firing with wood requires 4–5 days. with a high content of magnesium carbonate. It is less During firing, the bricks will shrink by as much easily slaked, but some of the remaining unslaked as 10  percent. As they are taken out of the kiln, magnesium oxide may carbonate and produce greater they should be sorted into different grades, the main strength than high-calcium lime. 2∙0 m Chapter 5 – Construction materials 69 Hydraulic lime is produced by mixing and grinding Chemistry of cement together limestone and clay material, and then burning The main components of standard portland cement are: it in a kiln. • lime (calcium oxide: 66%) in the form of limestone; It is stronger but less fat, or plastic, than non- • silica (silicum dioxide: 22%), a component in hydraulic lime. During burning, the calcium oxide from most quartz, which forms the particles of clays; the limestone reacts with siliceous matter from the clay • aluminium oxide (4%), found in large quantities in to form dicalcium silicate. This compound may react many clays. The proportion of aluminium oxide in with water, forming ‘mineral glue’ – tricalcium disilicate the clay can be adjusted by the addition of bauxite, hydrate. The reaction is slow and may take weeks or which is mainly water-soluble aluminium oxide; months, but after some time a very good degree of • iron oxide (3%), found in iron ore and in clay; strength is achieved. • magnesium oxide (2%); The reaction that forms dicalcium silicate requires • sulphur dioxide (2%); a very high temperature to be complete. In practical • miscellaneous components (1%). production, a lower temperature of 1  200  °C is used, leaving some of the ingredients in their original state. The manufacturing process aims to produce a At this temperature, the limestone will lose the carbon material with a high content of tricalcium silicate, usually dioxide and thus form quicklime. If the correct amount 55–62 percent of the crystals in the clinker. Other crystals of water is added, the quicklime will slake, forming a formed are: about 15  percent dicalcium silicate (the fine powder. Note, however, that excess water will lead same component as the hydraulic binder in hydraulic to premature hardening caused by hydraulic reaction. lime), 8–10  percent tricalcium aluminate and 9  percent tetracalcium aluminate ferrite. As cement sinters during Cement burning, it is very important for no calcium oxide Portland cement hardens faster and develops (quicklime) to remain in the finished product. considerably higher strength than hydraulic lime. This The quicklime will remain embedded in the clinker, is because cement contains tricalcium silicate. However, even after very fine grinding, and will not be available the manufacturing process is much more complicated for slaking until the hardening process of the cement than for lime. The ingredients are mixed in definite is quite far advanced. When the quicklime particles and controlled proportions, before being ground to are finally slaked, they expand and break the structure a very fine powder. The fine grinding is necessary already developed. The proportion of limestone in the because the formation of tricalcium silicate can only initial mix must therefore be no more than 0.1 percent. take place in a solid state, and therefore only the When cement is mixed with water, it initiates the surface of the particles in the mix is accessible for the chemical reactions that are so important for hardening. chemical reaction, which requires a temperature of The most important of these is the formation of 1 250–1 900 °C to be completed. tricalcium disilicate hydrate, ‘mineral glue’, from During burning, the small particles of limestone hydrated calcium oxide and silica. and clay are sintered together to form clinker. After cooling, the clinker is ground to cement powder, with 2(3CaO SiO2) + 6H2O = 3CaO 2SiO2 3H2O + 3Ca(OH)2 a small amount of gypsum being added during the grinding. The finer the cement particles, the larger the and surface area available for hydration by water, and the more rapidly setting and hardening occurs. Cement 2(2CaO SiO2) + 3H2O = 3CaO 2SiO2 3H2O + Ca(OH)2 is normally sold in 50  kg bags, but occasionally is available in bulk at a lower price. The reaction between dicalcium silicate and water Ordinary portland cement is the least expensive, is slow and does not contribute to the strength of and by far the most widely used, type of cement. It is the concrete until a considerable time has elapsed. suitable for all normal purposes. Aluminate would interfere with these processes, hence Rapid-hardening portland cement is more finely the addition of gypsum at the end of the manufacturing ground, which accelerates the chemical reaction with process. The gypsum forms an insoluble compound water and develops strength more rapidly. It has the with the aluminate. same strength after 7 days as ordinary portland cement During the hydration process, the cement chemically does after 28  days. Early hardening may be useful binds water corresponding to about one-quarter of its where early stripping of formwork and early loading of weight. Additional water evaporates, leaving voids, the structure is required. which reduce the density, and therefore the strength Low-heat portland cement develops strength very and durability, of the end products. slowly. It is used in very thick concrete work where the heat generated by the chemical reactions in ordinary Pozzolana portland cement would be excessive and lead to serious A pozzolana is a siliceous material which, in finely cracking. divided form, can react with lime in the presence of 70 Rural structures in the tropics: design and development moisture at normal temperatures and pressures to are sometimes included in the concrete mix to achieve form compounds possessing cementing properties. certain properties. Reinforcement steel is used for Unfortunately, the cementing properties of pozzolana added strength, particularly for tensile stresses. mixtures are highly variable and unpredictable. Concrete is normally mixed at the building site A wide variety of materials, both natural and and poured into formwork of the desired shape, in artificial, may be pozzolanic. Silica constitutes more the position that the unit will occupy in the finished than half the weight of the pozzolana. Volcanic ash structure. Units can also be precast, either at the was the first pozzolana that the Romans used to make building site or at a factory. concrete for a host of large and durable buildings. Deposits of volcanic ash are likely to be found wherever Properties of concrete there are active, or recently active, volcanoes. Concrete is associated with high strength, hardness, Other natural sources of pozzolana are rock or durability, imperviousness and mouldability. It earth in which the silica constituent contains the is a poor thermal insulator, but has high thermal mineral opal, and the lateritic soils commonly found capacity. Concrete is not flammable and has good fire in Africa. Artificial pozzolana includes fly ash from resistance, but there is a serious loss of strength at high the combustion of coal in thermoelectric power plants, temperatures. Concrete made with ordinary portland burnt clays and shales, blast furnace slag formed in the cement has low resistance to acids and sulphates but process of iron manufacture, rice husk ash and the ash good resistance to alkalis. from other agricultural wastes. Concrete is a relatively expensive building material The energy requirement for the manufacture of for farm structures. The cost can be lowered if some portland cement is very high. By comparison, lime of the portland cement is replaced with pozzolana. and hydraulic lime can be produced with less than half However, when pozzolanas are used, the chemical the energy requirement, and natural pozzolana may reaction is slower and strength development is delayed. be used directly without any processing. Artificial The compressive strength depends on the proportions pozzolana requires some heating, but less than half that of the ingredients, i.e. the cement/water ratio and the required for lime production. cement aggregate ratio. As the aggregate forms the bulk Pozzolana and lime can be produced with much less of hardened concrete, its strength will also have some sophisticated technology than portland cement. This influence. Direct tensile strength is generally low, only means that pozzolana can be produced at relatively 1/8 to 1/14 of the compressive strength, and is normally low cost and requires much less foreign exchange than neglected in design calculations, especially in the design cement. However, it takes 2–3  times the volume of of reinforced concrete. pozzolana to make a concrete with the same strength Compressive strength is measured by crushing as with portland cement, and this adds to the cost of cubes measuring 15 cm on all sides. The cubes are cured transport and handling. for 28 days under standard temperature and humidity The main use of pozzolanas is for lime-pozzolana conditions, before being crushed in a hydraulic press. mortars, for blended pozzolanic cements and as an Characteristic strength values at 28  days are those admixture in concrete. Replacing up to 30  percent below which not more than 5  percent of the test of the portland cement with pozzolana will produce results fall. The grades used are C7, C10, Cl5, C20, 65–95 percent of the strength of portland cement concrete C25, C30, C40, C50 and C60, each corresponding to a at 28  days. The strength nominally improves with age characteristic crushing strength of 7.0 N/mm2, 10.0 N/ because pozzolana reacts more slowly than cement, and mm2, 15.0 N/mm2, etc. at 1 year about the same strength is obtained. COnCRETE TABLE 5.11 Concrete is a building material made by mixing cement Typical strength development of concrete paste (portland cement and water) with aggregate (sand Average crushing strength and stone). The cement paste is the ‘glue’ that binds Ordinary Portland cement the particles in the aggregate together. The strength of Storage in air 18 °C the cement paste depends on the relative proportions Age RH 65% Storage in water at test (n/mm2) (n/mm2 of water and cement, with a more diluted paste being ) weaker. In addition, the relative proportions of cement 1 day 5.5 - paste and aggregate affect the strength, with a higher 3 days 15.0 15.2 proportion of paste making stronger concrete. 7 days 22.0 22.7 The concrete hardens through the chemical reaction between water and cement, without the need for air. 28 days 31.0 34.5 Once the initial set has taken place, concrete cures well under water. Strength is gained gradually, depending 3 months 37.2 44.1 on the speed of the chemical reaction. Admixtures (1 cement/6 aggregate, by weight, 0.6 water/cement ratio). Chapter 5 – Construction materials 71 TABLE 5.12 In some literature, the required grade of concrete is Suggested use for various concrete grades defined by the proportions of cement, sand and stone and nominal mixes (so called nominal mixes), rather than the compressive nominal strength. Therefore some common nominal mixes have Grade mix use been included in Table  5.12. Note, however, that the C7 1:3:8 Strip footings; trench fill foundations; amount of water added to such a mix will have a great C10 1:4:6 stanchion bases; non-reinforced influence on the compressive strength of the cured 1:3:6 foundations; oversite concrete and 1:4:5 bindings under slabs; floors with very concrete. 1:3:5 light traffic; mass concrete, etc. The leaner of the nominal mixes listed opposite the C15 1:3:5 Foundation walls; basement walls; C7 and C10 grades are workable only with very well C20 1:3:4 structural concrete; walls; reinforced graded aggregates ranging up to quite large sizes. 1:2:4 floor slabs; floors for dairy and beef 1:3:3 cattle, pigs and poultry; floors in grain and potato stores, hay barns, and Ingredients machinery stores; septic tanks and water storage tanks; slabs for farmyard manure; roads, driveways, paving and Cement walks; stairways. Ordinary Portland cement is used for most farm C25 1:2:4 All concrete in milking parlours, dairies, structures. It is sold in paper bags containing 50  kg, C30 1:2:3 silage silos, and feed and drinking or approximately 37  litres. Cement must be stored in C35 1:1.5:3 troughs; floors subject to severe wear 1:1:2 and weather conditions, or weak acid a dry place, protected from ground moisture, and the and alkali solutions; roads and paving storage period should not exceed a month or two. Even in frequent use by heavy machinery and lorries; small bridges; retaining damp air can spoil cement. It should be the consistency walls and dams; suspended floors, of powder when used. If lumps have developed, the beams and lintels; floors used by heavy, quality has decreased, although it can still be used if the small-wheeled equipment, such as lift trucks; fencing posts and precast lumps can be crushed between the fingers. concrete components. C40   Concrete in very severe exposure; Aggregate C50 prefabricated structural elements; Aggregate or ballast is either gravel or crushed stone. C60 prestressed concrete. Aggregates that pass through a 5  mm sieve are called fine aggregate or sand. and those retained are called coarse aggregate or stone. The aggregate should be of voids to be filled with the more costly cement hard, clean and free from salt and vegetable matter. paste. The particles also flow together readily, i.e. the Too much silt and organic matter makes the aggregate aggregate is workable, enabling less water to be used. unsuitable for concrete. The grading is expressed as a percentage by weight To test for silt, place 80 mm of sand in a 200 mm-high of aggregate passing through various sieves. A well- transparent bottle. Add water up to a height of 160 mm. graded aggregate will have a fairly even distribution Shake the bottle vigorously and allow the contents to of sizes. settle until the following day. If the silt layer, which will The moisture content in sand is important because settle on top of the sand, is less than 6 mm, the sand can the sand mixing ratio often refers to kilograms of dry be used without further treatment. If the silt content is sand, and the maximum amount of water includes the higher, the sand must be washed. moisture in the aggregate. The moisture content is To test for organic matter, place 80 mm of sand in determined by taking a representative sample of 1 kg. a 200  mm-high transparent bottle. Add a 3  percent The sample is accurately weighed and spread thinly solution of sodium hydroxide up to 120  mm. Note on a plate, soaked with spirit (alcohol) and burned that sodium hydroxide, which can be bought from a while stirring. When the sample has cooled, it is chemist, is dangerous to the skin. Cork the bottle and weighed again. The weight loss amounts to the weight shake it vigorously for 30 seconds, then leave it standing of the water that has evaporated, and is expressed as a until the following day. If the liquid on top of the sand percentage by dividing the weight lost by the weight turns dark brown or coffee-coloured, the sand should of the dried sample. The normal moisture content of not be used. A ‘straw’ colour is satisfactory for most naturally moist sand is 2.5–5.5 percent. The amount of jobs, but not for those requiring the greatest strength water added to the concrete mixture should be reduced or water resistance. Note, however, that some ferrous by the same percentage. compounds may react with the sodium hydroxide to Density is the weight per volume of the solid mass, cause the brown colour. excluding voids, and is determined by placing 1  kilo Grading of the aggregate refers to proportioning of of dry aggregate in 1  litre of water. The density is the the different sizes of the aggregate material and greatly weight of the dry aggregate (1 kg) divided by the volume influences the quality, permeability and workability of of water forced out of place. Normal values for density the concrete. With a well-graded aggregate, the various of aggregate (sand and stone) are 2  600–2  700  kg/m3 particle sizes intermesh, leaving a minimum volume and, for cement, 3 100 kg/m3. 72 Rural structures in the tropics: design and development Bulk density is the weight per volume of the influenced by the grading, shape and texture of the aggregate, including voids, and is determined by aggregate. weighing 1  litre of the aggregate. Normal values for Workability describes the ease with which the coarse aggregate are 1  500–1  650  kg/m3. Although concrete mix can be compacted. Workability can be completely dry and very wet sand have the same increased by adding water to a given mixing ratio since volume, the bulking characteristic of damp sand gives this will increase the water–cement ratio and thereby it greater volume. The bulk density of typical naturally reduce the strength. Instead it should be obtained moist sand is 15–25 percent lower than coarse aggregate by use of a well-graded aggregate (adjustment of the of the same material, i.e. 1 300–1 500 kg/m3. relative proportions of sand and stone), use of smooth The size and texture of aggregate affects the concrete. and rounded rather than irregular shaped aggregate or The larger particles of coarse aggregate should not by decreasing the aggregate–cement ratio. exceed one-quarter of the minimum thickness of the Batching measuring is done by weight or by volume. concrete member being cast. In reinforced concrete, Batching by weight is more exact but is only used at the coarse aggregate must be able to pass between the large construction sites. Batching by volume is used reinforcement bars, 20 mm being generally regarded as when constructing farm buildings. Accurate batching is the maximum size. more important for higher grades of concrete. Batching While aggregate with a larger surface area and rough by weight is recommended for concrete of grade C30 texture, i.e. crushed stone, allows greater adhesive and higher. Checking the bulk density of the aggregate forces to develop, it will give less workable concrete. will result in greater accuracy when grade  C20 or Stockpiles of aggregate should be situated close to the higher is batched by volume. A 50 kg bag of cement can mixing place. Sand and stone should be kept separate. If be split into halves by cutting across the middle of the a hard surface is not available, the bottom of the pile top side of a bag lying flat on the floor. The bag is then should not be used in order to avoid defilement with grabbed at the middle and lifted so that the bag splits soil. In hot, sunny climates, a shade should be provided, into two halves. or the aggregate should be sprinkled with water to cool A bucket or box can be used as a measuring unit. it. Hot aggregate materials make poor concrete. The materials should be placed loosely in the measuring unit and not compacted. It is convenient to construct Water a cubic box with 335  mm sides, as it will contain Water should be reasonably free of impurities such 37  litres, which is the volume of 1  bag of cement. If as suspended solids, organic matter and salts. This the box is made without a bottom and placed on the requirement is usually satisfied by using water which is mixing platform while being filled, it is easy to empty fit for drinking. Sea water can be used if fresh water is not by simply lifting it. The ingredients should never be available, but not for reinforced concrete, as the strength measured with a shovel or spade. of the concrete will be reduced by up to 15 percent. Batching 40 The concrete mix should contain enough sand to fill all the voids between the coarse aggregate, enough 30 cement paste to cover all particles with a complete film, Increasing workability and enough water to complete the chemical reaction. 20 Requirements for batching ordinary concrete mixes of various grades and workability are given in Appendix 10 V: 1-2. The water–cement ratio is an expression for the 0 0∙4 0∙5 0∙6 0∙7 0∙8 0∙9 1∙0 1∙1 relative proportions of water, including the moisture in the damp aggregate, and cement in the cement paste. Water / cement ratio The strongest concrete is obtained with the lowest water-cement ratio which gives a workable mix that can Figure 5.19 Relation between compressive strength be thoroughly compacted. Note that every 1 percent and water/cement ratio of water in excess of what is needed will reduce the strength by up to 5 percent. Water–cement ratio should however, not be below 0.4:1 since this is the minimum Calculating of the amount of ingredients is done required to hydrate the cement. from the number of cubic metres of concrete required. The aggregate–cement ratio will influence on the The sum of the ingredient volumes will be greater concrete price since the amount of cement used per than the volume of concrete, because the sand will fill cubic meter will be changed. It is not possible to give the voids between the coarse aggregate. The volume of a specific relationship between water-cement ratio, the materials will normally be 30–50  percent greater aggregate-cement ratio and workability, since it is than in the concrete mix  – 5  percent to 10  percent is Compressive strength N/mm2 Chapter 5 – Construction materials 73 TABLE 5.13 Requirements per cubic metre for batching nominal concrete mixes naturally moist aggregate1 Aggregate: Sand to total Cement Sand Stones cement aggregate Proportions (number of by volume 50 kg bags) (m³) (tonnes) (m³) (tonnes) (ratio) (%) 1:4:8 3.1 0.46 0.67 0.92 1.48 13.4 31 1:4:6 3.7 0.54 0.79 0.81 1.30 11.0 37 1 5:5 3.7 0.69 1.00 0.69 1.10 10.9 47 1:3:6 4.0 0.44 0.64 0.89 1.42 10.0 31 1:4:5 4.0 0.60 0.87 0.75 1.20 9.9 41 1:3:5 4.4 0.49 0.71 0.82 1.31 8.9 35 1:4:4 4.5 0.66 0.96 0.66 1.06 8.7 47 1:3:4 5.0 0.56 0.81 0.74 1.19 7.7 40 1:4:3 5.1 0.75 1.09 0.57 0.91 7.6 54 1:2:4 5.7 0.42 0.62 0.85 1.36 6.7 31 1:3:3 5.8 0.65 0.94 0.65 1.03 6.5 47 1:2:3 6.7 0.50 0.72 0.74 1.19 5.5 37 1:1:5:3 7.3 0.41 0.59 0.82 1.30 5.0 31 1:2:2 8.1 0.60 0.87 0.60 0.96 4.4 47 1:1:5:2 9.0 0.50 0.72 0.67 1.06 3.9 40 1:1:2 10.1 0.37 0.54 0.75 1.19 3.3 31 1 These quantities are calculated on the assumption that sand has a bulk density of 1 450 kg/m³ and stone has a density of 1 600 kg/m³. The density of the aggregate material is 2 650 kg/m³. allowed for waste and spillage. Adding cement does not Stone = (2.84 × 6) / 9 = 1.89 m³ noticeably increase the volume. The above assumptions are used in Example  5.2 to estimate roughly the Number of bags of cement required = amount of ingredients needed. Example  5.3 gives a 320 / 37 = 8.6 bags, i.e. 9 bags have to be bought. more accurate method of calculating the amount of concrete obtained from the ingredients. Weight of sand required = 0.95 m³ × 1.45 tonnes / m³ = 1.4 tonnes Example 5.2 Calculate the amount of materials needed to construct Weight of stone required = a rectangular concrete floor measuring 7.5 m by 4.0 m 1.89 m³ × 1.60 tonnes / m³ = 3.024 tonnes and 7  cm thick. Use a nominal mix of 1:3:6. Fifty kilograms of cement is equal to 37 litres. Maximum size of stones = 70 mm × 1 / 4 = 17.5 mm Total volume of concrete required = Example 5.3 7.5 m × 4.0 m × 0.07 m = 2.1 m³ Assume a 1:3:5 cement-sand-stone concrete mix by volume, using naturally moist aggregates and adding Total volume of ingredients, assuming 30  percent 62  litres of water. What will be the basic strength and decrease in volume when mixed and 5 percent waste = volume of the mix if 2 bags of cement are used? 2.1 m³ + 2.1 (30% + 5%) m³ = 2.84 m³ Additional assumptions: The volume of the ingredients is proportional to the Moisture content of sand: 4% number of parts in the nominal mix. In this case, there Moisture content of stones: 1.5% is a total of 10 parts (1+3+6) in the mix, but the cement Bulk density of the sand: 1 400 kg/m³ does not affect the volume, so only the 9 parts of sand Bulk density of the stones: 1 600 kg/m³ and stone are used. Solid density of aggregate materials: 2 650 kg/m³ Solid density of cement: 3 100 kg/m³ Cement = (2.84 × 1) / 9 = 0.32 m³ or 320 Density of water: 1 000 kg/m³ Sand = (2.84 × 3) / 9 = 0.95 m³ 74 Rural structures in the tropics: design and development 1. Calculate the volume of the aggregate in the mix. Total = 0.448 m³ Two bags of cement have a volume of The total volume of 1:3:5 mix obtained from 2 bags 2 × 37 litres = 74 litres of cement is 0.45 m³. The volume of sand is 3 × 74 litres = 222 litres Note that the 0.45 m³ of concrete is only two-thirds of the sum of the volumes of the components - 0.074 The volume of stones is 5 × 74 litres = 370 litres + 0.222 + 0.370. 2. Calculate the weight of the aggregates. Mixing Mechanical mixing is the best way of mixing concrete. Sand 222 / 1 000 m³ × 1 400 kg/m³ = 311 kg Batch mixers with a tilting drum for use on building sites are available in sizes of 85–400 litres. Power for drum Stones 370 / 1 000 m³ × 1 600 kg/m³ = 592 kg rotation is supplied by a petrol engine or an electric motor, whereas the drum is tilted manually. The pear- 3. Calculate the amount of water contained in the shaped drum has internal blades for efficient mixing. aggregate. Mixing should be continued for at least 2.5  minutes after all the ingredients have been added. For small- Water in the sand is 311 kg × 4 / 100= 12 kg scale work in rural areas it may be difficult and rather expensive to use a mechanical mixer. Water in the stones is 592 kg × 1.5 / 100= 9 kg 4. Adjust amounts in the batch for water content in TABLE 5.14 aggregate. Mixing-water requirements1 for dense concrete for different consistencies and maximum sizes of aggregate Cement 100 kg (unaltered) Maximum Water requirement (litres / m³) for concrete size of aggregate2 1/ 1 2–1/3 /3–1/ 1 6 /6–1/2 Sand 311 kg – 12 kg = 299 kg High Medium Plastic workability workability consistency Stones 592 kg – 9 kg = 583 kg 10 mm 245 230 210 14 mm 230 215 200 Total amount of dry aggregate = 20 mm 215 200 185 299 kg + 583 kg = 882 kg 25 mm 200 190 175 Water = 62 kg + 12 kg + 9 kg = 83 kg 40 mm 185 175 160 1 Includes moisture in aggregate. The quantities of mixing water 5. Calculate the water–cement ratio and the are the maximum for use with reasonably well-graded, well- shaped, angular, coarse aggregate. cement–aggregate ratio 2 For slump see Table 5.15. Water–cement ratio = (83 kg water) / 100 kg cement = 0.83 Aggregate–cement ratio = (882 kg aggregate) / 100 kg cement = 8.8 The water–cement ratio indicates that the mix has a basic strength corresponding to a C10  mix. See Appendix V: 12. 6. Calculate the ‘solid volume’ of the ingredients in the mix, excluding the air voids in the aggregate and cement. Cement 100 kg / 3 100 kg/m³ = 0.032 m³ Aggregate 882 kg / 2 650 kg/m³ = 0.333 m³ Water 83 kg / 1 000 kg / m³ = 0.083 m³ Figure 5.20 Batch mixer Chapter 5 – Construction materials 75 A simple hand-powered concrete mixer can be All tools and the platform should be cleaned with manufactured from an empty oil drum set in a frame water when there is a break in the mixing, and at the of galvanized pipe. Figure  5.21 shows a hand crank, end of the day. but the drive can be converted easily to machine power. Slump test The slump test gives an approximate indication of the workability of the wet concrete mix. Fill a conically shaped bucket with the wet concrete mix and compact it thoroughly. Turn the bucket upside down on the mixing platform. Lift the bucket, place it next to the concrete heap and measure the slump, as shown in Figure 5.22. Placing and compaction Concrete should be placed with a minimum of delay after the mixing is completed, and certainly within 30 minutes. Special care should be taken when Figure 5.21 Home-built concrete mixer transporting wet mixes, because the vibrations of a moving wheelbarrow may cause the mix to segregate. The mix should not be allowed to flow, nor should it Hand mixing is normally used for small jobs. be dropped into position from a height of more than Mixing should be done on a close-boarded platform 1  metre. The concrete should be placed with a shovel or a concrete floor near to where the concrete is to be in layers no deeper than 15 cm, and compacted before placed, and never on bare ground because of the danger the next layer is placed. of earth contamination. When slabs are cast, the surface is levelled with The following method is recommended for hand a screed board, which is also used to compact the mixing: concrete mix as soon as it has been placed, to remove 1. The measured quantities of sand and cement are any trapped air. The less workable the mix, the more mixed by turning them over with a shovel at least porous it is, and the more compaction is necessary. 3 times. The concrete loses up to 5  percent of its strength for 2. About three-quarters of the water is added to the every 1  percent of entrapped air. However, excessive mixture a little at a time. compaction of wet mixes brings fine particles to the 3. Mixing continues until the mixture becomes top, resulting in a weak, dusty surface. homogeneous and workable. Manual compaction is commonly used for the 4. The measured quantity of stones, after being construction of farm buildings. It can be used for mixes wetted with part of the remaining water, is with high and medium workability, and for plastic spread over the mixture and mixing continues, mixes. Wet mixes used for walls are compacted by with all ingredients being turned over at least punting with a batten, stick or piece of reinforcement 3 times during the process, using as little water as bar. Knocking on the formwork also helps. Less possible to obtain a workable mix. workable mixes, such as those used for floors and paving, are best compacted with a tamper. 100 Slump No slump Collapse slump 200 Bucket Too wet suitable Too dry Figure 5.22 Concrete slump test 300 76 Rural structures in the tropics: design and development TABLE 5.15 Concrete slump for various uses Consistency Slump use Method of compaction High workability 1/2–1/3 Constructions with narrow passages and/or complex Manual shapes. Heavily reinforced concrete. Medium workability 1/3–1/6 All normal uses. Non-reinforced and normally Manual reinforced concrete. Plastic 1/6–1/12 Open structures with fairly open reinforcement, which Manual or mechanical are heavily worked manually for compaction, such as floors and paving. Mass concrete. Stiff 0–1/2 Non-reinforced or sparsely reinforced open structures, Mechanical such as floors and paving, which are mechanically vibrated. Factory prefabrication of concrete goods. Concrete blocks. Damp 0 Factory prefabrication of concrete goods. Mechanical or pressure should be straight, either vertical or horizontal. When resuming work, the old surface should be roughened and cleaned before being treated with a thick mixture of water and cement. Formwork Formwork provides the shape and surface texture of concrete members and supports the concrete during setting and hardening. Figure 5.23 Manual compaction of foundation and The simplest type of form is sufficient for pavement floor slab edges, floor slabs, pathways, etc. Only mechanical vibrators are capable of compacting Expansion joint stiffer mixes thoroughly. For walls and foundations, a Screed board poker vibrator (a vibrating pole) is immersed in the poured concrete mix at points up to 50 cm apart. Floors Stop board and paving are vibrated with a beam vibrator. Stake Side boards thickness: 38−50 mm. width according to thickness of slab Figure 5.25 Simple type of formwork for a concrete slab In large concrete slabs, such as floors, cracks tend to occur early in the setting period. In a normal slab where watertightness is not essential, this can be controlled by laying the concrete in squares, with joints between them, allowing the concrete to move slightly without Figure 5.24 Mechanical vibrators causing cracks in the slab. The distance between the joints should not exceed 3  metres. The simplest type is a called a dry joint. The concrete is poured directly Construction joints against the already hardened concrete of another square. The casting should be planned in such a way that the A more sophisticated method is a filled joint. A work on a member can be completed before the end of minimum gap of 3 mm is left between the squares, and the day. If cast concrete is left for more than 2 hours, filled with bitumen or any comparable material. it will set so much that there is no direct continuation Forms for walls must be strongly supported because between the old and new concrete. Joints are potentially when concrete is wet it exerts great pressure on the weak and should be positioned where they will affect side boards. The greater the height, the greater the the strength of the member as little as possible. Joints pressure. A concrete wall will not normally be thinner Chapter 5 – Construction materials 77 than 10 cm, or 15 cm in the case of reinforced concrete. Although the formwork can be taken away after If it is higher than 1 metre, it should not be less than 3 days, leaving it for 7 days makes it easier to keep the 20 cm thick, to make it possible to compact the concrete concrete wet. properly with a tamper. The joints of the formwork In order to save on material for the formwork and must be tight enough to prevent loss of water and its supporting structure, tall silos and columns are cement. cast using a slip form. The form is not built to the full If the surface of the finished wall is to be visible, height of the silo, and may in fact be only a few metres and no further treatment is anticipated, tongued and high. As casting of the concrete proceeds, the form grooved boards, planed on the inside, can be used to is lifted. The work needs to proceed at a speed that provide a smooth and attractive surface. Alternatively, allows the concrete to set before it leaves the bottom of 12-mm plywood sheets can be used. The dimensions and the form. This technique requires complicated design spacing of studs and ties are shown in Figure 5.26. The calculations, skilled labour and supervision. proper spacing and installation of the ties is important to prevent distortion or complete failure of the forms. Curing concrete Not only must forms be well braced, they must also Concrete will set in 3 days, but the chemical reaction be anchored securely to prevent them from floating up, between water and cement continues for much longer. If allowing the concrete to run out from underneath. the water disappears through evaporation, the chemical The forms should be brushed with oil and watered reaction will stop. It is therefore very important to keep thoroughly before filling with concrete. This is done the concrete wet (damp) for at least 7 days. to prevent water in the concrete from being absorbed Premature drying out may also result in cracking by the wooden boards, and to stop the concrete from caused by shrinkage. During curing, the strength and sticking to the forms. Although soluble oil is best, used impermeability increases and the surface hardens against engine oil mixed with equal parts of diesel fuel is the abrasion. Watering of the concrete should start as soon as easiest and cheapest material in practice. the surface is hard enough to avoid damage, but not later If handled carefully, wooden forms can be used than 10–12 hours after casting. Covering the concrete with several times before they are abandoned. If there is a sacks, grass, hessian, a layer of sand or polythene helps to recurrent need for the same shape, it is advantageous to retain the moisture and protects the surface from dry make the forms of steel sheets. winds. This is particularly important in tropical climates. Wooden spacers length=wall thickness to be removed when filling concrete Studs, 50x100 mm 8 G wire ties vertical max. spacing 600 mm spacing 500 mm Support max. spacing 2 m Boards min. thickness 25 mm nailed to studs Figure 5.26 dimensions and spacing of studs and ties in formwork for walls 78 Rural structures in the tropics: design and development Temperature is also an important factor in curing. Reinforced concrete For temperatures above 0 °C and below 40 °C, strength Concrete is strong in compression but relatively weak development is a function of temperature and time. At in tension. The underside of a loaded beam, such as a temperatures above 40 °C, the stiffening and hardening lintel over a door, is in tension. may be faster than desired and result in lower strength. Figure  5.27 shows the approximate curing time needed to achieve characteristic compressive strength at various curing temperatures for concrete mixes using ordinary portland cement. 80 70 60 50 40 30 COMPRESSION TENSION 20 10 0 5 10 15 20 30 40 50 T °C Figure 5.28 Stresses in a concrete lintel Figure 5.27 Curing times for concrete Concrete subject to tension loading must be reinforced with steel bars or mesh. The amount and Finishes on concrete type of reinforcement should be carefully calculated or, The surface of newly laid concrete should not be alternatively, a standard design obtained from a reliable worked until some setting has taken place. The type of source should be followed without deviating from the finish should be compatible with the intended use. In design. the case of a floor, a non-skid surface for humans and Important factors affecting reinforced concrete: animals is desirable. 1. The steel bars should be cleaned of rust and dirt Tamped finish: The tamper leaves a coarse, rippled before they are placed. surface when it has been used to compact the concrete. 2. In order to obtain good adhesion between the Tamper-drawn finish: A less pronounced ripple can concrete and the steel bars, the bars should be be produced by moving a slightly tilted tamper on its overlapped where they join by at least 40  times tail end over the surface. the diameter. When plain bars are used, the ends Broomed finish: A broom of medium stiffness is of the bars must be hooked. drawn over the freshly tamped surface to give a fairly 3. The reinforcement bars should be tied together rough texture. well and supported so that they will not move Wood-float finish: For a smooth, sandy texture the when concrete is placed and compacted. concrete can be wood-floated after tamping. The float is 4. The steel bars must be in the tensile zone and be used with a semicircular sweeping motion, the leading covered with concrete to a thickness of 3  times edge being slightly raised; this levels out the ripples and the diameter, or by at least 25  mm, to protect produces a surface with a fine, gritty texture, a finish them from water and air, which causes rusting. often used for floors in animal houses. 5. The concrete must be well compacted around Steel trowel finish: Steel trowelling after wood the bars. floating gives a smoother surface with very good 6. Concrete should be at least C20 or 1:2:4 nominal wearing qualities. However, it can be slippery in wet mix, and have a maximum aggregate size of 20 mm. conditions. Surfaces with the aggregate exposed can be used Concrete floors are sometimes reinforced with for decorative purposes, but can also give a rough, welded steel mesh or chicken wire, placed 25  mm durable surface on horizontal slabs. This surface can beneath the upper surface of the concrete, to limit the be obtained by removing cement and sand by spraying size of any cracking. However, such load-distributing water on the new concrete, or by positioning aggregate reinforcement is necessary only when loadings are by hand in the unset concrete. heavy, the underlying soil is not dependable, or when cracking must be minimized, as in water tanks. Days Chapter 5 – Construction materials 79 Rods tied with wire Rods should be lapped 40 times diameter Temporary blocks Figure 5.30 Wooden mould for solid concrete blocks Distance to surface should be at least 3 times diameter The mould illustrated in Figure 5.31 has a steel plate Figure 5.29 Placing reinforcement bars cut to the shape of the block, which is used as a lid and held down as the hollow-making pieces are withdrawn. Bolts are then loosened and the sides of the mould are COnCRETE BLOCkS, SAnd And CEMEnT BLOCkS removed with a swift motion. All parts of the mould It is faster to build with concrete blocks than with should be slightly tapered so that they can be removed bricks, and using concrete blocks reduces the mortar easily from the block. requirement by half or more. If face-shell bedding is As from the day after the blocks have been made, used, in which the mortar is placed only along the edges water is sprinkled on the blocks for 2  weeks during of the blocks, the consumption of mortar is reduced curing. After 48 hours, the blocks can be removed for by a further 50 percent. However, the total amount of stacking, but wetting must continue. After curing, the cement required for the blocks and mortar is far greater blocks are dried. If damp blocks are placed in a wall, than that required for the mortar in a brick wall. they will shrink and cause cracks. To ensure maximum Concrete blocks are often made of 1:3:6 concrete drying, the blocks are stacked interspaced, exposed to with a maximum aggregate size of 10 mm, or a cement- the prevailing wind and, in the case of hollow blocks, sand mixture with a ratio of 1:7, 1:8 or 1:9. If properly with the cavities laid horizontal to form a continuous cured, these mixtures produce concrete blocks with passage for the circulating air. compression strength well above what is required in a one-storey building. The blocks may be solid, cellular decorative and ventilating blocks or hollow. Cellular blocks have cavities with one end Decorative concrete or sand-cement blocks serve closed, while in hollow blocks the cavities pass through. several purposes: Lightweight aggregate, such as cracked pumice stone, is • to provide light and security without installing sometimes used. windows or shutters; Blocks are made to a number of coordinating sizes, • to provide permanent ventilation; the actual sizes being about 10  mm less in order to • to give an attractive appearance. allow for the thickness of the mortar. In addition, some are designed to keep out rain, Block manufacturing while others include mosquito proofing. Blocks can be made using a simple block-making While blocks with a simple shape can be made in a machine driven by an engine, or operated by hand. wooden mould by inserting pieces of wood to obtain They can also be made using simple wooden moulds on the desired shape, more complicated designs usually a platform or floor. The mould can be lined with steel require a professionally made steel mould. plates, to prevent damage during tamping and to reduce wear on the mould. Steel moulds are often used in large-scale production. The wooden mould is initially MORTAR oiled overnight and need not be oiled each time it is Mortar is a plastic mixture of water and binding filled. It is sufficient to wipe it clean with a cloth. The materials, used to join concrete blocks, bricks or other concrete, with a stiff or plastic consistency, is placed in masonry units. the mould in layers, and each layer is compacted with It is desirable for mortar to hold moisture, be plastic a 3-kg rammer. enough to stick to the trowel and the blocks or bricks, The mould in Figure 5.30 has a lid made so that it and to develop adequate strength without cracking. can pass through the rest of the mould. The slightly Mortar need not be stronger than the units it joins. tapered sides can be removed by lifting the handles, In fact, cracks are more likely to appear in the blocks or while holding down the lid with one foot. bricks if the mortar is excessively strong. 80 Rural structures in the tropics: design and development Mould made of 25 mm timber 460 140 390 80 90 Concrete block 55 75 140 Figure 5.31 Mould for hollow or cellular concrete blocks There are several types of mortar, each suitable for Cement mortar is stronger and more waterproof particular applications and varying in cost. Most of than lime mortar, but it is difficult to work with because these mortars include sand as an ingredient. In all cases, it is not ‘fat’ or plastic and falls away from the blocks the sand should be clean, free of organic material, well or bricks during placement. In addition, cement mortar graded (a variety of sizes) and not exceed 3 mm of silt is more costly than other types. Consequently, it is in the sedimentation test. In most cases, particle size used in only a few applications, such as a damp-proof should not exceed 3 mm, as this would make the mortar course or in some limited areas where heavy loads are ‘harsh’ and difficult to work with. expected. A 1:3 mix using fine sand is usually required Lime mortar is typically mixed using 1 part lime to to obtain adequate plasticity. 3 parts sand. Two types of lime are available. Hydraulic Compo mortar is made with cement, lime and sand. lime hardens quickly and should be used within an In some localities, a 50:50 cement-lime mix is sold as hour. It is suitable for both above- and below-ground mortar cement. The addition of the lime reduces the cost applications. Non-hydraulic lime requires air to harden, and improves workability. A 1:2:9, cement-lime-sand and can only be used above ground. If it is smoothed mix is suitable for general purposes, while a 1:1:6 is better off while standing, a pile of this type of lime mortar can for exposed surfaces, and a 1:3:12 can be used for interior be stored for several days. walls, or stone walls, where the extra plasticity is helpful. Figure 5.32 Ventilating and decorative concrete blocks 190 190 Chapter 5 – Construction materials 81 Mortar can also be made using pozzolana, bitumen, Cement plaster can be used on most types of wall, cutback or soil. A 1:2:9 lime-pozzolana-sand mortar is but it does not adhere well to soil-block walls, as roughly equivalent to a 1:6 cement-sand mortar. Adobe shrinking and swelling tend to crack the plaster. The and stabilized soil blocks are often laid in a mortar of mixing ratio is 1 part cement and 5 parts sand and, if the the same composition as the blocks. plaster is too harsh, 0.5–1 part of lime can be added. The Tables  5.16 and  5.17 provide information on the wall is first moistened and then the plaster is applied materials required for a cubic metre of various mortars, in two coats of about 5  mm each, allowing at least and the amount of mortar per square metre, for several 24 hours between layers. Cement plaster should not be building units. applied on a wall while it is exposed to the sun. Starting with cement mortar, strength decreases Dagga plaster is a mixture of clay soil (such as red with each type, although the ability to accommodate or brown laterite), stabilizer and water. The plaster movement increases. is improved by adding lime or cement as a stabilizer and bitumen for waterproofing. A good mixture is 1  part lime or cement, 3  parts clay, 6  parts sand, TABLE 5.16 0.2  parts bitumen and water. Dagga plaster is applied Materials required per cubic metre of mortar on previously moistened earth or adobe brick walls in a Cement Lime Sand layer 10–25 mm thick. Type bags (kg) (m³) Cement mortar 1:5 6.0 - 1.1 FERROCEMEnT Compo mortar 1:1:6 5.0 100.0 1.1 Ferrocement is a highly versatile form of reinforced Compo mortar 1:2:9 3.3 13.5 1.1 concrete made with closely spaced light reinforcing Compo mortar 1:8 3.7 - 1.1 rods or wire mesh, and a cement and sand mortar. It can be worked with relatively unskilled labour. Compo mortar 1:3:12 2.5 150.0 1.1 The function of the wire mesh and reinforcing rods Lime mortar 1:3 - 200.0 1.1 is first to act as a lath, providing the form to support the mortar in its plastic state, while, in the hardened state, it absorbs the tensile stresses in the structure, which the TABLE 5.17 mortar alone is not able to withstand. Mortar required for various types of wall The reinforcing can be assembled in any desired Type of wall Amount required per m² wall shape, and the mortar applied in layers to both sides. 11.5 cm brick wall 0.25 m³ Simple shapes, such as water tanks, can be assembled 22.2 cm brick wall 0.51 m³ using wooden sticks as support for the reinforcing 10 cm sand-cement block wall 0.008 m³ while the first coat of mortar is applied. The mortar should have a mixing ratio of 1:2  to 15 cm sand-cement block wall 0.011 m³ 1:4  cement/sand by volume, using the richer mix for 20 cm sand-cement block wall 0.015 m³ the thinnest structures. The water/cement ratio should be below 0.5/1.0. Lime can be added in the proportion 1  part lime to 5  parts cement in order to improve Finishing mortar workability. This is sometimes used on floors and other surfaces to The mechanical behaviour of ferrocement depends give a smooth finish, or as an extremely hard coating to on the type, quantity, orientation and strength of the increase resistance to wear. While such a top coating is mesh and reinforcing rods. The most common types of prone to cracking, it seldom increases strength, and is mesh used are illustrated in Figure 5.33. difficult to apply without causing loose or weak parts. Standard galvanized mesh (galvanized after weaving) Concrete floors can normally be cast to the finished is adequate. Although non-galvanized wire has adequate level directly, and be given a sufficiently smooth and strength, the problem of rusting limits its use. hard surface without a top coating. A construction similar to ferrocement has recently For coating, a mix of 1  part cement and 2–4  parts been developed for small water tanks, sheds, huts, etc. sand is used. The coating is placed in a 1–2  cm thick It consists of welded 150  mm-square reinforcement layer with a steel trowel. Before application, the surface mesh (6 mm rods), covered with hessian and plastered of the underlying concrete slab should be cleaned and in the same way as ferrocement. moistened. FIBRE-REInFORCEd COnCRETE Plastering and rendering Fibre-reinforced concrete members can be made thinner The term ‘plastering’ is usually applied to interior than those with conventional reinforcement because walls and ceilings to give jointless, hygienic and usually there is no need for a corrosion-protection covering smooth surfaces, often over uneven backgrounds. over the steel bars. The fibres improve flexible strength Exterior plastering is usually called ‘exterior rendering’. and resistance to cracking. 82 Rural structures in the tropics: design and development Commonly used fibres include asbestos, steel Asbestos cement (AC) (0.25 mm diameter), sisal and elephant grass. Asbestos, which is a silicate of magnesium, is found as a rock that can be split into extremely thin fibres ranging from 2 mm to 900 mm long. These have good resistance to alkalis, neutral salts and organic solvents, and the varieties used for building products have good resistance to acids. Asbestos is non-combustible and able to withstand high temperatures without alteration. Inhalation of asbestos dust causes asbestosis (a disease of the lungs) and asbestos is now used only where no alternative fibre is available. Workers must wear masks and take great care not to inhale any asbestos dust! As the fibres are strong in tension and flexible, they are used as reinforcement with Portland cement, lime a. Hexagonal wire mesh (chicken wire mesh) and bitumen binders, in asbestos-cement and asbestos- silica-lime products, vinyl floor tiles and in bitumen felts. Asbestos-cement is used in farm structures for corrugated roofing sheets, ridges and sanitary pipes. Sisal-fibre-reinforced cement (SFRC) Sisal and other vegetable fibres have only recently come into use for cement reinforcement. Sisal fibre can be used as short, discontinuous fibres (15–75  mm in length), or as continuous long fibres exceeding 75 mm in length. Sometimes both short and long fibres are used together. The manner in which the fibres are incorporated into the matrix affects the b. Welded wire mesh - strongest properties of the composite, both in the fresh state and in the hardened state. Sisal fibres may deteriorate if not treated. Although the alkalinity of the concrete helps to protect the fibres from outside attack, it may itself attack the fibres chemically by decomposing the lignin. Sisal fibre reinforcing is used with various cement- sand mixing ratios, depending on the use: Wall plastering 1:3 Guttering 1:2 Roofing tiles 1:1 Corrugated roofing sheets 1:0.5 c. Woven mesh - strong The sand should be passed through a sieve with 1.5  mm to 2  mm holes (e.g. mosquito netting). The mixing water must be pure and the mix kept as dry as possible, while still being workable. Between 16 grams and 17 grams of short (25 mm), dry sisal fibres are added to the mix for each kilogram of cement. The short fibres are mixed into the dry cement and sand before adding water. As sisal fibres have a high water-absorption capacity, some extra water may have to be added to the mix to compensate for this. When mixing, there is a tendency for the fibres to ball and separate out from the rest of the mix. This tendency will increase with longer fibres but, if fibres d. Expanded wire mesh shorter than 25 mm are used, the reinforcing effect will be reduced. In most cases, the mix is then trowelled Figure 5.33 Reinforcement meshes for ferrocement onto a mesh of full-length sisal fibres. Chapter 5 – Construction materials 83 Making corrugated reinforced roofing sheets with mortar and another mat made from the Home-made reinforced corrugated roofing is usually remaining two bundles. Finally, all the sisal is cast to standard width, but to only 1  metre in length covered with the remaining mortar, and the because of its additional weight. Commercial asbestos- surface is screeded even with the edge strips on cement roofing is heavier than corrugated steel, and the the plywood. home-made sheets are still heavier. Special attention 4. Cover with the top sheet of polythene, ensuring must therefore be given to rafter or truss sizes to ensure that the mortar is of even thickness all over and a safe structure. that no air bubbles remain under the polythene. Although the casting procedure for sisal-fibre- 5. While holding the batten strip at the fold in the reinforced cement is tricky, once the proper equipment polythene, carefully remove the plywood sheet has been assembled and several sheets have been made, to allow the new sisal-cement sheet to fall onto the process becomes much easier. the asbestos-cement sheet. At the same time, A concrete block cast over a 1-metre length of press the new sheet into the corrugations using a asbestos-cement roofing is needed as a face for casting PVC drain pipe 90 mm in diameter. Compact the the roof sheets. The block is cast within a 100 mm-high new sheet by placing another asbestos sheet on form, which will give a block of sufficient strength top, and treading on it. Holes for mounting are after a few days curing. Two or more 1-metre lengths punched with a 5 mm dowel 25 mm from the end of asbestos-cement roofing will be needed, as well in the gulleys (crests when mounted on the roof) as a piece of 18  mm plywood, measuring 1.2  metres of the fresh sheet. by 1.2  metres, and a sheet of heavy-duty polythene, 6. Remove from the moulding block the asbestos measuring 2.25  metres long and 1  metre wide. The sheet bearing the sisal-cement sheet, and leave polythene is folded in the middle and a thin batten, it until the cement in the new sheet has set measuring 9 mm by 15 mm, is stapled at the fold. Strips (preferably 2  days). Then carefully remove the of 9 mm plywood or wood are nailed along two edges new sheet, peel off the polythene and cure of the plywood sheet, leaving exactly 1 metre between the new sheet for at least 1  week, preferably them, as shown in Figure 5.34. immersed in a water tank. Below are the steps to follow in the casting procedure: 7. If more polythene and asbestos-cement sheets 1. Fit an asbestos cement sheet onto the moulding are available, casting can proceed immediately. block and cover with the piece of plywood, with the edge strips at the ends of the sheet. The polythene is placed over the plywood and the top Walls using the sisal-cement plastering sheet is folded back off the plywood. technique 2. Prepare a mix of 9  kg cement, 4.5  kg sand, Soil blocks can be used for inexpensive walls with good 150  grams of short sisal fibres (25  mm) and thermal insulation. However, they are easily damaged 4.5  litres of water. Also prepare four 60-gram by impact and eroded by rain. One way of solving these bundles of sisal fibres that are as long as possible. problems is to plaster the face of the wall. Ordinarily, 3. Use one-third of the mortar mix to trowel a mortar plaster tends to crack and peel off, as it does thin, even layer over the polythene. Take two of not expand at the same rate as the soil. This can be the four sisal bundles and distribute the fibres overcome by letting long sisal fibres pass through the evenly, with the second bundle at right angles to wall, to be incorporated into the mortar on each face. the first, forming a mat of fibres. This is covered The double skin so formed provides sufficient strength Top layer of polythene PVC - Pipe Lower layer of polythene Flat wooden Wire Wooden sticks moulding board Concrete moulding block Figure 5.34 Plywood casting board and polythene ‘envelope’ 84 Rural structures in the tropics: design and development and waterproofing to the wall to enable soil blocks to and promoting good drainage, by avoiding contact be laid without mortar between the blocks to join them. between dissimilar metals, and by using corrosion- inhibiting coatings. Sisal fibres Corrosion-inhibiting coatings (200 m2 wall) Copper, aluminium, stainless steel and cast iron tend to form oxide coatings that provide a considerable amount of self-protection from corrosion. However, most other steels require protective coatings if they are exposed to moisture and air. Methods used include zinc coating (galvanizing), vitreous enamel glazing and painting. Painting is the only practical method for field application, although grease and oil will provide Plaster 1:3 containing temporary protection. 16-17 g short sisal fibres per kg cement Before painting, the metal surface must be clean, dry and free from oil. Both bituminous and oil-based paints Soil block with metallic oxide pigments offer good protection, if they are carefully applied in continuous layers. Two to Figure 5.35 Sisal-cement plastering technique three coats provides the best protection. BuILdInG HARdWARE METALS nails Several ferrous metals (those containing iron) are useful A nail relies on the grip around its shank and the shear in the construction of farm buildings and other rural strength of its cross-section to give strength to a joint. structures. Cast iron is used for making sanitary waste It is important to select the right type and size of nail pipes and fittings. for any particular situation. Nails are specified by their Steel consists of iron, plus a small percentage of type, length and gauge (the higher the gauge number, carbon in chemical combination. High-carbon or ‘hard’ the smaller the shank diameter). See Table  5.18. Most steel is used for tools with cutting edges. Medium- nails are made from mild steel wire. In a corrosive carbon steel is used for structural members such as environment, galvanized, copper-plated, copper or I-beams, reinforcing bars and implement frames. Low- aluminium nails are used. A large number of nail carbon or ‘mild’ steel is used for pipes, nails, screws, types and sizes are available on the market. Below is a wire, screening, fencing and corrugated roof sheets. description of the nails most commonly used in farm Non-ferrous metals, such as aluminium and copper, buildings. are corrosion-resistant and are often chosen for this Round plain-headed nails or round wire nails are quality. Copper is used for electric wire, tubing for used for general carpentry work. As they have a water supply and for flashing. Aluminium is most tendency to split thin members, the following rule is commonly used for corrugated roofing sheets, gutters often used: the diameter of the nail should not exceed and the accompanying nails. Using nails of the same 1/7 of the thickness of the timber. material avoids the problem of corrosion caused by electrolytic action. Brass is a corrosion-resistant alloy of copper and zinc used extensively for building hardware. TABLE 5.18 dimensions and approximate number per kilogram Corrosion of commonly used sizes of round wire nails Air and moisture accelerate corrosion in ferrous Length diameter Approximate materials unless they are protected. Acids tend to (inches) (mm) (mm) number/kg corrode copper, while alkalis, such as that found in 6 150 6.0 29 animal waste, portland cement and lime, as well as in 5 125 5.6 42 some soils, cause rapid corrosion of aluminium and 4 100 4.5 77 zinc. Electrolytic action, caused by slight voltages set up when dissimilar metals are in contact with 3 75 3.75 154 each other in the presence of water, also encourages 2.5 65 3.35 230 corrosion in some metals. Aluminium is particularly 2 50 2.65 440 prone to electrolytic corrosion. 1.5 40 2.0 970 Corrosion can be reduced by the careful selection of 1 25 1.8 1 720 metal products for the application, by reducing the time that the metal will be wet by preventing condensation Chapter 5 – Construction materials 85 Lost-head nails have a smaller head, which can be set inserted by rotation, and not by being driven with a below the surface of the wood. Their holding power is hammer. It is usually necessary to drill a pilot hole lower because the head can be pulled through the wood for the shank of the screw. Screws made of mild steel more easily. are normally preferred because they are stronger. A Panel pins are fine wire nails with small heads, used wide range of finishes, such as galvanized, painted and for fixing plywood and hardboard panels. plated, are available. Clout or slate nails have large heads and are used for Screws are classified according to the shape of their fixing tiles, slates and soft boards. Felt nails have even head, as countersunk, raised, round or recessed (not larger heads. slotted across the full width). Coach screws have a Concrete nails are made from harder steel, which square head and are turned with a spanner. They are allows them to be driven into concrete or masonry work. used for heavy construction work and should have a Staples are U-shaped nails with two points, and are metal washer under the head to prevent damage to the used mainly to fasten wires. wood surface. Screws are sold in boxes containing a Roofing nails have a square, twisted shank and a gross (144 screws), and are specified by their material, washer attached to the head. Roofing felt or rubber finish, type, length and gauge. Unlike the wire gauge may be used under the washer to prevent leakage. The used for nails, the larger the screw-gauge number, the nail and the washer should be galvanized to prevent greater the diameter of the shank. corrosion. They are used for fixing corrugated-sheet Bolts provide even stronger joints than either nails materials and must be long enough to penetrate at least or screws. As the joint is secured by tightening the nut 20  mm into the wood. Alternatively, wire nails with onto the bolt, in most cases the load becomes entirely used bottle caps for washers can be used. a shear force. Bolts are used for heavy loads, such as at the joints in a gantry hoist frame, at the corners of a ring beam installed for earthquake protection, or to secure the hinges of heavy doors. Most bolts used with wood Grid rings have a rounded head, with a square shank just under Shank the head. Only one spanner is required for these ‘coach’ bolts. Square-head bolts, requiring two spanners, are also available. Washers help to prevent the nuts from sinking into the wood. Point ROUND PLAIN HEAD NAIL OVAL WIRE NAIL PANEL PIN Slot Head D Shank RECESSED HEAD OR PHILLIPS SCREW Thread ROUND HEAD RAISED COUNTERSUNK HEAD CLOUT NAIL FELT NAIL Core Point COUNTERSUNK WOODEN PLUG HEAD SCREW The plug in the drilled hole will expand as the screw is driven PLASTIC PLUG STAPLE DUPLEX HEADNAIL Squared Hexagonal Cup for concrete formwork head head head Square neck ROOFING NAIL with felt washer (Different types for iron sheet and Asbestos sheets) WASHER WASHER Figure 5.36 Types of nails Hexagonal Square nut nut LUG or MACHINE BOLT COACH BOLT COACH SCREW Screws and bolts Wood screws have a thread, which gives them greater holding power and resistance to withdrawal than nails, PLAIN WASHER SPRING WASHER and they can be removed easily without damaging the wood. For a screw to function properly, it must be Figure 5.37 Types of wood screws and bolts 25−40 mm 15−200 mm 15−200 mm 15−30 mm 45−100 mm 15−75 mm 50−200 mm 15−75 mm 2/3 L 86 Rural structures in the tropics: design and development TABLE 5.19 Conversion of screw gauge to millimetres Screw gauge 0 1 2 3 4 5 6 7 8 9 10 12 14 16 18 20 24 28 32 Nominal 1.52 1.78 2.08 2.39 2.74 3.10 3.45 3.81 4.17 4.52 4.88 5.59 6.30 7.01 7.72 8.43 9.86 11.28 12.70 diameter of unthreaded shank (mm) Hinges Hinges are classified by their function, length of nap Staple fixed Strinking plate Backplate to frame fixed to frame Lever and the material from which they are made, and come Latch bolt Latch bolt handle in many different types and sizes. Hinges for farm Axle buildings are manufactured mainly from mild steel and Case Case are provided with a corrosion-inhibiting coating. The Forend most common types are described below. Lock bolt Lock bolt The steel butt hinge is commonly used for windows, RIM LOCK MORTICE LOCK BACKPLATE shutters and small doors, as it is cheap and durable. If the pin can be removed from the outside, it is not Locking cylinder burglarproof. The flaps are usually set in recesses in the Spindel door or window and the frame. The H-hinge is similar to the butt hinge but is Bolt Staple usually surface mounted. Turning Staple knob The T-hinge is used mostly for hanging match- RIM LATCH BARREL BOLT FOR PADLOCK boarded doors. For security reasons, the strap of the T-hinge should be fixed to the door with at least one Staple Hasp coach bolt, which cannot be unscrewed easily from the outside. Door Frame The band-and-hook hinge is a stronger type of T-hinge and is used for heavy doors and gates. This type is suitable for fabrication at the site or by the local HASP & STAPLE DOUBLE STAPLE blacksmith. Figure 5.39 Types of locks and latches Tee TEE HINGE Hinge Knuckle GLASS Glass suitable for general window glazing is made 150−600 mainly from soda, lime and silica. The ingredients are heated in a furnace to about 1 500 °C, and fuse together Hook Band in the molten state. Sheets are then formed by a process Flap Pin of drawing, floating or rolling. The ordinary glazing 150−600 quality is manufactured by drawing in thicknesses ranging from 2  mm to 6  mm. It is transparent, with PLATE WITH HOOK BAND & HOOK 90 percent light transmission. As the two surfaces are never perfectly flat or parallel, there is always some Figure 5.38 Types of hinges visual distortion. Plate glass is manufactured with ground and polished surfaces, and should be free of imperfections. Locks and latches Glass in buildings is required to resist loads, Any device used to keep a door in the closed position including wind loads, impacts from people and animals, can be classified as a lock or latch. A lock is activated and sometimes thermal and other stresses. Generally by means of a key, whereas a latch is operated by a lever the thickness increases with the area of the glass pane. or bar. Locks can be obtained with a latch bolt so that Glass is elastic right up to its breaking point but, as it the door can be kept in a closed position without using is also completely brittle, there is no permanent set or the key. Locks in doors are usually fixed at a height of warning of impending failure. The support provided 1 050 mm. Some examples of common locks and latches for glass will affect its strength performance. Glass used in farm buildings are illustrated in Figure 5.39. should be cut to give a minimum clearance of 2 mm all around the frame to allow for thermal movements. 140−250 75−140 25−100 Chapter 5 – Construction materials 87 PLASTICS In most cases, epoxy resins are provided in two parts: Plastics are among the newest building materials, a resin and a curing agent. They are extremely tough ranging from materials strong enough to replace metal, and stable, and adhere well to most materials. Silicone to foam-like products. Plastics are considered to be resins are water-repellent and used for waterproofing mainly organic materials derived from petroleum and, in masonry. Note that fluid plastics can be very toxic. to a small extent, coal, which at some stage in their processing are plastic when heated. Plastics used for seepage protection in dams The range of properties is so great that generalizations Seepage from dams is a common problem. Occasionally, are difficult to make. However, plastics are usually light site conditions and the lack of local clay may require in weight and have a good strength-to-weight ratio, but the use of synthetic liners, also called geomembranes, rigidity is lower than that of virtually all other building to line the dam and overcome the problem of seepage. materials, and creep is high. If the correct product is selected and good installation Plastics have low thermal conductivity and thermal procedures are used, only normal maintenance will be capacity, but thermal movement is high. They resist a needed during the service life of the dam. wide range of chemicals and do not corrode, but they Three types of lining material are available: low- tend to become brittle with age. density plastic sheeting; woven polyethylene fabric; Most plastics are combustible and may release and high density polyethylene (HDPE) sheeting. All poisonous gases in a fire. Some are highly flammable, of these products are susceptible to degradation by while others are difficult to burn. introduced chemicals. The most suitable product for Plastics lend themselves to a wide range of a specific job depends on a number of factors. These manufacturing techniques, and products are available include: site conditions, cost, resistance to sunlight, in many forms, both solid and cellular, from soft and strength, resistance to puncturing and the method of flexible to rigid, and from transparent to opaque. joining. Various textures and colours are available (many of which fade if used outdoors). Plastics are classified as: Low-density plastic sheeting • Thermoplastics, which always soften when heated Commonly referred to as ‘builder’s plastic’, these sheets and harden again on cooling, provided they are are normally black or orange in colour. Low-density not overheated. plastic sheeting is manufactured in various thicknesses, • Thermosetting plastics, which undergo an with the recommended thickness being 0.2–0.3  mm. irreversible chemical change in which the This sheeting punctures easily, as it is relatively thin. molecular chains crosslink so that subsequently It is suitable for use on low slopes (less than 2:1,  i.e. they cannot be appreciably softened by heat. 2 metres horizontal to 1 metre vertical) and on sites free Excessive heating causes charring. of sticks, small stones and abrasive materials. Extreme care needs to be taken during installation. Low-density Thermoplastics plastic breaks down quickly in sunlight if it is not Polythene is tough, waterproof and oilproof, and can covered with soil. be manufactured in many colours. In buildings, it is used for cold water pipes, plumbing and sanitary ware, Woven polyethylene fabric and polythene film (translucent or black). Film should This has a polyethylene coating on both sides, and is not be subjected unnecessarily to prolonged heat over blue or green in colour. Woven polyethylene fabrics are 50  °C, or to direct sunlight. The translucent film will generally the most suitable as dam liners in temperate last for only 1–2 years if exposed to sunlight, but the countries, and are sold according to weight, not carbon pigmentation of black film increases resistance thickness. Fabric weight of about 250 grams per square to sunlight. metre is normally selected for farm dams. Heavier Polyvinyl chloride (PVC) will not burn and can be grades are required where puncturing is a concern. made in rigid or flexible form. It is used for rainwater Although polyethylene fabrics are not UV resistant, goods, drains, pipes, ducts, electric cable insulation, etc. their life expectancy can be increased to 15–20  years Acrylics, a group of plastics containing polymethyl with soil cover. methacrylate, which transmit more light than glass and It is normal practice to cover these fabrics with are easy to mould or curve into almost any shape. at least a 300 mm layer of soil. Owing to the need to provide this cover, sites with steep batters (greater than Thermosetting plastics 2:1) are not suitable. For woven polyethylene fabrics The main uses for thermosetting plastics in buildings used as dam liners, the batter slopes of the embankment are as impregnants for paper fabrics, binders for particle and excavations should not exceed 2:1, although a boards, adhesives, paints and clear finishes. Phenol gentler slope is preferable to ensure that any soil cover formaldehyde (bakelite) is used for electrical insulating stays on the liner. accessories, and urea formaldehyde is used for particle board manufacture. 88 Rural structures in the tropics: design and development High-density polyethylene (HDPE) sheeting geocomposites. Several combinations are possible High-density polyethylene sheeting, which is black in and this area has attracted interest of many research colour, does not require soil cover, but it is the most establishments. The main uses of geocomposites expensive of lining materials. Installation is generally embrace the entire range of uses of the geosynthetics undertaken using fusion-weld joining equipment. discussed above, e.g. reinforcement, drainage, Thicknesses range from 0.4 mm to 2.5 mm. It is suitable liquid barrier, etc. for sites where puncturing of cheaper products cannot be avoided, or where steep slopes (steeper than 2:1) (b) Degradable erosion mats: preclude the use of other products. These are made of flexible erosion control blankets HDPE is the most widely used geomembrane, and that are used to keep soil and seeds stable until offers the most cost-effective liner for large, exposed, vegetation completely covers the dam catchment. lining projects. This product has been used in landfills, As they are made of organic materials, they wastewater treatment lagoons, animal waste lagoons, eventually breakdown and become part of the soil. mining applications and water storage. It has the following advantages: RuBBER • soil covering is not required; Rubbers are similar to thermosetting plastics. In the • it has high overall chemical resistance and is manufacturing process, a number of substances are resistant to ozone and UV; mixed with latex, a natural polymer. Carbon black is • it is cost effective for large projects; added to increase strength in tension and to improve • it is suitable for potable water. wearing properties. After forming, the product is vulcanized by heating under pressure, usually with sulphur present. This Plastic components used with dam liners process increases the rubber’s strength and elasticity. In the process of lining dams, there may be needed one Ebonite is a fully vulcanized, hard rubber. or more additional components that will ensure the Modified and synthetic rubbers (elastomers) are longevity of the earth dam. These components include: increasingly being used for building products. Unlike natural rubbers, they often have good resistance to (a) Geosynthetics: oil and solvents. One such rubber, butyl, is extremely These are synthetic materials made from tough, has good weather resistance, excellent resistance polymers (geomembranes are also classified under to acids and very low permeability to air. Synthetic geosynthetics). When these materials are used rubber fillers and nail washers are used with metal together with dam liners (geomembranes), the roofing. service life of the earth dam is extended. The geosynthetics commonly used together with dam BITuMInOuS PROduCTS liners include: These include bitumen (asphalt in the United States), coal tar and pitch. They are usually dark brown or 1. Geonets: These are open grid-like materials black and, in general, they are durable materials that formed by a continuous extrusion of parallel are resistant to many chemicals. They resist the passage sets of polymeric ribs intersecting at a constant of water and water vapour, especially if they have been acute angle. They are used in the design of applied hot. drainage systems, particularly on slopes and Bitumen occurs naturally as rock asphalt or lake are a viable alternative to the common sand asphalt, or can be distilled from petroleum. It is used and gravel systems. for road paving, paint, damp-proof membranes, joint 2. Geocells: These are constructed from filler, stabilizer in soil blocks, etc. polymeric strips, which are joined together to form a 3-dimensional network. They are PAInTS used to stabilize the side slopes of dams and Paint preserves, protects and decorates surfaces, and other earth structures. This usually involves enables them to be cleaned easily. All paints contain a filling the cells with soil. binder that hardens. Other ingredients found in various 3. Geogrids: These are stiff or flexible polymer paints include: pigments, strainers, extenders, driers, grid-like sheets with large uniformly hardeners, thinners, solvents and gelling agents. Some distributed apertures. These apertures allow water-thinned paints contain emulsifiers. direct contact between soil particles on either Owing to the cost involved, few buildings in rural side of the sheet. Their main use is to reinforce areas are painted. When paint can be afforded, priority unstable soils. should be given to painting surfaces likely to rust, rot or decay because of exposure to rain or dampness, and Combination of two or more of the geosynthetics, to rooms such as a kitchen or a dairy, where hygiene e.g. geogrid and geomembrane, are referred to as demands easily cleaned surfaces. White and other light Chapter 5 – Construction materials 89 colours reflect more light than dark colours, and can Assuming 38.1 microns dry is desired, then: be used in a sitting room or a workshop to make the room lighter. 420 m2 / litre / 38.1 = 11.0 m2 / litre Painting A coating with 42  percent volume solids, applied Adequate preparation of the surface to be painted is at 11.0  m2  /  litre, will produce a dry film that is essential. The surface should be smooth (not shiny, 38.1 microns thick. because this would not give good adhesion), clean, dry and stable. Old, loose paint should be brushed off Example 5.4 before a new coat is applied. Most commercial paints The living room walls require painting, excluding the are supplied with directions for use, which should be ceiling. The walls are 3  metres high, with a total of read carefully before the work is started. The paint film 18 metres of wall length. The total door and window is usually built up in two or more coats; area is 3 m2. If a spreading rate of 11 square metres per Priming paints are used for the first coat, to seal and litre is used, and only two coats of paint are required, protect the surface and to give a smooth surface for work out how much paint is needed.  subsequent coats. They are produced for application to To work out how much paint is needed: wood, metal and plaster. • Take the surface area = (3×18) -3 = 51 m2 Undercoating paints are sometimes used to obscure • The spreading rate is 11 m2 / litre the primer, as a further protective coating and to • The number of coats needed = 2 provide the correct surface for the finishing paint. • The required litres of paint = (51m2 / 11m2 / litres) Finishing paints are produced in a wide range of × 2 coats = 9 litres in total (i.e. a 10-litre pail) colours and finishes (e.g. matt, semi-matt or gloss). Some commonly used types of paint for farm structures Oil- and resin-based paints are detailed below, but many others are manufactured Oil paints are based on naturally drying oils (e.g. with special properties, making them water- and linseed oil). They are being gradually replaced by alkyd chemical-resistant, heat-resistant, fire-retardant, anti- and emulsion paints. condensation, fungicidal or insecticidal, for example. Alkyd paints are oil-based paints, modified by the addition of synthetic resins to improve durability, Estimation of quantities of paint required flexibility, drying and gloss. They are quite expensive. The volume of paint required for a particular paint job Synthetic resin paints contain substantial proportions can be determined from knowledge of the following: of thermosetting resins, such as acrylics, polyurethane 1. Surface area of the surface(s) to be painted. or epoxides, and are often packed in two parts. They 2. Spreading rate of the paint being used. have excellent strength, adhesion and durability, but are 3. The number of coats needed. very expensive. Bituminous paints are used to protect steelwork and Spreading rates iron sheeting from rust, and to protect wood from decay. The spreading rate of paint is the area that a specific They are black or dark in colour, and tend to crack in volume of paint will cover at a specified film thickness. hot sunlight. They can be overpainted with ordinary Two standard measurements are used to describe the film paint only after a suitable sealer has been applied. thickness of a coating: mils and microns. A micron is a Varnishes are either oil/resin or spirit-based and metric system measurement equal to 0.001 millimetres. used mainly to protect wood with a transparent finish, The spreading rate in microns may be calculated as but protection is inferior to opaque finishes. Spirit- follows: based varnish is used only for interior surfaces. Any liquid will cover 1 000 square metres per litre at 1 micron wet. Therefore, a 100 percent volume/solids Water-based paints material will cover 1 000 m2 per litre dry when applied Non-washable distemper consists of chalk powder, at 1 micron wet and, because it is 100  percent solids, mixed with animal glue dissolved in hot water. It is it will yield a 1 micron-thick dry film. However, if a cheap, but easily rubbed or washed off, and therefore coating is less than 100 percent solids, then the dry film suitable only for whitening ceilings. thickness will be thinner because the volatile portion of Washable distemper (water paint) consists of drying the volume will evaporate and leave the film, thereby oil or casein, emulsified in water with the addition reducing the dried film volume or thickness. of pigments and extenders. Hardening is slow but, Assuming a material has 42 percent volume of solids, after a month, it can withstand moderate scrubbing. It the area that it will cover when a dry film thickness of weathers fairly well outdoors and is reasonably cheap. 38.1 microns is required may be calculated as follows: Whitewash (limewash) consists of lime mixed with water. It can be used on all types of wall, including earth 1  000  m2  /  litre × 0.42 = 420  m2  /  litre at 1  micron walls, and is cheap, but its lack of water resistance and thickness. poor weathering properties make it inferior to emulsion 90 Rural structures in the tropics: design and development paint for outdoor surfaces. However, the addition of REVIEW quESTIOnS tallow or cement gives some degree of durability for 1. (a) Explain how the following factors affect external use. Whitewash can be made in the following construction material choice: way: (i) resource utilization in the choice of • Mix 8  litres (9  kg) of quicklime with about construction materials; 18 litres of boiling water, adding the water slowly (ii) social costs and shadow prices. and stirring constantly until a thin paste results. (b) Define the following for cement: • Add 2 litres of salt and stir thoroughly. (i) hydration; • Add water to bring the whitewash to a suitable (ii) setting. consistency. (c) Briefly describe the Pozzolana as a building • If external quality is required, add a handful of material. cement per 10 litres of whitewash just before use. 2. (a) Outline three disadvantages of soil as a In emulsion paints, the pigments and binder (vinyl, construction material. acrylic, urethane or styrene polymers) are dispersed as (b) During the bar shrinkage test the following small globules in water. They harden quickly, are quite results were obtained: tough and weather-resistant, and the cost is moderate. • Length of wet bar = 600 mm Although they adhere well to most supports, because • Length of dry bar = 420 mm they are permeable an oil-based primer may be required Find the shrinkage ratio and state the conclusion to seal porous exterior surfaces. that may be drawn from this result. Cement-based paints are often used for exteriors, (c) Briefly describe how burnt (soil) bricks are and are quite inexpensive. They contain white portland made. cement, pigments (if other colours are desired) and water-repellents, and are sold in powder form. Water is 3. (a) Name five methods for seasoning wood. added just before use to obtain a suitable consistency. (b) Briefly outline the Bethel full-cell process of Paint that has thickened must not be thinned further. It timber preservation. adheres well to brickwork, concrete and renderings, but not to timber, metal or other types of paint. Surfaces 4. Briefly describe: geonets, geocells and degradable should be dampened before painting. erosion mats. Cement slurries make economical surface coatings on masonry and concrete, but earth walls that shrink 5. The tensile strength of blue gum timber is and swell will cause the coating to peel off. Slurries are 50  MPa at a moisture content of 12  percent. If mixtures of cement and/or lime, clean fine sand and the strength determined in its green state was enough water to make a thick liquid. A good slurry 42 MPa, and its fibre saturation point occurs at can be made using 1 part cement, 1 part lime and up to a moisture content of 25 °C, find the strength of 4 parts sand. It is applied on the dampened surface with a this timber at a moisture content of 8 percent. large brush or a used bag, hence the name ‘bag washing’. If the density of the wood was 1.4  g/cm3 at moisture content of 12 percent, find the density at a moisture content of 8 percent. 6. (a) Briefly describe glass as a building material. (b) Briefly describe three main types of paint. (i) Assuming paint has 42 percent volume of solids, find the area that 1 litre will cover when a dry film thickness of 38.1 microns is required. (ii) The walls of a room are 3 metres high, with a total of 30 metres of wall length. The total door and window area is 3 square metres. If the spreading rate is 11  square metres per litre, and only two coats of paint are required, work out how much paint is needed. Chapter 5 – Construction materials 91 FuRTHER REAdInG Shetty, M.S. 2001. Concrete technology:theory and Barnes, M.M. 1971. Farm construction: buildings. practice. 4th edition, New Delhi, S. Chand & Co. Slough, Cement and Concrete Association. Ltd. Bathurst, R.J. 2007. Geosynthetics Classification. Smith, M.J. 1992. Soil mechanics. 4th edition. Essex, IGS Leaflets on Geosynthetics Applications. IGS United Kingdom, ELBS Longman. Education Committee (available at http://www. Spence, R.J.S. & Cook, D.J. 1983. Building materials geosyntheticssociety.org). in developing countries. Chichester, John Wiley & Colorado Lining International Inc. 2002. Geosynthetic Sons Ltd. materials (available at http://www.coloradolining. Storrs, A.E.G. 1979. Know your trees. Ndola, The com/services/Materials.html). Forest Department. Eldridge, H.J. 1974. Properties of building materials. Storrs, A.E.G. 1982. More about trees. Ndola, The Lancaster, Medical and Technical Publishing Co. Forestry Department. Ltd. Stulz, R. 1981. Appropriate building materials. SKAT Erwine, B. 2009. Which glass should I use? Sorting it all No. 12. St. Gallen, Swiss Center for Appropriate out (available at http://www.lightingdesignlab.com). Technology. Everett, A. 1981. Materials. Mitchell’s Building Series. Swift, D.G. & Smith, R.B.L. 1979. The construction London, Batsford Academic and Educational Ltd. of corrugated roofing sheets using sisal-cement. Forest Products Laboratory. 1999. Wood handbook-- Nairobi, Kenyatta University College. Wood as an engineering material. Madison, WI: U.S. Tsoumis, G. 1991. Science and technology of wood: Department of Agriculture, Gen. Tech. Rep. FPL- structure, properties, utilization. Chapman & Hall, GTR-113., Forest Service New York. Fullerton, R.L. 1977-1979. Building construction in United Nations. 1972. The use of bamboo and reeds in warm climates. Part 1-3. Oxford, Oxford University building construction. New York. Press. Volunteers in Technical Assistance (VITA). 1977. Hodgkinson, A. 1982. AJ handbook of building Making building blocks with the CINVA. Mt. structure. London, The Architectural Press Ltd. Rainier, Ram Block Press. International Labour Office. 1984. Small-scale brickmaking. Geneva. Lindley, J.A. & Whitaker, J.H. 1996. Agricultural buildings and structures. Revised edition. American Society of Agricultural Engineers (ASAE). Lippsmeier, G. 1969. Tropenbau  –  building in the Tropics. Munich, Callwey Verlag. Lundborg, N. 1976. To choose timber for building. Dar-es-Salaam, National Housing and Building Research Unit. Lunt, M.G. 1980. Stabilized soil blocks for building. Overseas Building Notes No. 184. Watford, Building Research Establishment, Overseas Division. McKay, W.B. 1975. Carpentry. London, Longman Group Ltd. National Academy of Science. 1973. Ferro-cement: applications in developing countries. Washington, D.C. National Vocational Training Institute, Accra. Rural building: 1.  Reference book, 2.  Basic knowledge, 3.  Construction. 4.  Drawing book. Maastricht, Stichting Kongretatie F.l.C. Palmeira, E.M., Tatsuoka, F., Bathurst, R.J., Stevenson, P.E. & Zornberg, J.G. 2008. Advances in Geosynthetics Materials and Applications for Soil Reinforcement and Environmental Protection Works. Electronic Journal of Geotechnical Engineering, Vol. 13, Special Issue State of the Art in Geotechnical Engineering, December, pp. 1-38. Paterson, D.N. 1971. The strength of Kenya timbers: their derivation and application. Nairobi, Kenya Forest Department. 93 Chapter 6 Basic mechanics BASIC PRInCIPLES OF STATICS All objects on earth tend to accelerate toward the Statics is the branch of mechanics that deals with the centre of the earth due to gravitational attraction; hence equilibrium of stationary bodies under the action of the force of gravitation acting on a body with the mass forces. The other main branch – dynamics – deals with (m) is the product of the mass and the acceleration due moving bodies, such as parts of machines. to gravity (g), which has a magnitude of 9.81 m/s2: Static equilibrium F = mg = vrg A planar structural system is in a state of static equilibrium when the resultant of all forces and all where: moments is equal to zero, i.e. F = force (N) m = mass (kg) g = acceleration due to gravity (9.81m/s2) y ∑Fx = 0 ∑Fx = 0 ∑Fy = 0 ∑Ma = 0 v = volume (m³) ∑Fy = 0 or ∑Ma = 0 or ∑Ma = 0 or ∑Mb = 0 r = density (kg/m³) x ∑Ma = 0 ∑Mb = 0 ∑Mb = 0 ∑Mc = 0 Vector where F refers to forces and M refers to moments of Most forces have magnitude and direction and can be forces. shown as a vector. The point of application must also be specified. A vector is illustrated by a line, the length of Static determinacy which is proportional to the magnitude on a given scale, If a body is in equilibrium under the action of coplanar and an arrow that shows the direction of the force. forces, the statics equations above must apply. In general, three independent unknowns can be determined from Vector addition the three equations. Note that if applied and reaction The sum of two or more vectors is called the resultant. forces are parallel (i.e. in one direction only), then only The resultant of two concurrent vectors is obtained by two separate equations can be obtained and thus only constructing a vector diagram of the two vectors. two unknowns can be determined. Such systems of The vectors to be added are arranged in tip-to-tail forces are said to be statically determinate. fashion. Where three or more vectors are to be added, they can be arranged in the same manner, and this is Force called a polygon. A line drawn to close the triangle A force is defined as any cause that tends to alter the or polygon (from start to finishing point) forms the state of rest of a body or its state of uniform motion resultant vector. in a straight line. A force can be defined quantitatively The subtraction of a vector is defined as the addition as the product of the mass of the body that the force is of the corresponding negative vector. acting on and the acceleration of the force. P = ma P where P = applied force m = mass of the body (kg) a = acceleration caused by the force (m/s2) The Système Internationale (SI) units for force are Q therefore kg m/s2, which is designated a Newton (N). The following multiples are often used: A 1 kN = 1 000 N, 1 MN = 1 000 000 N 94 Rural structures in the tropics: design and development Q Concurrent coplanar forces P Forces whose lines of action meet at one point are said to be concurrent. Coplanar forces lie in the same plane, whereas non-coplanar forces have to be related to a three-dimensional space and require two items of directional data together with the magnitude. Two P + Q R = coplanar non-parallel forces will always be concurrent. Equilibrium of a particle When the resultant of all forces acting on a particle is A zero, the particle is in equilibrium, i.e. it is not disturbed P from its existing state of rest (or uniform movement). The closed triangle or polygon is a graphical expression of the equilibrium of a particle. Q The equilibrium of a particle to which a single force P + is applied may be maintained by the application of a R = second force that is equal in magnitude and direction, but opposite in sense, to the first force. This second force, which restores equilibrium, is called the equilibrant. Q When a particle is acted upon by two or more forces, the A equilibrant has to be equal and opposite to the resultant of the system. Thus the equilibrant is the vector drawn closing the vector diagram and connecting the finishing Q point to the starting point. P P S R = P + Q + S A Q Resolution of a force A In analysis and calculation, it is often convenient to consider the effects of a force in directions other than Q that of the force itself, especially along the Cartesian P (xx-yy) axes. The force effects along these axes are called vector components and are obtained by reversing the vector addition method. ANT ULT RES y A Q F P Fy T LIB RAN θ x 0 EQUI Fx Fy is the component of F in the y direction Fy = F sinθ Fx is the component of F in the x direction Fx = F cosθ A TAB TAC A Chapter 6 – Basic mechanics 95 980 N Free body diagram for point A Free-body diagram of a particle A sketch showing the physical conditions of a problem is known as a space diagram. When solving a problem it TAB is essential to consider all forces acting on the body and to exclude any force that is not directly applied to the 980 N body. The first step in the solution of a problem should therefore be to draw a free-body diagram. T A free-body diagram of a body is a diagrammatic AC representation or a sketch of a body in which the body is shown completely separated from all surrounding bodies, including supports, by an imaginary cut, and the action of each body removed on the body being Example 6.2 considered is shown as a force on the body when A rigid rod is hinged to a vertical support and held drawing the diagram. at 50° to the horizontal by means of a cable when a To draw a free-body diagram: weight of 250 N is suspended as shown in the figure. 1. Choose the free body to be used, isolate it from Determine the tension in the cable and the compression any other body and sketch its outline. in the rod, ignoring the weight of the rod. 2. Locate all external forces on the free body and clearly mark their magnitude and direction. This should include the weight of the free body, which 75° A is applied at the centre of gravity. 3. Locate and mark unknown external forces and reactions in the free-body diagram. 250 N 4. Include all dimensions that indicate the location and direction of forces. 50° Space diagram The free-body diagram of a rigid body can be reduced to that of a particle. The free-body of a particle is used to represent a point and all forces working on it. Example 6.1 65° Determine the tension in each of the ropes AB and AC 40° B C Free-body diagram for point A A Tension 180 N 75° 65° Compression Space diagram 40° 265 N 250 N TAB Force triangle TAC The forces may also be calculated using the law of sines: A Compression in rod Tension in cable 250 N = = sin 75° sin 40° sin 65° 980 N Free body diagram Point of concurrency for point A Three coplanar forces that are in equilibrium must all pass through the same point. This does not necessarily apply for more than three forces. TAB 980 N TAC 96 Rural structures in the tropics: design and development If two forces (which are not parallel) do not meet at supports will react against the tendency of the applied their points of contact with a body, such as a structural forces (loads) to cause the member to move. The forces member, their lines of action can be extended until they generated in the supports are called reactions. meet. In general, a structural member has to be held or supported at a minimum of two points (an exception to Collinear forces this is the cantilever). Anyone who has tried ‘balancing’ Collinear forces are parallel and concurrent. The sum of a long pole or a similar object will realize that, although the forces must be zero for the system to be in equilibrium. only one support is theoretically necessary, two are needed to give satisfactory stability. Coplanar, non-concurrent, parallel forces Three or more parallel forces are required. They will be Resultant of gravitational forces in equilibrium if the sum of the forces equals zero and The whole weight of a body can be assumed to act at the sum of the moments around a point in the plane the centre of gravity of the body for the purpose of equals zero. Equilibrium is also indicated by two sums determining supporting reactions of a system of forces of moments equal to zero. that are in equilibrium. Note that, for other purposes, the gravitational forces cannot always be treated in this way. Reactions Structural components are usually held in equilibrium Example 6.3 by being secured to rigid fixing points; these are often A ladder rests against a smooth wall and a person other parts of the same structure. The fixing points or weighing 900 N stands on it at the middle. The weight TABLE 6.1 Actions and reactions Flexible cable or rope Force exerted by the cable or rope is always tension away from the fixing, in the direction of the tangent to the cable curve. θ θ Smooth surfaces Reaction is normal to the surface, i.e., at right angles to the tangent. N Rough surfaces Rough surface is capable of supporting a tangental F force as well as a normal reaction. Resultant reaction is vectorial sum of these two. N Roller support Reaction is normal to the supporting surface only. Pin support A freely hinged support is fixed in position, hence the two reaction forces, but is not restrained in direction - it R is free to rotate. x Ry Built-in support The support is capable of providing a longitudinal y y y reaction (H), a lateral or transverse reaction (V), and a moment (M). The body is fixed in position and fixed in direction. H M V Chapter 6 – Basic mechanics 97 of the ladder is 100 N. Determine the support reactions (A) can then be found, giving the direction of the at the wall (RW) and at the ground (RG). ground reaction force. This in turn enables the force vector diagram to be drawn, and hence the wall and ground reactions determined. Example 6.4 A pin-jointed framework (truss) carries two loads, as shown. The end A is pinned to a rigid support, W = (900 + 100) N while the end B has a roller support. Determine the supporting reactions graphically: 12 kN 3 m Space diagram A B 15 kN Rw A 1. Combine the two applied forces into one and find the line of action. 2. Owing to the roller support reaction RB will be vertical. Therefore the resultant line (RL) must R x 1 000 N G be extended to intersect the vertical reaction of support B. This point is the point of concurrency for the resultant load, the reaction at B and the RGy reaction at A. Free-body diagram of ladder 12 A RL 15 RL RB C RG = 1 030.8 1 000 N 3. From this point of concurrency, draw a line through the support pin at A. This gives the line Rw = 250 N of action of the reaction at A. Force diagram RA As the wall is smooth, the reaction RW must be at RL R right angles to the surface of the wall and is therefore B horizontal. A vertical component would have indicated RA R R L B a friction force between the ladder and the wall. At the C bottom, the ladder is resting on the ground, which is not smooth, and therefore the reaction RG must have both a vertical and a horizontal component. 4. Use these three force directions and the magnitude As the two weight forces in this example have the of RL to draw the force diagram, from which RA same line of action, they can be combined into a single and RB can be found. force, reducing the problem from one with four forces Answer: RA = 12.2 kN at 21° to horizontal. to one with only three forces. The point of concurrency RB = 12.7 kN vertical. 6 m 98 Rural structures in the tropics: design and development The link polygon (see an engineering handbook) Using the first condition of equilibrium it can be seen may also be used to determine the reactions to a beam that the horizontal component of RG must be equal but or a truss, though it is usually quicker and easier to opposite in direction to RW, i.e. obtain the reactions by calculation, the method shown in Example 6.4, or a combination of calculation and drawing. RGX = 250 N However, the following conditions must be satisfied. 1. All forces (apart from the two reactions) must be Because RG is the third side of a force triangle, where known completely, i.e. magnitude, line of action the other two sides are the horizontal and vertical and direction. components, the magnitude of RG can be calculated as: 2. The line of action of one of the reactions must be known. (1 0002 + 2502)½ = 1 030.8 N 3. At least one point on the line of action for the other reaction must be known (2 and 3 reduce the Resultant of parallel forces number of unknowns related to the equations of If two or more parallel forces are applied to a equilibrium to an acceptable level). horizontal beam, then theoretically the beam can be held in equilibrium by the application of a single force Moments of forces (reaction) that is equal and opposite to the resultant R. The effect of a force on a rigid body depends on its point The equilibrant of the downward forces must be equal of application, as well as its magnitude and direction. It and opposite to their resultant. This provides a method is common knowledge that a small force can have a for calculating the resultant of a system of parallel large turning effect or leverage. In mechanics, the term forces. However, two reactions are required to ensure ‘moment’ is used instead of ‘turning effect’. the necessary stability, and a more likely arrangement The moment of force with a magnitude (F) about will have two or more supports. a turning point (O) is defined as: M = F × d, where The reactions RA and RB must both be vertical because d is the perpendicular distance from O to the line of there is no horizontal force component. Furthermore, action of force F. The distance d is often called lever the sum of the reaction forces RA and RB must be equal arm. A moment has dimensions of force times length to the sum of the downward-acting forces. (Nm). The direction of a moment about a point or axis is defined by the direction of the rotation that the Beam reactions force tends to give to the body. A clockwise moment is usually considered as having a positive sign and an anticlockwise moment a negative sign. 80 kN 70 kN 100 kN 30 kN The determination of the moment of a force in a coplanar system will be simplified if the force and its point of application are resolved into its horizontal and vertical components. 2 m 2 m 3 m 3 m 2 m Example 6.5 As the ladder in Example 6.3 is at rest, the conditions RA RB of equilibrium for a rigid body can be used to calculate the reactions. By taking moments around the point where the ladder rests on the ground, the moment of The magnitude of the reactions may be found by the the reaction RG can be ignored as it has no lever arm application of the third condition for equilibrium, i.e. (moment is zero). According to the third condition the algebraic sum of the moments of the forces about for equilibrium, the sum of moments must equal zero, any point must be zero. therefore: Take the moments around point A, then: (6 × RW) - (900 N × 1.5 m) - (100 N × 1.5 m) = 0 (80 × 2) + (70 × 4) + (100 × 7) + (30 × 10) - (RB × 12) = 0; RW = 250 N Giving RB = 120 kN The vertical component of RG must, according to the second condition, be equal but opposite to the sum of RA is now easily found with the application of the the weight of the ladder and the weight of the person second condition for equilibrium. on the ladder, because these two forces are the only vertical forces and the sum of the vertical forces must RA - 80 - 70 - 100 - 30 + RB=0; with RB = 120 kN gives: equal zero, i.e. RA=160 kN. RGy = 1 000 N Chapter 6 – Basic mechanics 99 Couples This technique must not be used for calculation of Two equal, parallel and opposite but non-collinear shear force, bending moment or deflection. forces are said to be a couple. A couple acting on a body produces rotation. Note Example 6.6 that the couple cannot be balanced by a single force. Consider a suspended floor where the loads are supported To produce equilibrium, another couple of equal and by a set of irregularly placed beams. Let the load arising opposite moment is required. from the weight of the floor itself and the weight of any material placed on top of it (e.g. stored grain) be 10 kPa. Determine the UDL acting on beam A and beam C. FLOOR SECTION BEAM A B C D 150 mm FLOOR PLAN 150 mm F=20N 2·0 m 3·0 m 2·0 m F=20N It can be seen from the figure below that beam A carries the floor loads contributed by half the area between the Loading systems beams A and B, i.e. the shaded area L. Beam C carries Before any of the various load effects (tension, the loads contributed by the shaded area M. compression, bending, etc.) can be considered, the applied loads must be rationalized into a number of ordered systems. Irregular loading is difficult to deal 2·5 m with exactly, but even the most irregular loads may 1·0 m 1·0 m be reduced and approximated to a number of regular systems. These can then be dealt with in mathematical terms using the principle of superposition to estimate the overall combined effect. Concentrated loads are those that can be assumed to L M act at a single point, e.g. a weight hanging from a ceiling, or a person pushing against a box. Concentrated loads are represented by a single arrow drawn in the direction, and through the point of action, of the force. The magnitude of the force is always indicated. 2·0 m 3·0 m 2·0 m Uniformly distributed loads, written as UDL, are those that can be assumed to act uniformly over an area or along the length of a structural member, e.g. roof loads, wind loads, or the effect of the weight of water Therefore beam A carries a total load of: on a horizontal surface. For the purpose of calculation, a UDL is normally considered in a plane. 1 m × 4 m × 10 kPa = 40 kN, or 40 kN / 4 = 10 kN / m. In calculating reactions, uniformly distributed loads can in most, but not all, cases be represented by a concentrated In the same way, the loading of beam C can be load equal to the total distributed load passing through the calculated to 25 kN / m. The loading per metre run can centre of gravity of the distributed load. then be used to calculate the required size of the beams. 4·0 m 4·0 m 100 Rural structures in the tropics: design and development 10kN/m taken about an axis passing through the centroid of the cross-section, of all the forces applied to the beam on either side of the chosen cross-section. Consider the cantilever AB shown in (A). For equilibrium, the reaction force at A must be vertical and 4·0 m equal to the load W. The cantilever must therefore transmit the effect of load W to the support at A by developing resistance Loading of beam A (on vertical cross-section planes between the load and the support) to the load effect called shearing force. 25 kN/m Failure to transmit the shearing force at any given section, e.g. section x-x, will cause the beam to fracture as in (B). (A) W 4·0 m X A B Loading of beam C C X Distributed load with linear variation is another common load situation. The loading shape is triangular and is the result of such actions as the pressure of water R=W on retaining walls and dams. (B) W X A B C X R Distributed loads with linear variation The bending effect of the load will cause the beam to deform as in (C). To prevent rotation of the beam at the support A, there must be a reaction moment at A, shown as MA, which is equal to the product of load W Shear force and bending moment of beams and the distance from W to point A. A beam is a structural member subject to lateral loading The shearing force and the bending moment in which the developed resistance to deformation is of a transmitted across the section x-x may be considered as flexural character. The primary load effect that a beam the force and moment respectively that are necessary to is designed to resist is that of bending moments but, in maintain equilibrium if a cut is made severing the beam addition, the effects of transverse or vertical shearing at x-x. The free-body diagrams of the two portions of forces must be considered. the beam are shown in (D). Shear force (V) is the algebraic sum of all the Then the shearing force between A and C = Qx = transverse forces acting to the left or to the right of the W and the bending moment between A and C = Mx = chosen section. W × AC. Bending moment (M) at any transverse cross-section Note: Both the shearing force and the bending of a straight beam is the algebraic sum of the moments, moment will be zero between C and B. Chapter 6 – Basic mechanics 101 (C) force remains constant in between. When the load is W uniformly distributed, however, the shear force will X A vary at a uniform rate. Thus it will be seen that uniform MA B loads cause gradual and uniform change of shear, while concentrated loads bring a sudden change in the value X C of the shear force. Bending moment variation Concentrated loads will cause a uniform change of the (D) bending moment between the points of action of the Q W loads. In the case of uniformly distributed loads, the X MX rate of change of the bending moment will be parabolic. MA QX Maximum bending moment values will occur where the MX shear force is zero or where it changes sign. X Shear-force (SF) and bending-moment (BM) diagrams R Representative diagrams of the distribution of shearing forces and bending moments are often required at several stages in the design process. These diagrams Definitions are obtained by plotting graphs with the beams as the Shear force (Q) is the algebraic sum of all the transverse base and the values of the particular effect as ordinates. forces acting to the left or to the right of the chosen It is usual to construct these diagrams in sets of three, section. representing the distribution of loads, shearing forces Bending moment (M) at any transverse cross section and bending moments respectively. These graphical of a straight beam is the algebraic sum of the moments, representations provide useful information regarding: taken about an axis passing through the centroid of the 1. The most likely section where a beam may fail in cross section, of all the forces applied to the beam on shear or in bending. either side of the chosen cross section. 2. Where reinforcement may be required in certain Table 6.2 shows the sign convention for shear types of beam, e.g. concrete beams. force (Q) and bending moment (M) used in this book. 3. The shear-force diagram will provide useful Shearing forces, which tend to make the part of the information about the bending moment at any beam to the left move up and the right part move point. down, are considered positive. The bending moment is 4. The bending-moment diagram gives useful considered positive if the resultant moment is clockwise information on the deflected shape of the beam. on the left and anticlockwise on the right. These tend to make the beam concave upwards and are called sagging Some rules for drawing shear-force and bending- bending moments. If the moment is anticlockwise on moment diagrams are: the left and clockwise on the right, the beam will tend 1. In the absence of distributed loads, the shear- to become convex upwards – an effect called hogging. force diagram consists of horizontal steps and the bending-moment diagram is a series of straight lines. TABLE 6.2 2. For a beam (or part of a beam) carrying a UDL Shearing and bending forces only, the shear-force diagram is a sloping straight Sign convention line and the bending diagram is a parabola. Load effect Symbol Positive (+) negative (–) units 3. At the point where the shear-force diagram passes through zero (i.e. where the SF changes Shearing N sign), the BM has a maximum or minimum value. force Q kN 4. Over a part of the span for which SF is zero, the Up on the left Down on the left bending moment has a constant value. 5. At a point where the bending-moment diagram Bending Nm passes through zero, the curvature changes from moment M Sagging Hogging kNm (top fibre in compression) (top fibre in tension) Nmm concave upwards to concave downwards or vice versa. This point is referred to as point of inflexion. 6. If a beam is subjected to two or more different Shear-force variation systems of loading, the resulting shear and Concentrated loads will change the value of the shear bending moment at a given section is the algebraic force only at points where they occur, i.e. the shear sum of the values at the section. This is referred to 102 Rural structures in the tropics: design and development as the principle of superposition and applies also 1. Consider a section through the beam just to to bending stresses, reactions and deflections. the left of D, and find the algebraic sum of all vertical forces to the left of this section. ∑Fy = 0, The following example demonstrates the construction therefore, shear force to the left of D is zero. of diagrams representing shearing forces and bending 2. Consider a section just to the right of D, algebraic moments. sum of forces to the left of this section is 10 kN down to the left. Hence, shear force to the right Example 6.7 of D is 10 kN (negative). The distribution of loads in a simply supported beam is 3. The same result as in point 2 above will be found as given in the diagram below. Determine the reactions for any such section between D and E. The at the supports and draw the shear-force and bending- shear-force diagram between D and E is thus a moment diagram. horizontal line at -10 kN. 4. Consider a section just to the right of E; the algebraic sum of forces to the left of this section a = 10 m b = 10 m c = 10 m is made up of P and RE given that the shear force W2 = 4 kN/m equals (-10 + 40)  kN = + 30  kN, i.e. up to the W1 = 2 kN/m left of section. Thus at E the shear-force diagram P = 10 kN changes from -10 kN to + 30 kN. D F 5. As we approach the right-hand end of the beam E G we find the mathematics easier to consider on the right-hand side of any section. Section just Solution: to the left of F. Shear force = (4 kN / m × 10 m) - (30 kN) using the sign convention to determine (a) Draw the free-body diagram of the beam. positive or negative. Shear force here equals + 40 - 30 = + 10 kN. 6. Section just to the right of F. Shear force = + 40 - a = 10 m b = 10 m c = 10 m 30 = + 10 kN (i.e. no sudden change at F). 4 kN/m 7. Section just to the left of G. Shear force = -30 kN 2 kN/m P = 10 kN 8. Variation of shear under a distributed load must be linear. D E F G RE = 40 kN RG = 30 kN a = 10 m b = 10 m c = 10 m 4 kN/m (b) Determine the reactions at the supports. First use 2 kN/m the condition for equilibrium of moments about P = 10 kN a point: D E F G RE = 40 kN RG = 30 kN ∑ ME = 0 kN + 30 kN ME = (P × a) + (w1 × b × b / 2 ) + w2 × c(b+c / 2) 30 - RG (b + c) = 0 20 ME = -(10 × 10) + (2 × 10 × 5) + 4 × 10 × (15) + 10 kN S.F.D. 10 - RG (20) = 0 H 0 m RG = 30 kN 0 10 20 30 -10 -10 kN ∑Fy = 0 hence -20 ∑Fy = RE + RG - P-(w1 × b) - ( w2 × c) = 0 -30 -30 kN ∑ Fy = RE + 30 -10 - (2 × 10) - (4 × 10) = 0 RE = 40 kN (c) Draw the shear-force diagram (SFD) directly Note the following from the shear-force diagram: below the loading diagram and choose a • Maximum shear force occurs at E and G where convenient scale to represent the shear force. the values are + 30 kN and - 30 kN respectively. These two transverse sections are the two most Calculate the values of the shear force to the left and likely points for failure in shear. to the right of all critical points. Critical points are: • The maximum bending moment will occur where • at concentrated loads; the shear force is zero or where the shear force • at reactions; changes sign. However, note that cantilevered • at points where the magnitude of a distributed beams will always have maximum bending at the load changes. fixed end. Chapter 6 – Basic mechanics 103 The shear-force diagram in the example has two a = 10 m b = 10 m c = 10 m 4 kN/m points where the shear force is zero. One is at E and 2 kN/m the other is between H and G. The position of H can P = 10 kN be calculated from the fact that at F the shear force D E F G is 10  kN and, under the action of UDL to the right RE = 40 kN RG = 30 kN of F, it reduces at the rate of 4 kN / m. It will read a kN + 30 kN value of zero after 2.5 m, i.e. the point H is 2.5 m to 30 the right of F. 20 10 + + 10 kN S.F.D. (d) Draw the bending-moment diagram directly 0 m 0 - 10 20 - 30 under the shear-force diagram and choose -10 2∙5 m a convenient scale to represent the bending -20 moment. Calculate values of the bending moment -30 at all critical points. Critical points for bending moment are: • ends of the beam; +112.5 kNm 100 • where the shear force is zero or changes sign; 50 • other points that experience has shown to be + B.M.D. critical. 0 m 30 - Values of bending moment are calculated using the -50 definition and sign convention, and considering each -100 -100 kNm load (to one side of the point) separately. It is the effect that one load would have on the bent shape at the chosen point that determines the sign. 1. For the bending moment at D consider the left Note the following from the bending-moment side of this point MD = 0 diagram: • The maximum negative bending-moment hogging 2. For the bending moment at E consider the left (100 kNm) occurs at E and the maximum positive side of this point ME = P × a and the beam would bending moment sagging (112.5 kNm) occurs at a assume a hogging shape: point between F and G. When designing beams in materials such as concrete, the steel reinforcement ME = -(10 × 10) = -100 kNm would have to be placed according to these moments. 3. For the bending moment at F consider the loads • The bending-moment diagram will also give to the right of this point, a sagging beam results an indication as to how the loaded beam will and: deflect. Positive bending moments (sagging) cause compression in the top fibres of the beam, hence MF = -(4 × 10 × 10 / 2) + (30 × 10) = 100 kNm they tend to bend the beam with the concave side downwards. 4. The bending moment at G is obviously zero • At the supported ends of a simple beam and at the free end of a cantilevered beam, where there can 5. At point H we have the maximum bending be no resistance to bending, the bending moment moment: considering the forces to the right of is always zero. this point gives Forces in pin-jointed frames MH = -(4 × 7 512 × 7 5) + (30 × 7 5) Designing a framework necessitates finding the forces = 112.5 (sagging) in the members. For the calculation of primary stresses, each member is considered to be pin-jointed at each 6. The variation of the bending moment under a end so that it can transmit an axial force only in the UDL is parabolic direction of the line connecting the pin joints at each end. The force can be a pure tension (conventionally 7. If the inclusion of other points would be helpful designated positive), in which case the member is called in drawing the curve, they should also be plotted. a tie, or a pure compression (conventionally designated negative), when the member is called a strut. These are internal forces that must be in equilibrium with the external applied forces. 104 Rural structures in the tropics: design and development INTERNAL FORCE ∑Mc = 0 (FHG × CG) + (9 × CD) - (RE × 20) = 0 TIE EXTERNAL FORCE STRUT CG = FX = 10 tan 30° = 5.774 CD = DE = FE / cos 30° A number of different techniques can be used to determine the forces in the members. FE = EX / cos 30° = 11.547 m Joint analysis: This is based on considering the equilibrium of each joint in turn and using the free- CD = 11 547 / cos 30° = 13.333 m body diagram for each joint. Method of sections: The free-body diagram RE = (9 + 12 + 12 ) / 2 = 15 kN considered is for a portion of the framework to one side or the other of a cut section. The forces in the members Hence (FHG × 5.774) + (9 × 13.333) - (15 × 20) = 0 cut by the section are included in the free-body FHG = 31.17 diagram. Application of the equations of equilibrium will solve the unknown forces in the cut section. This provides an analytical solution and is most useful when 12 kN 9 kN 2 requiring the answers for one or two members only. C D Example 6.8 H G F 9 kN 12 kN 9 kN 2 E B C D RE H G F A 30° E Take section 2-2. 10 m 10 m 10 m 10 m HC = FE = 11.547 (FBH × 11.547) + (9 × 13.333) - (15 × 20) = 0 FBH = 15.59 kN ( all 30°, 60°, 90° triangle) It can therefore be seen that FGH and FBH must be clockwise to have equilibrium about point C. The Find the forces and their direction in the members BH members GH and HB are therefore in tension. and HG by using the method of sections. MECHAnICS OF MATERIALS direct stress 1 12 kN 9 kN When a force is transmitted through a body, the body tends to change its shape. Although these deformations C D are seldom visible to the naked eye, the many fibres or particles that make up the body transmit the force throughout the length and section of the body, and the G F fibres doing this work are said to be in a state of stress. E Thus, a stress may be described as a mobilized internal 1 R reaction that resists any tendency towards deformation. E As the effect of the force is distributed over the cross- section area of the body, stress is defined as force transmitted or resisted per unit area. FHG is found by taking a moment about point C, considering the right hand section (RHS) of the cut Thus Stress Force = 1-1 is in equilibrium. The forces FHC and FBC have no Area moment about point CBL because they intersect at and pass through the point. The SI unit for stress is Newtons per square metre (N / m²). This is also called a Pascal (Pa). However, it is often more convenient to use the multiple N / mm². Chapter 6 – Basic mechanics 105 Note that 1 N / mm² = 1 MN / m² = 1 MPa 490 kN Tensile and compressive stress, which result from forces acting perpendicular to the plane of cross-section in question, are known as normal stress and are usually symbolized with σ (the Greek letter sigma), sometimes given a suffix t for tension (σt) or c for compression (σc). Shear stress is produced by forces acting parallel or tangential to the plane of cross-section and is symbolized with τ (Greek letter tau). Tensile stress Example 6.9 0∙7 m m 0∙7 45 kN 12 m Solution a σ 45 kN t = = 150 MPa m 300 mm2 24 Cross-section area = 0.49 m² mm 490 kN Stress = σc = = 1 000 kPa or 1 MPa 0.49 m2 kN 45 Solution b Weight of pier = 0.7 m × 0.7 m × 3.0 m × 19 kN / m³ = 28 kN Consider a steel bar that is thinner at the middle of its Total load = 490 + 28 = 518 kN and length than elsewhere, and that is subject to an axial pull of 45 kN. If the bar were to fail in tension, it would be as a 518 kN Stres s = σc = = 1 057 kPa 0.49 m2 result of breaking where the amount of material is at a minimum. The total force tending to cause the bar to fracture is 45 kN at all cross-sections but, whereas the effect of the force is distributed over a cross-sectional Shear stress area of 1 200 mm² for part of the length of the bar, it is Shear stress = τ 6 kN = = 76 MPa Example 6.11 78.5 mm2 distributed over only 300 mm² at the middle position. Thus, the tensile stress is greatest in the middle and is: A rivet is used to connect two pieces of flat steel. If the loads are large enough, the rivet could fail in shear, i.e. not breaking but sliding of its fibres. Calculate the shear σ 45 kN t = = 150 MPa stress of the rivet when the steel bars are subject to an 300 mm2 Direacxti aslt rpauin Change in length ∆L ll =of 6 kN. = ε = origina l length L Compressive stress 490 kN Stress = σc = = 1 000 kPa or 1 MPa 0.49 m2 Example 6.10 6 kN 6 kN A brick pier is 0.7 metres square and 3 metres high and weighs 19 kN / m³. It is supporting an axial load from a column of 490 kN. The load is spread uniformly over the top of the pier, so the arrow shown in the diagram merely represents the resultant of the load. Calculate 6 kN 6 kN (a)  the stress in the brickwork immediately under the 10 mm column, and (b) the stress at the bottom of the pier. 150 mm 25 mm 3 m 106 Rural structures in the tropics: design and development Note that although the rivets do, in fact, strengthen elasticity (E), or Young’s modulus and should be the connection by pressing the two steel bars together, considered as a measure of the stiffness of a material. this strength cannot be calculated easily owing to friction and is5 1th8 ekrNefore neglected,  i.e. the rivet is assSutmreesd s =to σ gc i=ve all the =str1e n0g5t7h k tPoa the connection. Modulus of elasticity Stress FL 0.49 m2 = E = = Strain A∆L Cross-section area of rivet = 1/4 × π × 102 = 78.5 mm² The modulus of elasticity will have the same units as stress (Pa). This is because strain has no units. Shear stress = τ 6 kN = = 76 MPa A convenient way of demonstrating elastic behaviour 78.5 mm2 is to plot a graph of the results of a simple tensile test carried out on a thin mild steel rod. The rod is hung vertically and a series of forces are applied at the lower Strain end. Two gauge points are marked on the rod and the When loads of any Direct strain Chan tgye p ien aleren gatphplied to a body, the body distance between them is measured after each force will always =u ndergo dimension =chε ∆L a=origina l length ngeLs; this is called increment has been added. The test is continued until deformatsion. Ten518 kN Stre s = σ = sile and= 1c o0m57p rkePsasive stresses cause the rod breaks. changes in lecngt0h.4, 9 tmor2sional-shearing stresses cause twisting, and bearing stresses cause indentation in the bearing surface. In farm structures, where a uniaxial state of stress Ultimate or maximum stress Upper yield point Plastic Stress at is Sthhee aurs ustarle ssst r=e sτs 6 kN =c onsidered , =th 7e6 mMajPoa r deformation is failure Lower yield point in the axial directio7n8. .A5 mlthmo2ugh there are always small deformations present in the other two dimensions, they Elastic limit are seldom significant. Limit of proportionality Stress (σ) Direct strain Change in length = = ε ∆L = origina l length L Srain (ε) Figure 6.1 Behaviour of a mild steel rod under tension Original length L Elongation ∆L Strained length Example 6.12 L + ∆L Two timber posts, measuring 150  millimetres square and 4  metres high, are subjected to an axial load of 108  kN each. One post is made of pine timber (E = By definition strain is a ratio of change and thus it is a 7 800 MPa) and the other is Australian blackwood (E dimensionless quantity. = 15 300 MPa). How much will they shorten because of the load? Elasticity FL 108 000 × 4 000 All solid materials deform when they are stressed and, Cross-section area A = 2∆2L 5=00 = = 2.5 A mEm²;2 l2e 5n0g0t h× 7L 8 =00 4 000 mm as the stress increases, the deformation also increases. In many cases, when the load causing the deformation FL 108 000 × 4 000 is removed, the material returns to its original size and Pine: ∆L = = = 2.5 mm shape and is said to be elastic. If the stress is steadily AE 22 500 × 7 800 increased, a point is reached when, after the removal of the load, not all of the induced strain is recovered. This Australian blackwood: FL 108 000 × 4 000 ∆L = = = 1.3 mm limiting value of stress is called the elastic limit. AE 22 500 × 15 300 Within the elastic range, strain is proportional to the stress causing it. This is called the modulus of elasticity. Factor of sFaLfe 108 000 × 4 000 ∆L = =ty = 1.3 mm The greatest stress for which strain is still proportional The permisAsiEble s2t2r e5s0s0e ×s 1m5 3u0s0t, of course, be less than is called the limit of proportionality (Hooke’s law). the stresses that would cause failure of the members of Thus, if a graph is drawn of stress against strain as the structure – in other words there must be an ample the load is gradually applied, the first portion of the safety margin. (In 2 000 BC, a building code declared graph will be a straight line. The slope of this straight the life of the builder to be forfeit should the house line is the constant of proportionality, modulus of collapse and kill the owner). Elastic Chapter 6 – Basic mechanics 107 Also, deformations must be limited because excessive STRuCTuRAL ELEMEnTS And LOAdInG deflection may give rise to problems such as cracking of ceilings, partitions and finishes, as well as adversely Applied loads affecting the functional needs. Applied loads fall into three main categories: dead Structural design is not an exact science and, while loads, wind loads and other imposed loads. calculated values of reactions, stresses, etc. may be Dead loads are loads resulting from the self- mathematically correct for the theoretical structure (i.e. weight of all permanent construction, including roof, the model), they may be only approximate as far as the walls, floor, etc. The self-weight of some parts of a actual behaviour of the structure is concerned. structure, e.g. roof cladding, can be calculated from the For these and other reasons, it is necessary to ensure manufacturer’s data sheets, but the self-weight of the that the design stress, working stress, allowable stress structural elements cannot be accurately determined and permissible stress are less than the ultimate stress until the design is completed. Hence estimates of or the yield stress. This margin is called the factor of self-weight of some members must be made before safety. commencing a design analysis and the values checked upon completion of the design. Wind loads are imposed loads, but are usually treated Design stress Ultimate (or yield) stress = as a separate category owing to their transitory nature factor of safety and their complexity. Very often wind loading proves to be the most critical load imposed on agricultural In the case of a material such as concrete, which does buildings. Wind loads are naturally dependent on wind not have a well defined yield point, or brittle materials speed, but also on location, size, shape, height and that behave in a linear manner up to failure, the factor construction of a building. of safety is related to the ultimate stress (maximum Specific information concerning various load types stress before breakage). Other materials, such as steel, is presented in Chapter 8. have a yield point where a sudden increase in strain When designing a structure, it is necessary to occurs, and at which point the stress is lower than consider which combination of dead and imposed loads the ultimate stress. In this case, the factor of safety is could give rise to the most critical loading condition. related to the yield stress in order to avoid unacceptable Not all the imposed loads will necessarily reach their deformations. maximum values at the same time. In some cases (for The value of the factor of safety has to be chosen example, light open sheds), wind loads may tend to with a variety of conditions in mind, such as the: cause the roof structure to lift, producing an effect • accuracy in the loading assumptions; opposite in direction to that of the dead load. • permanency of the loads; Imposed loads are loads related to the use of the • probability of casualties or big economic losses in structure and to the environmental conditions,  e.g. case of failure; weight of stored products, equipment, livestock, vehicles, • purpose of the building; furniture and people who use the building. Imposed • uniformity of the building material; loads include earthquake loads, wind loads and snow • workmanship expected from the builder; loads where applicable, and are sometimes referred to as • strength properties of the materials; superimposed loads because they are in addition to the • level of quality control ensuring that the materials dead loads. are in accordance with their specifications; Dynamic loading results from a change of loading, • type of stresses developed; resulting directly from the movement of loads. For • cost of building materials. example, a grain bin may be affected by dynamic loading if filled suddenly from a suspended hopper; it Values of 3 to 5 are normally chosen when the factor of is not sufficient to consider the load solely when the bin safety is related to ultimate stress, and values of 1.4 to is either empty or full. 2.4 are chosen when related to yield-point stress. In the case of building materials such as steel Principle of superposition and timber, different factors of safety are sometimes This principle states that the effect of a number of loads considered for common loading systems and for applied at the same time is the algebraic sum of the exceptional loading systems, in order to save materials. effects of the loads applied singly. Common loadings are those that occur frequently, whereas a smaller safety margin may be considered for exceptional loadings, which occur less frequently 4 kN 2 kN 6 kN 2 kN 4 kN 6 kN + = and seldom at full intensity, e.g. wind pressure, 3 kN earthquakes, etc. 1 kN 4 kN 4 kN 7 kN 5 kN 108 Rural structures in the tropics: design and development Using standard load cases and applying the principle Structural elements of superposition, complex loading patterns can be solved. Standard case values of shear force, bending Cable moment or deflection at particular positions along a Cables, cords, strings, ropes and wires are flexible member can be evaluated, after which the total value because of their small lateral dimensions in relation to of such parameters for the actual loading system can be their length, and therefore have very limited resistance found by algebraic summation. to bending. Cables are the most efficient structural elements because they allow every fibre of the cross- Effects of loading section to resist the applied loads up to any allowable After the loads have been transformed into definable stress. However, their application is limited by the fact load systems, the designer must consider how the loads that they can be used only in tension. will be transmitted through the structure. Loads are not transmitted as such, but as load effects. It is usual practice to orientate the Cartesian z-z axis along the length of the member and the x-x and y-y axes along the horizontal and vertical cross-sectional axes respectively, when considering a structural member that occupies a certain space (see the figure below). Rod Z Rods, bars and poles are used to resist tensile or Y compressive loads. In a rod or a bar under axial tension, the full cross section can be considered and the full allowable stress for the material can be used in design calculations. X Column Rods or bars under compression are the basis for vertical structural elements such as columns, stanchions, piers Z X and pillars. They are often used to transfer load effects Y from beams, slabs and roof trusses to the foundations. They may be loaded axially or they may have to be designed to resist bending when the load is eccentric. Primary load effects A primary load effect is defined as being the direct result of a force or a moment, which has a specific orientation with respect to the three axes. Any single load or combination of loads can give rise to one or more of these primary load effects. In most cases, a member will be designed basically to sustain one load effect, usually the one producing the greatest effect. In more complex situations, the forces and moments are resolved into their components along the axes, after which the load effects are first studied separately for one axis at a time, and subsequently their combined effects are considered when giving the member its size and shape. The choice of material for a member may be influenced to some extent by the type of loading. For instance, concrete has little or no strength in tension Ties and struts and is therefore unsuitable for use alone as a tie. When bars are connected with pin joints and the Tension, compression, shear, bending and torsion resulting structure loaded at the joints, a structural are all primary load effects. Secondary load effects, framework called a pin-jointed truss or lattice frame such as deflection, are derived from the primary load is obtained. The members are subjected only to axial effects. loads and members in tension are called ties, while members in compression are called struts. Chapter 6 – Basic mechanics 109 The load on a beam causes longitudinal tension and compression stresses, and shear stresses. Their S S magnitudes will vary along, and within, the beam. T T S S The span that a beam can usefully cover is limited by S S the self-weight of the beam, i.e. it will eventually reach a T T T length when it is capable of supporting only itself. To a degree, this problem is overcome with the hollow web beam and the lattice girder or frame. The safe span for long, lightly loaded beams can be increased somewhat S S S S S S by removing material from the web, even though the shear capacity will be reduced. T T T T T S S S S S S S T T T T T T Web Flanges Hallow web beam Beam A beam is a member used to resist a load acting across its longitudinal axis by transferring the effect over a distance between supports – referred to as the span. Arch The arch can be shaped such that, for a particular loading, all sections of the arch are under simple compression with no bending. Arches exert vertical and horizontal thrusts on their supports, which can prove troublesome in the design of supporting walls. This problem of horizontal thrust can be eliminated by connecting a tension member between the support points. n Spa Simple arch Built - in ends TIE Deflection shape BOW STRING ARCH Simply supported Frames Plane frames are also made up of beams and columns, the only difference being that they are rigidly connected at the joints. Internal forces at any cross-section of the plane frame member are: bending moment, shear force Cantilever and axial force. 110 Rural structures in the tropics: design and development y C x x y Moment of inertia The area moment of inertia (I), or to use the more PROPERTIES OF STRuCTuRAL SECTIOnS correct term, second moment of area, is a property that When designing beams in bending, columns in measures the distribution of area around a particular buckling, etc., it is necessary to refer to a number of axis of a cross-section, and is an important factor in its basic geometrical properties of the cross-sections of resistance to bending. Other factors, such as the strength structural members. of the material from which a beam is made, are also important for resistance to bending, and are allowed for Area in other ways. The moment of inertia measures only Cross-section areas (A) are generally calculated in how the geometric properties or shape of a section affect square millimetres, because the dimensions of most its value as a beam or slender column. The best shape structural members are given in millimetres, and values for a section is one that has the greater part of its area as for design stresses found in tables are usually given in distant as possible from its centroidal, neutral axis. Newtons per millimetre square (N / mm²). For design purposes, it is necessary to use the moment of inertia of a section about the relevant axis Centre of gravity or centroid or axes. This is a point about which the area of the section is evenly distributed. Note that the centroid is sometimes Calculation of moment of inertia outside the actual cross-section of the structural element. Consider a rectangle that consists of an infinite number of strips. The moment of inertia about the x-x axis of Reference axes such a strip is the area of the strip multiplied by the It is usual to consider the reference axes of structural square of the perpendicular distance from its centroid sections as those passing through the centroid. In general, to the x-x axis, i.e. b × ∆y × y2 the x-x axis is drawn perpendicular to the greatest lateral dimension of the section, and the y-y axis is drawn perpendicular to the x-x axis, intersecting it at the centroid. b ∆y y d/2 y C x x x x d/2 y y The sum of all such products is the moment of inertia about the x-x axis for the whole cross-section. By applying calculus and integrating as follows, the exact value for the moment of inertia can be obtained. C x x +d 2 bd3 I = ∫ by2 xx dy = −d 2 12 y πD4 Ixx = 64 +d 2 bd3 I = ∫ by2 xx dy = Chapter 6 – B −d 2 1a2sic mechanics 111 For a circular cross-section: and from the principle of parallel axes, the Ixx of one flange equals: πD4 Ixx = (7.2 × 106) + (86 × 100 × 2002) = 351.2 × 106 mm4 64 Thus the total Ixx of the web plus two flanges equals: Moments of inertia for other cross-sections are given later and in Table 4.3. For structural rolled-steel sections, the Ixx = (22.5 × 106) + (351.2 × 106) + (351.2 × 106) moment of inertia can be found tabulated in handbooks. = 725 × 106 mm4 Some examples are given in Appendix V.3. The Iyy of the above beam section is most easily found Principle of parallel axes by adding the Iyy of the three rectangles of which According to the principle of parallel axes, if the it consists, because the y-y axis is their common moment of inertia of any area (e.g. top flange of the neutral axis, and moments of inertia may be added or beam shown below) about any axis is parallel to its subtracted if they are related to the same axis. centroidal axis, then the product of the area of the shape and the square of the perpendicular distance between the axes must be added to the moment of inertia about 100 300 100 the centroidal axis of that shape. Example 6.13 y Determine the moment of inertia about the x-x axis and the y-y axis for the I-beam shown in the figure. The beam has a web of 10 mm plywood and the flanges are made of 38 mm by 100 mm timber, which are nailed and glued to the plywood web. 100 × 863 300 × 103 Iyy = 2 × + 12 12 = 2 × 5.3 × 106 + 0.025 × 106 Timber 38x100 86 = 10.6 × 106 mm4 F F Section modulus In problems involving bending stresses in beams, a property called section modulus (Z) is useful. It is the Plywood x x ratio of the moment of inertia (I) about the neutral axis 10 of the section to the distance (C) from the neutral axis to the edge of the section. Unsymmetrical cross-sections Sections for which a centroidal reference axis is not an axis of symmetry will have two section moduli for that axis. Solution: I I The entire cross-section of both the beam and the Zxx1 = xx ; Z = xx y xx2 cross-section of the web have their centroids on the x-x 1 y2 axis, which is therefore their centroidal axis. Similarly, the F-F axis is the centroidal axis for the top flange. y bd3 10 × 3003 bd3 10 × 3003 = = 22.5 × 106 mm4 Ixx of the web us1in2g =1 2 = 22.5 × 106 mm4 y1 12 12 The moment of inertia of one flange about its own x x y2 centroidal axis (F-F): 86 × 1003 y IFF of one flange = 86 × 1003 = 7.2 × 106 = 7.2 ×m1m064mm4 12 12 500 300 100 200 10 86 112 Rural structures in the tropics: design and development TABLE 6.3 Properties of structural sections Section Area Moment Section Radius Distance from (mm2) of inertia modulus of gyration extreme fibre or (m2) (mm4) or (m4) (mm3) or (m3) (mm) or (m) to centroid (mm) or (m) A Ixx Iyy Zxx Zyy rxx ryy y x Rectangle bd bd 3 db3 bd 2 db d b d b y = 12 12 6 6 12 12 2 d b d y = G d 12 12 2 y a Square a2 a4 a4 a3 a3 a a a y = x = 12 12 6 6 12 12 2 y y a x G x y x y Square a2 a4 a4 a3 a3 a a a y = x = a with 12 12 6 2 6 2 12 12 2 y diagonal x G x axes y x y Circle π D2 x π D4 π D4 π D3 π D3 D D D y = 4 64 64 32 32 4 4 2 D x = x G x 2 y y y Annulus π (D2 − d2) π (D4 − d4) π (D4 − d4) π (D3 − d3) π (D3 − d3) D x 4 (D2 + d2) 4 (D2 + d2) y = 4 64 32 32 32 2 y D x = d x G x 2 y D Radius of gyration I I I I Radius of gyration (r) is the property of a cross-section Thereforrex,x r=xx = xx xx and r r= = yy yy that measures the distribution of the area of the cross- A A yy yy A A section in relation to the axis. In structural design, it is used in relation to the length of compression (general relationship I = Ar2) members, such as columns and struts, to estimate their slenderness ratio and hence their tendency to buckle. Slender compression members tend to buckle about the axis for which the radius of gyration is a minimum value. From the equations, it will be seen that the least radius of gyration is related to the axis about which the least moment of inertia occurs. Chapter 6 – Basic mechanics 113 REVIEW quESTIOnS FuRTHER REAdInG 1. Sketch the shear and bending moment diagrams Al Nageim, H., Durka, F., Morgan, W. & Williams for the beams below, indicating values of shear D.T. 2010. Structural mechanics: loads, analysis, force and bending moment at the key points. materials and design of structural elements. 7th edition. London, Pearson Education. a) Nelson, G.L., Manbeck, H.B. & Meador, N.F. 1988. Light agricultural and industrial structures: analysis and design. AVI Book Co. Prasad, I.B. 2000. A text book of strength of materials. 20th edition. 2B, Nath Market, Nai Sarak, Delhi, Khanna Publishers. Roy, S.K & Chakrabarty, S. 2009. Fundamentals of structural analysis with computer analysis and b) W N/m applications. Ram Nagar, New Delhi, S. Chand and Company Ltd. Salvadori, M. & Heller, R. 1986. Structure in architecture: the building of buildings. 3rd edition. Englewood Cliffs, New Jersey, Prentice-Hall. 2 1 Whitlow, R. 1973. Materials and structures. New York, 3 L 3 L Longman. 2. Find the reactions on beam BC. A 2 3 60˚ 60˚ 1 B C 4 5 P L/2 L/2 3. Two concentrated loads of 100 kN and 200 kN advance along a girder with a 20-metre span, the distance between the loads being 8 metres. Find the position of the section that has to support the greatest bending moment, and calculate the value of the bending moment. 4. A load of 100  kN, followed by another load of 50  kN, at a distance of 10  metres, advances across a girder with a 100-metre span. Obtain an expression for the maximum bending moment at a section of the girder at a distance of z metres from an abutment. 115 Chapter 7 Structural design InTROduCTIOn thrust, shear, bending moments and twisting moments), Structural design is the methodical investigation of the as well as stress intensities, strain, deflection and stability, strength and rigidity of structures. The basic reactions produced by loads, changes in temperature, objective in structural analysis and design is to produce shrinkage, creep and other design conditions. Finally a structure capable of resisting all applied loads without comes the proportioning and selection of materials for failure during its intended life. The primary purpose the members and connections to respond adequately to of a structure is to transmit or support loads. If the the effects produced by the design conditions. structure is improperly designed or fabricated, or if the The criteria used to judge whether particular actual applied loads exceed the design specifications, proportions will result in the desired behavior reflect the device will probably fail to perform its intended accumulated knowledge based on field and model tests, function, with possible serious consequences. A well- and practical experience. Intuition and judgment are engineered structure greatly minimizes the possibility also important to this process. of costly failures. The traditional basis of design called elastic design is based on allowable stress intensities which are chosen Structural design process in accordance with the concept that stress or strain A structural design project may be divided into three corresponds to the yield point of the material and should phases, i.e. planning, design and construction. not be exceeded at the most highly stressed points of Planning: This phase involves consideration of the the structure, the selection of failure due to fatigue, various requirements and factors affecting the general buckling or brittle fracture or by consideration of the layout and dimensions of the structure and results in permissible deflection of the structure. The allowable the choice of one or perhaps several alternative types – stress method has the important disadvantage in that of structure, which offer the best general solution. The it does not provide a uniform overload capacity for all primary consideration is the function of the structure. parts and all types of structures. Secondary considerations such as aesthetics, sociology, The newer approach of design is called the strength law, economics and the environment may also be design in reinforced concrete literature and plastic design taken into account. In addition there are structural and in steel-design literature. The anticipated service loading constructional requirements and limitations, which is first multiplied by a suitable load factor, the magnitude may affect the type of structure to be designed. of which depends upon uncertainty of the loading, the Design: This phase involves a detailed consideration possibility of it changing during the life of the structure of the alternative solutions defined in the planning phase and for a combination of loadings, the likelihood, and results in the determination of the most suitable frequency, and duration of the particular combination. In proportions, dimensions and details of the structural this approach for reinforced-concrete design, theoretical elements and connections for constructing each capacity of a structural element is reduced by a capacity- alternative structural arrangement being considered. reduction factor to provide for small adverse variations Construction: This phase involves mobilization of in material strengths, workmanship and dimensions. personnel; procurement of materials and equipment, The structure is then proportioned so that depending including their transportation to the site, and actual on the governing conditions, the increased load cause on-site erection. During this phase, some redesign fatigue or buckling or a brittle-facture or just produce may be required if unforeseen difficulties occur, such yielding at one internal section or sections or cause as unavailability of specified materials or foundation elastic-plastic displacement of the structure or cause the problems. entire structure to be on the point of collapse. Philosophy of designing design aids The structural design of any structure first involves The design of any structure requires many detailed establishing the loading and other design conditions, computations. Some of these are of a routine nature. which must be supported by the structure and therefore An example is the computation of allowable bending must be considered in its design. This is followed by the moments for standard sized, species and grades of analysis and computation of internal gross forces, (i.e. dimension timber. The rapid development of the 116 Rural structures in the tropics: design and development computer in the last decade has resulted in rapid adoption of Computer Structural Design Software that has now replaced the manual computation. This has greatly reduced the complexity of the analysis and T1 T2 h1 design process as well as reducing the amount of time required to finish a project. 60° Standard construction and assembly methods have A evolved through experience and need for uniformity in the construction industry. These have resulted in standard details and standard components for building if P=100N construction published in handbooks or guides. P T1=T2=58 design codes Many countries have their own structural design codes, T2 codes of practice or technical documents which perform a similar function. It is necessary for a designer to become familiar with local requirements or recommendations in regard to correct practice. In this chapter some examples are given, occasionally in a simplified form, in order to demonstrate procedures. They should not be assumed T1 to apply to all areas or situations. dESIGn OF MEMBERS In dIRECT TEnSIOn And COMPRESSIOn P Tensile systems Tensile systems allow maximum use of the material FORCE DIAGRAM FOR POINT A because every fibre of the cross-section can be extended to resist the applied loads up to any allowable stress. As with other structural systems, tensile systems require depth to transfer loads economically across a span. As the sag (h) decreases, the tensions in the cable (T1 and T2) increase. Further decreases in the sag would again increase the magnitudes of T1 and T2 h until the ultimate condition, an infinite force, would be 2 T required to transfer a vertical load across a cable that is 1 T 120º 2 horizontal (obviously an impossibility). A A distinguishing feature of tensile systems is that vertical loads produce both vertical and horizontal reactions. As cables cannot resist bending or shear, they transfer all loads in tension along their lengths. P The connection of a cable to its supports acts as a pin joint (hinge), with the result that the reaction (R) must be exactly equal and opposite to the tension in the cable (T). The R can be resolved into the vertical and horizontal directions producing the forces V and H. The horizontal reaction (H) is known as the thrust. The values of the components of the reactions can be obtained by using the conditions of static equilibrium if P=100N and resolving the cable tensions into vertical and then horizontal components at the support points. T1=T2=100N Example 7.1 Two identical ropes support a load P of 5 kN, as shown in the figure. Calculate the required diameter of the rope, if its ultimate strength is 30  MPa and a safety factor of 4.0 is applied. Also determine the horizontal FORCE DIAGRAM FOR POINT A support reaction at B. Chapter 7 – Structural design 117 60° At support B, the reaction is composed of two components: 30° B Bv = T2 sin 30° = 2.5 sin 30° = 1.25 kN A BH = T2 cos 30° = 2.5 cos 30° = 2.17 kN Short columns A column which is short (i.e. the height is small P=5kN compared with the cross-section area) is likely to fail because of crushing of the material. Note, however, that slender columns, which are tall T1 T2 compared with the cross-section area, are more likely to fail from buckling under a load much smaller than that needed to cause failure from crushing. Buckling is dealt with later. P Free body diagram Short columns T= 4.3 kN 5 kN T2= 2.5 kN The allowable stress in the rope is 30 = 7.5 N3/0mm2 = 7.5 MPa 4 30 = 7.5 N/mm2 = 7.5 MPa = 47.5 N/mm2 = 7.5 MPa 4 Force Stress = Area Force StresFs o=r ce Slender columns Stress = A rea Area Therefore: Example 7.2 4.3 × 103 Area required = = 573 mm2 A square concrete column, which is 0.5 m high, is made 4.3 × 103 Area requ7.i5re4d.3=× 103 = 573 mm2 of a nominal concrete mix of 1:2:4, with a permissible Area required = 7.5= 573 mm2 π π d2 direct compression stress of 5.3 MPa (N / mm²). What A= r2 = 7.5 4 π π d2 is the required cross-section area if the column is A= r2 = required to carry an axial load of 300 kN? 4 Thus: F 300 000 N A= = = 56 604 mm2 σ 5.3 N/mm2 4 × 573 d = = 27 mm (min) 4π × 573 d = = 27 mm (min) i.e. the column should be minimum 238 mm square. π 118 Rural structures in the tropics: design and development dESIGn OF SIMPLE BEAMS Bending stresses C C When a sponge is put across two supports and gently pressed downwards between the supports, the pores N A h at the top will close, indicating compression, and the pores at the bottom will open wider, indicating tension. T T Similarly, a beam of any elastic material, such as wood or steel, will produce a change in shape when external loads are acting on it. The moment caused by the external loads acting on the beam will be resisted by the moment of this internal couple. Therefore: M = MR = C (or T) × h where: M = the external moment MR = the internal resisting moment C = resultant of all compressive forces on the cross- section of the beam T = resultant of all tensile forces on the cross-section of the beam h = lever arm of the reaction couple Now consider a small element with the area (R) at a distance (a) from the neutral axis (NA). Compression fc Tension fa a ymax N A Figure 7.1 Bending effects on beams The stresses will vary from maximum compression at the top to maximum tension at the bottom. Where ft the stress changes from compressive to tensile, there will be one layer that remains unstressed and this is called the neutral layer or the neutral axis (NA). Note that it is common practice to use the symbol f This is why beams with an I-section are so effective. for bending stress, rather than the more general symbol. The main part of the material is concentrated in Maximum compressive stress (fc) is assumed to occur in the flanges, away from the neutral axis. Hence, the this case at the top of the beam. Therefore, by similar maximum stresses occur where there is maximum triangles, the stress in the chosen element is: material to resist them. If the material is assumed to be elastic, then the fa f f stress distribution can be represented by two triangular = c , fa = a × c a ymax y shapes with the line of action of the resultant force of max each triangle of stress at its centroid. The couple produced by the compression and As force  = stress × area, then the force on the tension triangles of stress is the internal-reaction couple element = fa × R = a × (fc / ymax) × R of the beam section. The resisting moment of the small element is: force × distance (a)  = a × (fc / ymax) × R × a  = Ra2 × (fc / ymax) Chapter 7 – Structural design 119 The total resisting moment of all such small elements C in the cross-section is: N A h MR = ∑ Ra2 × (fc / ymax) T But ∑ Ra2  = I, the moment of inertia about the neutral axis, and therefore Reinforced-concrete T-beams MR = I × (fc / ymax) As the section modulus Zc = I / ymax, therefore In summary the following equation is used to test for safe bending: MR = fc × Zc = M; fw ≥ f = Mmax / Z Similarly where: MR = ft × Zt = M fw = allowable bending stress f = actual bending stress The maximum compressive stress (fc) will occur in Mmax = maximum bending moment the cross-section area of the beam where the bending Z = section modulus moment (M) is greatest. A size and shape of cross- section, i.e. its section modulus (Z), must be selected so Horizontal shear that the fc does not exceed an allowable value. Allowable The horizontal shear force (Q) at a given cross-section working stress values can be found in building codes or in a beam induces a shearing stress that acts tangentially engineering handbooks. to the horizontal cross-sectional plane. The average As the following diagrams show, the concept of value of this shear stress is: a ‘resisting’ couple can be seen in many structural members and systems. τ Q = A C where A is the transverse cross-sectional area. N A h This average value is used when designing rivets, bolts T and welded joints. The existence of such a horizontal stress can be Rectangular beams illustrated by bending a paper pad. The papers will slide relative to each other, but in a beam this is prevented by C the developed shear stress. N A h T Girders and I –beams (1/6 web area can be added to each flange area for moment resistance) Sliding of layers N A C h T No sliding of layers Rectangular reinforced-concrete beams (note that the steel bars are assumed to carry all the tensile forces). Figure 7.2 Shearing effects on beams 120 Rural structures in the tropics: design and development τ 3Q 3Q Q max = = = 1.5 2bd 2A A τ 3Q 3Q Q max = = = 1.5 However, the shear stresses are not equal across the 2bd 2A A cross-section. At the top and bottom edge of the beam For rectangular sections τ 3Q 3Q Q max = = = 1.5 they must be zero, because no horizontal shear stresses τ 3Q 2bd Q2A A max = = 1.5 can develop. τ 3Q 3Q 2a2Q A max = = = 1.5 2bd 2A A Q For square sections τ 3Q If the shear stresses at a certain distance from max = = 1.5 the neutral axis are considered, their value can be 2a2 A τ 3Q Q determined according to the following formula: max = = 1.5 τ 16Q 2a42Q A For circular sections τ 3Q max = Q = 3πD2 3A max = = 1.5 2a2 A16Q 4Q τ Q×∆A× y = τmax = = I×b 3πD2 3A For I-shaped sections of steel beams, a convenient approximation is to assume τ 16Q 4Q τ thaQt all shearing resistance where: is afforded by the τ 1 6wQeb p4lu max≈ max = = 3πD2 3A Qs thde ×ptart of the flanges that τ = shear stress forms a contminaxu=a = 3tiπoDn 2of τt3hAe weQb. max≈ Q = shear force d×t ∆A = area for the part of the section being sheared off Thus: τ Q max≈ y = perpendicular distance from the centroid of PA to d×t For I-sections τ Q the neutral axis max≈ I = moment of inertia for the entire cross-section d×t b = width of the section at the place where shear stress where: is being calculated. d = depth of beam t = thickness of web y If timber and steel beams with spans normally used in buildings are made large enough to resist the tensile and compressive stresses caused by bending, they are usually strong enough to resist the horizontal shear stresses also. However, the size or strength of short, heavily loaded timber beams may be limited by these stresses. x G x deflection of beams Excessive deflections are unacceptable in building y construction, as they can cause cracking of plaster in ceilings and can result in jamming of doors and windows. Most building codes limit the amount of Centroid ∆A for area ∆A b allowable deflection as a proportion of the member’s length, i.e. 1/180, 1/240 or 1/360 of the length. y For standard cases of loading, the deflection formulae can be expressed as: WL3 δmax = Kc × EI Q where: δmax = maxiMmum deflection (mm) Kfc w= ≥cofn=stant mdaxepending on the type of loading and the end supZport conditions W = total load (N) L = effective span (mm) Maximum horizontal shear force in beams E = modulus of elasticity (N/mm²) It can be shown that the maximum shear stress τmax in a I = moment of inertia (mm4) beam will occur at the neutral axis. Thus, the following relations for the maximum shear stress in beams of It can be seen that deflection is greatly influenced different shapes can be deduced, assuming the maximum by the span L, and that the best resistance is provided shear force (Q) to be the end reaction at a beam support by beams which have the most depth (d), resulting in a (column). large moment of inertia. Mmax f = max I y NA max INA = Z ymax 3 ×Q 3Q τ max w≥ τ = max = 2 ×A 2bd Chapter 7 – Structural design 121 4 × Qmax 16Q τ τ = max w≥ = 3 × A 3πd2 Note that the effective span is greater than the clear For I-shaped cross-sections of steel beams span. It is convenient to use the centre to centre distance of the supports as an approximation of the effective span. Q Some standard cases of loading and resulting τw≥ τ = max deflection for beams can be found later in this section. A design criteria where: The design of beams is dependent upon the following τw = allowable shear stress factors: τ = actual shear stress 1. Magnitude and type of loading Qmax = maximum shear force 2. Duration of loading A = cross-section area 3. Clear span 4. Material of the beam Like allowable bending stress, allowable shear stress 5. Shape of the beam cross-section varies for different materials and can be obtained from a building code. Maximum shear force is obtained from WL3 δBeams are designed using the following formulae: the shear-force diagram. max = Kc × EI 1. Bending stress 3. Deflection In addition, limitations are sometimes placed on M maximum deflection of the beam (δmax): f max w ≥ f = Z δ WL3 max = Kc × where: EI fw = allowable bending stress f = actual bending stress Mmax = maximum bending moment Example 7.3 Z = section modulus Consider a floor where beams are spaced at 1 200 mm and have a span of 4 000 mm. The beams are seasoned This relationship derives from simple beam theory and cypress with the following properties: Mmax f M = max fw = 8.0 N/mm², τw = 0.7 MPa (N/mm²), E = 8.400 MPa I max y f NA = mamxax I y (N/mm²), density 500 kg/m³ NA max M Loading on floor and including floor is 2.5 kPa. max f andM = max f max I = ymax I NA y max Allowable deflection is L/240 NA max INA I = Z ymaNx A = Z 1∙2 m ymax The mIaximum bending stress will be found in the section IofN Ath=e ZyNA beam where the maximum bending moment ocmcax= Z ymax urs. The maximum moment can be obtained from the bending-moment diagram. 3 ×Q τ ma 3Qmax w≥ τ = x = 2. S23h××eaAQr stress2bQτ max 3 dma w≥ τ = = x For 2re×ctAangular2 bcdross-sections: 3 ×Q 3Q τ max w≥ τ =3 ×Q max = τ = max 3Q w≥ τ 2 ×A = 2bmdax 2 ×A 2bd 4 × Q Q τ ≥ τ = max 16 1∙2 m max w 4 × Q = τ 3 × A max 31π6dQτ 2 w≥ =For circula=r crossm-saxections: 3 × A 3πd2 4 × Qmax 16Q τ τ = max w≥ =4 × Q 1∙2 m 1∙2 m 1∙2 m 1∙2 m 1∙2 m τw≥ τ = max 16Q 3 × A = 3πmda2x 3 × A 3πd2 Q τw≥ τ = max Q τ ≥ τ = A max w A Q τw≥ τ =Q max τw≥ τ = mAax A 4 m 122 Rural structures in the tropics: design and development (i) Beam loading: w = 1.2 m × 2.5 kN/m2 = 3 kN/m Choose a 100 mm by 225  mm timber. The timber required is a little less than that assumed. No Assume a 100 mm by 250  mm cross-section for the recalcul6atZions a6re× r0e.7q8ui×re1d0 6unless it is estimated that a beams. smda=ller size= timber w = 216 mm b 6Z 6 ×10o00uld be adequate if a smaller size .78 × 106 had bde=en assum= ed initially. = 216 mm (ii) Beam mass = 0.1 × 0.25 × 500 × 9.81 = 122.6 N/m b 100 6Z 6 × 06.Z78 × 1066× 0.78 × 106 = 0.12 kN/m d = =vdi) =Check f=or she=ar2 l1o6adminmg: = 216 mm b b100 100 Total w = 3 + 0.12 = 3.12 kN/m 3Q τ max 3 × 6.24 × 103 = = = 0.42 MPa (iii) Calculate reactions and draw shear-force and 2A3Q 2 × 100 ×225 τ max 3 × 6.24 × 103 bending-moment diagrams = = = 0.42 MPa 2A 2 × 100 ×225 3Q 3Q τ max τA3 × 6.24 × 033× 6.24 × 103 = = =s the msaaxfe=1 load for the tim = 0.42 MPa=b0e.4r 2isM 0.P7a N/mm² (MPa) the 2A se2c×ti1o20nA0 is× a2d2e52q×u1a0te0 i×n2 r2e5sistance to horizontal shear. W= 3∙12 kN/m vii) Check deflectio δ −n5 to eWnsLu3re that it is less than 1/240 of the span (fromma xT=ab × 3l8e 47.1) 4 m − 5 EIWL3 δmax = × 384 EI 6∙24 kN δ − 5 δWL3 − 5 WL3 max = × 384 mEax = × I 384 EI bd3 100 × 2253 wIh=ere: = = 95×106 mm4 12 bd3 10102 × 2253 SFD E = 8 400 MPa (N/mm²) - 6∙24 I = = = 95×106 mm4 12 12 bd3 100 ×b2d2353 100 × 2253 I = = I = = = 95×106 m=m945×106 mm4 12 1212 12 W = 3.12 kN/m × 4 m = 12.48 kN = 12.48 × 103 N δ L = 4− 5× 10132m.4m8 × 103 × 43 ×109 max = × = −13 mm 384 8400 × 95 × 106 δ − 5 12.48 × 103 × 43 ×109 max = × = −13 mm wl2 384 8400 × 95 × 106 BMD M max= 8 = 6∙24 kN δ − 5 δ12.48 ×−1503 ×124.348××101903 × 43 ×109 max = × max = × = −13 mm = −13 mm 384 8403084× 95 ×814006 0 × 95 × 106 The allowable deflection, 400/240 = 16.7 >13. The beam is therefore satisfactory. iii) Calculate maximum bending moment (Mmax) using the equation for a simple beam, uniformly loaded (see Bending moments caused by askew loads Table 7.1) If the beam is loaded so that the resulting bending moment is not about one of the main axes, the moment has to be resolved into components acting about the main wL2 3.12 × 42 MwL2 = 3.12 =× 42 max = 6.24 kNm= 6.24 ×106/ Nmm axes. The stresses are then calculated separately relative Mmax = = 8 =86.24 kNm= 6.24 ×106/ Nmm 8 8 to each axis and the total stress is found by adding the stresses caused by the components of the moment. iv) Find the required section modulus (Z) Example 7.4 Mmax 6.24 × 106 MZ = 6.24 ×=106 = 0.78 × 6 Design a timber purlin that will span rafters 2.4 m on req 10 mm3 Z max 0.78 6 req = = fw =8 × 10 mm3 centre. The angle of the roof slope is 30° and the purlin fw 8 will support a vertical dead load of 250  N/m and a wind load of 200 N/m acting normal to the roof. The v) Find a suitable beam depth, assuming 100  mm allowable bending stress (fw) for the timber used is 8 breadths: MPa. The timber density is 600 kg/m³. From Table 6.3, the section modulus for a rectangular 1. Assume a purlin cross-section size of 50 mm × shape is Z = 1/6 × bd2 125 mm. Find an estimated self-load. W = 0.05 × 0.125 × 600 × 9.81 = 37 N/m 6Z 6 × 0.78 × 106 d = = = 216 mm b 100 The total dead load becomes 250 + 37 = 287 N/m 3Q τ 3 × 6.24 × 103 = max = = 0.42 MPa 2A 2 × 100 ×225 δ − 5 WL3 max = × 384 EI bd3 100 × 2253 I = = = 95×106 mm4 12 12 δ − 5 12.48 × 103 × 43 ×109 max = × = −13 mm 384 8400 × 95 × 106 M ax x M f = m + max z ≤ f Zx Z w y M x M f = max + max z ≤ f Zx Z w y M f = max x M + max z ≤ f Z w bx Z d2 50y× 1252 Zx = = = 130×103 mm3 6 6 bd2 50 × 1252 Z 3 x = = = 130×103 mm 6 6 bd2 50 × 1252 Zx = = = 130×103 mm3 6 6 bd2 125× 502 Zy = = = 52×103 mm3 6 6 bd2 125× 502 Zy = = = 52×103 mm3 6 6 bd2 125× 502 Zy =323×1=03 103×10=3 52×103 mm3 f = 6 + 6 = 2.5 + 2 = 4.5 N/mm2 = 4.5 MPa 130×103 52×103 323×103 103×103 f = + = 2.5 + 2 = 4.5 N/mm2 = 4.5 MPa Chapter 7 – Structural design 130×103 52×103 123 323×103 103×103 f = + = 2.5 + 2 = 4.5 N/mm2 = 4.5 MPa 130×103 52×103 bd2 50 × 1002 Zx = = = 83×103 mm3 6 6 2. Find the components of the loads relative to the This size is safe, but a smaller size may be satisfactory. main axes. Try 5b0 dm2 m ×5 01×001 0m0m2 . Zx = = = 83×103 mm3 6 6 Wx = 200 N/m + 287 N/m × cos 30° = 448.5 N/m bd2 50 ×b1d2002100× 502 Z Z x = = y = = = 83×10=3 m42m×3103 mm3 W 6 66 6 y = 287 N/m × sin 30° = 143.5 N/m bd2 100× 502 Zy = = = 42×103 mm3 W 0 N/m W1=250 N/m 6 6 2=20 2b3d 2 ×1 1000×35×0 2 Z 103 f y==3 =03 1 = 42×103 mm3 6 + = 3.9 + 2.5 = 6.4 N/mm2 = 6.4 MPa 83×103 462×103 y x 323×103 This 1i0s 3m×1u03 f = + ch =c3lo.9se+r 2t.o5 =th6e.4 aNllo/wmamb2le= s6t.r4esMs.P Tao save 83×10m3 one4y2, ×501 0m3 m × 75  mm should also be tried. In this x 323×10c3ase 1f 0>3 f×w1 a0n3d therefore 50 mm × 100 mm is chosen. f = + = 3.9 + 2.5 = 6.4 N/mm2 = 6.4 MPa 83×103 42×103 y universal steel beams Steel beams of various cross-sectional shapes are commercially available. Even though the properties of their cross-sections can be calculated with the formulae given in the section ‘Design of members in direct WL wL2 tension and compression’, it is easier to obtain them Mmax = = 8 8 from handbook tables. These tables will also take into consideration the effect of rounded edges, welds, etc. 3. Calculate the bending moments about each axis for Sections of steel beams are indicated with a a uniformly dMistribuWtedL loawdL. 2The purlin is assumed combination of letters and a number, where the letters to be a simple bm = = eaxam. 8 8 represent the shape of the section and the number represents the dimension, usually the height, of the wx × L2 448.5× 2.42 M section in millimetres, e.g. IPE 100. In the case of HE max x =WL wL=2 = 323×103 Nmm M 8= 8 sections, the number is followed by a letter indicating max = 8 8 the thickness of the web and flanges, e.g. HE 180B. M f = max x Mmax z f An example of an alternative method of notation is wx × L2 448.Z5× 2+.42 ≤ w M x Z=y323×103 Nmm 305 × 102 UB 25, i.e. a 305 mm by 102 mm universal max x = = 8 8 beam weighing 25 kg/m. The following example demonstrates another w M× L2 y 14M3.m5a×x z2.42 w × L2Mm4a4x 8y .=f = max 5× 2.42 = x + ≤ f = 103×103 Nmm method of taking into account the self-weight of the M x 8 Z w max x = = =x 323×Z1083 y Nmm structural member being designed. 8 8 4. The awctu×alL bs2dtr2ess in the t22imber must be no greater Example 7.5 M y 14530.5××122.54 mtahx yan= thZex a=llow=a=ble st8ress. == 113003××110033mNmm3 m 8 Design a steel beam, to be used as a lintel over a door 6 6 M opening, which is required to span 4.0  m between f = max x M + max z ≤ f MZ Z w M centres of simple supports. The beam will be carrying w × L2 f =14b3.dm52a×x x2+.5420 ×m x 1ax2 z5≤2 f y a 220  mm thick and 2.2  m high brick wall, weighing M y Z max y = Z= y ×10 x = x = Z= 103 =w 133N0×m1m03 mm3 8 6 8 6 20 kN/m³. Allowable bending stress is 165 MPa. Uniformly distributed load caused by brickwork is 5. Try the assumed purlin size of 50 × 125 mm. 0.22 × 2.2 × 4.0 × 20 = 38.7 kN. Assumed self-weight for the beam is 1.5 kN. bd2 125× 502 e triangular load distribution for bricks above bd2 Zy 5=0 × 12=52 = 52×103 mm3 (Note: th Z = = 6 = 16 3 the lintel would result in a slightly lower load value). x 30×103 mm bd2 506× 1252 6 Z 3 WL 40.2 ×m4l.y0 distributed load W = 38.7 + 1.5 = x = = = 130×10 mm3 Total unifor M 6 ma4x = = = 20.1 kNm = 20.1×106 Nmm 0.2 k8N bd26 125× 502 8 Zy = = = 52×103 mm3 6 6 WL 40.2 × 4.0 Mmax = = = 20.1 kNm = 20.1×106 Nmm 8 8 323×103 103×103 f = + = 2.5 + 2 = 4.5 N/mm2 = 4.5 MPa 130×103 52×103 20.1× 106 Zreq = = 0.122 × 106 mm3 = 122 cm2 bd2 125× 502 165 Zy = = = 52×103 mm3 323×b1d02 3 11 20.1× 106 = 20653××510023 6 f Z 2. Zreq = = 0.122 × 106 mm3 = 122 cm2 y = =+ = 525×+1203=m4.m5 3N/mm2 = 4.5 MPa 130×6103 526×103 165 bd2 50 × 1002 Zx = = = 83×103 mm3 6 6 323×103 103×103 f = + = 2.5 + 2 = 4.5 N/mm2 = 4.5 MPa 323×1013301×0130×3 10b35d2×15003 × 1002 f = + Zx = = 2=.5 + 2 = 4.5 N= 8/m3×m120=3 m4.5mM3 Pa 130×103 52×1036 6 bd2 100× 502 Zy = = = 42×103 mm3 6 6 bd22 1500= 0××1500022 ZZ 3 mm33 bd2yx = == ==4823××11003 5066× 1002 66 Z 3 x = = = 83×103 mm 6 323×1603 103×103 f = + = 3.9 + 2.5 = 6.4 N/mm2 = 6.4 MPa 83×103 42×103 bd2 100× 502 323×10Z3 y =103×1=03 = 42×103 mm3 f = bd2 + 1060× 502 = 36.9 + 2.5 = 6.4 N/mm2 = 6.4 MPa Z83×103 2×103 y = = 4 = 42×103 mm3 6 6 323×103 103×103 f = + = 3.9 + 2.5 = 6.4 N/mm2 = 6.4 MPa 323×10383×110033×10342×103 f = + = 3.9 + 2.5 = 6.4 N/mm2 = 6.4 MPa 83×103 42×103 124 Rural structures in the tropics: design and development Suitable sections as found in a handbook would be: Although the total value of the load has increased, the maximum shear force remains the same but the Section Zx-x Mass maximum bending is reduced when the beam is INP 160 117 cm³ 17.9 kg/m cantilevered over the supports. IPE 180 146 cm³ 18.8 kg/m HE 140A 155 cm³ 24.7 kg/m HE 120A 144 cm³ 26.7 kg/m Choose INP 160 because it is closest to the required section modulus and has the lowest weight. Then recalculate the required Z using the INP 160 weight: 4.0 × 17.9 × 9.81 = 702 N, which is less than the assumed Continuous beam self-weight of 1.5  kN. A recheck on the required Z reveals a value of 119 cm³, which is close enough. Continuous beams A single continuous beam extending over a number of supports will safely carry a greater load than a series of simple beams extending from support to support. Consider the shear force and bending moment diagrams for the following two beam loadings: Simple beam 8 m 5 kN/m Although continuous beams are statically indeterminate and the calculations are complex, WL BM = approximate values 20 kN 20 kN can be found with simplified equati6ons. Conservative +20 equations for two situations are as follows: WL Load concentrated between supports: BM = 6 WL Load uniformly distributed: BM = 12 M max = 40 kNm It is best to treat the two end sections as simple beams. WL STAndARd CASES OF BEAM LOAdBMInG= 12 A number of beam loading cases occur frequently and it is useful to have standard expressions available for them. Several of these cases will be found in Table 7.1. 2 m 8 m 2 m 5 kN/m COMPOSITE BEAMS In small-scale buildings the spans are relatively small and, with normal loading, solid rectangular or square sections are generally the most economical beams. However, where members larger than the available sizes 30 kN/m 30 kN/m +20 and/or length of solid timber are required, one of the +10 following combinations may be chosen: 1. Arranging several pieces of timber or steel into a structural frame or truss. 2. Universal steel beams. -10 3. Built-up timber sections with the beam members -20 nailed, glued or bolted together into a solid M max = 30 kNm section, or with the beam members spaced apart and only connected at intervals. 4. Strengthening the solid timber section by the addition of steel plates to form a ‘flitch-beam’. 5. Plywood web beams with one or several webs. -10 -10 6. Reinforced-concrete beams. Chapter 7 – Structural design 125 TABLE 7.1 Beam equations Loading diagram Shear force at x: Qx Bending moment at x: Mx deflection at x: δx W Wb Wab QA = M L c = L a b When a = b Wa2b2 L δ c = 3EIL A B Wa WL a + b = L QB = - M L c = 4 Total W =wL W WL 5WL3 QA = M 2 max = δ max = 8 384EI x W L L QB = - at x = at x = A L B 2 2 2 wL4 Total W = wL 2W wL 2 QA = = M = L2 3 3 max 0.064m δ max = 0.00652 EI W W wL QB = - = - at x = 0.577L A L at x = 0.519 x B 3 6 Total W = wL W wL 2 QA = = wL2 wL4 M 2 4 max = δ 12 max = 120EI W W wL L L Q A L B B = - = - at x = at x = 2 4 2 2 A W QA = QB =W MA = -WL WL3 δ B = 3EI L Total W = wL QA =W WL wL2 WL3 wL4 MA = - = - δ B = = A B QA = 0 2 2 3EI 3EI L Total W = wL 2 QA =W WL wL2 wL4 W MA = - = - δ B = A B QA = 0 3 6 30EI L W Wb Wab2 QA = MA = - L L2 A a b Wa3b3 B δ C = L Wa 3EIL Q a + b = L B = - Wa2b MB = - L L2 Total W = wL W QA = 2 WL WL3 MA = MB = - δ C = A B W 12 384EI L QB = - 2 Total W = wL 2W Q WL wL2 wL4 2 A = M 3 A = - = - δ max = 10 20 764EI W A B W Q WL wL2 B = - M 3 B = - = - at x = 0.475L x L 15 30 W W W W WL wL3 QA = Mmax = 6 * δ max = 2 L 192EI R1 R2 W W W W WL wL3 QA = Mmax = 2 12 * δ max = L 384EI R1 R2 126 Rural structures in the tropics: design and development Built-up timber beams COLuMnS When designing large members, there are advantages in Although the column is essentially a compression building up solid sections from smaller pieces because member, the manner in which it tends to fail and the these are less expensive and easier to obtain. Smaller amount of load that causes failure depend on: pieces also season properly without checking. The 1. The material of which the column is made. composite beams may be built up in ways that minimize 2. The shape of cross-section of the column. warping and permit rigid connections between columns 3. The end conditions of the column. and beams. At the same time the importance of timber defects is decreased, because the load is distributed to The first point is obvious: a steel column can carry a several pieces, not all with defects in the same area. greater load than a timber column of similar size. Columns with a large cross-section area compared with the height are likely to fail by crushing. These ‘short columns’ have been discussed earlier. Built-up solid beam Buckling of slender columns If a long, thin, flexible rod is loaded axially in compression, it will deflect a noticeable amount. This Built-up solid column phenomenon is called buckling and occurs when the stresses in the rod are still well below those required to cause a compression/shearing-type failure. Buckling is dangerous because it is sudden and, once started, is progressive. Rafte r m embers s paced apart Tie member Figure 7.3 Built-up beams and trusses Built-up solid beams are normally formed by using vertical pieces nailed or bolted together: Nailing is satisfactory for beams up to about 250  mm in depth, although these may require the use of bolts at the ends if the shear stresses are high. Simply multiplying the strength of one beam by the number of beams is satisfactory, provided that the staggered joints occur over supports. Although the buckling of a column can be compared Built-up sections with the members spaced apart with the bending of a beam, there is an important are used mainly where the forces are tensile, such as difference in that the designer can choose the axis about in the bottom chords of a truss. Where used in beams which a beam bends, but normally the column will take designed to resist bending, buckling of the individual the line of least resistance and buckle in the direction members may have to be considered if those members where the column has the least lateral unsupported have a large depth-to-width ratio. However, this can dimension. be avoided by appropriate spacing of stiffeners that As the loads on columns are never perfectly axial connect the spaced members at intervals. and the columns are not perfectly straight, there will Where the loading is heavy, the beam will require always be small bending moments induced in the considerable depth, resulting in a large section modulus column when it is compressed. to keep the stresses within the allowable limit. If There may be parts of the cross-section area where sufficient depth cannot be obtained in one member, it the sum of the compressive stresses caused by the may be achieved by combining several members, such load on the column could reach values larger than the as gluing the members together to form a laminate. allowable or even the ultimate strength of the material. Chapter 7 – Structural design 127 Therefore the allowable compressive strength δcw P is reduced by a factor kλ, which depends on the slenderness ratio and the material used. Pbw = kλ × δcw × A where: θ Rotation Pbw = allowable load with respect to buckling kλ = reduction factor, which depends on the slenderness ratio δcw = allowable compressive stress A = cross-section area of the column When the load on a column is not axial but eccentric, a bending stress is induced in the column as well as a direct compressive stress. This bending stress will need to be considered when designing the column with 2. Fixed in position but not in direction (pinned). respect to buckling. Slenderness ratio As stated earlier, the relationship between the length of the column, its lateral dimensions and the end fixity P P conditions will strongly affect the column’s resistance to buckling. An expression called the slenderness ratio has been developed to describe this relationship: λ KL l = = r r where: λ = slenderness ratio K = effective length factor whose value depends on how the ends of the column are fixed L = length of the column 3. Fixed in direction but not in position. r = radius of gyration (r = I / A) l = effective length of the column (K × L) There are four types of end condition for a column or strut: P P θ Rotation side movement 4. Fixed in position and in direction. The consideration of the two end conditions together 1. Total freedom of rotation and side movement – results in the following theoretical values for the effective like the top of a flagpole. This is the weakest end length factor (Kp is the factor usually used in practice). condition. 128 Rural structures in the tropics: design and development Columns and struts with both ends fixed in position and effectively restrained in direction would theoretically have an effective length of half the actual length. However, in practice this type of end condition is almost never perfect and therefore somewhat higher L K=1∙0 values for K are used and can be found in building codes. In fact, in order to avoid unpleasant surprises, the ends are often considered to be pinned (Kp = 1.0) even when, in reality, the ends are restrained or partially restrained in direction. The effective length can differ with respect to the different cross-sectional axes: Both ends pinned y x ly K=2∙0 ly lx One end fixed in direction and position, the other free 1. A timber strut that is restrained at the centre has only half the effective length when buckling about the y-y axis as when buckling about the x-x axis. Such a strut can therefore have a thickness of less K=0∙5 than its width. Kp=0∙65 A Both ends fixed in direction and position l d l l B 0∙7 Kp 0∙85 2. In the structural framework, the braces will reduce the effective length to l when the column A-B is buckling sideways but, as there is no bracing restricting buckling forwards and backwards, the effective length for buckling in these directions is 3l. Similarly, the bracing struts have effective lengths of One end pinned, the other fixed in direction and position 1/2 d and d respectively. Chapter 7 – Structural design 129 load of 15 kN. Allowable compressive stress (σcw) for the timber is 5.2 MPa. 15 kN Pin 3. The leg of a frame, which is pinned to the foundation, d has the effective length l = 2 L but, if the top is effectively secured for sideways movement, the effective length is reduced to l  = L. b d y 4. In a system of post and lintel where the bottom of the post is effectively held in position and secured x in direction by being cast in concrete, the effective b length l = 2 L. Axially loaded timber columns Timber columns are designed with the following 1. In this case, the end conditions for buckling about formulae: the x-x axis are not the same as about the y-y axis. Therefore both directions must be designed for buckling (Where the end conditions are the same, λ KL = and P = r bw kλ × δcw × A it is sufficient to check for buckling in the direction that has the least radius of gyration). Note that in some building codes a value of Find the effective length for buckling about both axes. slenderness ratio in the case of sawn timber is taken Buckling about the x-x axis, both ends pinned: as the ratio between the effective length and the least lateral width of the column l/b. lx = 1.0 × 3 000 = 3 000 mm Example 7.6 Buckling about the y-y axis, both ends fixed: Design a timber column that is 3 metres long, supported as shown in the figure and loaded with a compressive ly = 0.65 × 3 000 = 1 950 mm TABLE 7.2 Reduction factor (kλ) for stresses with respect to the slenderness ratio for wood columns Slenderness ratio l/b 2.9 5.8 8.7 11.5 14.4 17.3 20.2 23.0 26.0 28.8 34.6 40.6 46.2 52.0 l/r 10 20 30 40 50 60 70 80 90 100 120 140 160 180 kλ 1.0 1.00 0.91 0.81 0.72 0.63 0.53 0.44 0.35 0.28 0.20 0.14 0.11 0.40 b = least dimension of cross-section; r = radius of gyration L L l = 2L l = L l = 2L L = 3 000 λ l 3 000 = x x = = 83 rx 36.1 130 Rural structures in the tropics: design and development 2. Choose a trial cross-section, which should have λ lx 3 000 xl= = λ y 1 950 = 83 its largest lateral dimension resisting the buckling y = =rx 36.=1 90 gives kλy = 0.16 ad 125 rx = bou=t the a=x3is6 .w1 imthm the largest effective length. Try ry 21.7 2503 mm2 ×3 125 mm. The section properties are: Pwx = 0.41 × 5.2 × 9 375 = 19 988 N, say 20 kN d 125 rx =A d 125 r  = x = b =× d= = 5=03 ×6= .1132m65. m1=m 6 m250 mm² Pwy = 0.35 × 5.2 × 9 375 = 17 063 N, say 17 kN 2 32 32 32 3 b 50 The allowable load with respect to buckling on the ry = d 125 rx = = = = 1=4.346m.1mmm σ colF l λu y 1 950 ym=n1 5w 0i=t0h0 cross=-s9e0ction 75 mm × 125 mm is therefore 2 3 2 3 c = = = 1.6 MPa 2 3 2 3 d 125 17 AkN. 9rA y37lt5h2o1u.7gh this is bigger than the actual load, rx = = = 36.1 mm 2 b3 b2 530 50 further iterations to find the precise section to carry the ry =ry = = = = 14=.41m4.4mmm 15 kN are not necessary. 2 d32 3212352 3 r The compressive stress in the chosen cross-section x = = = 36.1 mm 2 3 b 2 3 50 will be: 3ry.λ F= lx 3 000 x =in = = 14.4 mm 2d 3t=he 2allo3w=a8b3le load with regard to buckling on bther xco5l0u3m6n.1ry = = = 1 f4o.4r mbumckling in both directions. σ F 15 000 c = = = 1.6 MPa 2 3 2 3 A 9 375 λ lx 3 000 gi x b=λ l 5x 3 000 x == 0= = 83= 83ves kλx = 0.41 (from Table 7.2) r r y = rx= 36.1 x =361.41.4 mm This is much less than the allowable compressive 2 3 2 3 λl l stress, which makes no allowance for slenderness. λ y x1 9530000 y = x == = = 13=583 gives kλy = 0.16 (from Table 7.2) rx 36.1 Axially loaded steel columns λ l ry x 3 01040.4 x = = = 83 lr λP yx l 1369.51 The allowable loads for steel columns with respect to 0 y w= λ= k y 1 950 y =λ= × σ=c × A= 13=5135 buckling can be calculated in the same manner as for Pλ rlx r 3 000 wxx= =y 0.=411 4×. 4y 51.42.= 4×8 63 250 mm² = 13 325 N timber. However, the relation between the slenderness Pwy =r 0x .1 36.1 l 6 × 5.2 × 6 250 mm² = 5 200 N ratio and the reduction factor (kλ) is slightly different, λ 125y 1 950 rx =y = ==36.1 m=m135 as seen in Table 7.3. 4. A ry 14.4 l s2 th3 λ y 1e9 a5l0lowable load with respect to buckling is y = s = = 135 rmalle Example 7.7 y 11 r 24 5. t4han the actual load, a bigger cross-section ne1e2d5r s to be chosen. Try 75 mm × 125 mm and repeat x =rxl = = 36=.13m6.m1 mm Calculate the safe load on a hollow square steel stanchion λ y 21 9350 y =st2ep3s= 2 and 3=. 135 whose external dimensions are 120 mm × 120 mm. The ry 7514.4 walls of the column are 6 mm thick and the allowable r Secy = 125= 21.7 mm trixon=2 p3rope=rt3ie6s.1: mm compressive stress σcw = 150 MPa. The column is 2 3 125 4 metres high and both ends are held effectively in rAx = = 757 5×= 13265. 1=m 9m 375 mm² ry =2 3 75 position, but one end is also restrained in direction. ry = = 21=.72m1.7mmm 122532 3 The effective length of the column l = 0.85L = 0.85 I BD×3 −4 b0d030 = 3 40102 m04m−. 1084 rx = = 36.1 mm 2 3 rx = r 75 y = = = = 46.6 mm A 12(BD− bd ) 12(1202 − 1082 ry = = 21.7 mm ) 2 3 75 I BD3 − bd3 1204 − 1084 ry = = 21.7 mm rx = ry = = = = 46.6 mm 2 3 A 12(BD− bd ) 12(1202 − 1082) 75 ry = = 21.7 mm Find t2he3 allowable buckling load for the new cross- λ l 3 400 = = = 73 gives kλ = 0.72 by interpolation section: r 46.6 λ lx 3 000 x = = = 83 gives kλx = 0.41 Pw = kλ × σcw × A = 0λ l 3 400 .7=2 × =150 (120=27 -3r 46.6 1082) = 295 kN. rx 36.1 D b = × 12 ≈ 0.87D 4 D TABLE 7.3 b = × 12 ≈ 0.87D Reduction factor (kλ) for stresses with respect to the slenderness ratio for steel colum4 l ns λ = y 1 950 λy =10 = 90 ry 21.7 20 30 40 50 60 70 80 90 100 110 120 130 140 kλ 0.97 0.95 0.92 0.90 0.86 0.81 0.74 0.67 0.59 0.51 0.45 0.39 0.34 0.30 λ 150 160 170 180 190 200 210 220 230 240 250 300 350   kλ 0.26 0.23 0.21 0.19 0.17 0.15 0.14 0.13 0.12 0.11 0.10 0.07 0.05   σ F 15 000 c = = = 1.6 MPa A 9 375 l 4000 = = 13.3 b 300 l 4000 Chapter 7 – Structural design = = 13.3 131 b 300 l 4000 = = 13.3 b 300 I BD3 − bd3 1204 − 1084 rx = ry = = Axially lo=aded concrete co=lu4m6.6nms m σObviofusly, by the law of superposition, the added A 12(MBDos−t bbdu)ilding1 2c(o1d2e0s2 p−e1rm08i2t) the use of plain concrete stresse+s ≤ 1 i.e. Pcw fof the two load effects must be below the only in short columns, that is to say, columns where the allowablew stress. σ f + ≤ 1 i.e. ratio of the effective length to least lateral dimension Pcw fw does not exceed 15, i.e. l/r ≤ 15. If the slenderness ratio is between 10 and 15, the allowable compressive strength Therefore σ f + ≤ 1 i.e. mλu l 3 400 P =st be= reduced=.7 T3he tables of figures relating to l/b in cw fw place rof a4 t6r.u6e slenderness ratio are only apapxriaolx ciommaptere, ssive stress bending stress + ≤ 1 as radii of gyration depend on both b anadll odw vaablluee cso minp ressive stress allowable bending stress the cross-section and must be used with caution. In the axial compressive stress bending stress + ≤ 1 case of a circular column: allowable compressive stress allowable bending stress D axial compressive stress bending stress b = × 12 ≈ 0.87D , where + ≤ 1 4 allowable comσpressive stress allowable bending stress c f + ≤ 1 which can be transferred to: kλ ×σcw fw σc f D = the diameter of the column. + ≤ 1 kλ ×σcw fw P1 σ M + cσw c× ≤ f + σ K ≤ 1 TABLE 7.4 λ × A kf λ w×σ Z cw cw fw Permissible compressive stress (Pcc) in concrete for columns (MPa or n/mm2) Example 7.9 Concrete mix Slenderness ratio, l/b Determine within 25  mm the required diameter of a   ≤ 10 11 12 13 14 15 timber post loaded as shown in the figure. The bottom Prescribed of the post is fixed in both position and direction by C10 3.2 3.1 3.0 2.9 2.8 2.7 being cast in a concrete foundation. Allowable stresses πfDor2 theπ t×im2b0e0r2 used are σcw = 9 MPa and fw = 10 MPa. C15 3.9 3.8 3.7 3.6 3.5 3.4 A= = = 31 400 mm2 C20 4.8 4.6 4.5 4.3 4.2 4.1 4 4 Nominal P = 30 kN 1:3:5 3.1 3.0 2.9 2.8 2.7 2.6 1:2:4 3.8 3.7 3.6 3.5 3.4 3.3 e = 500 F = 5 kN 1:1.5:3 4.7 4.5 4.4 4.2 4.1 4.0 Higher stress values may be permitted, depending on the level of π D3 π × 2003 work supervision. Z = = = 785 400 mm2 32 32 Example 7.8 A concrete column with an effective length of 4 metres has a cross-section of 300  mm by 400  mm. Calculate the allowable axial load if a nominal 1:2:4 concrete mix D 200 r = = = 50 mm is to be used. 4 4 Slenderness ratio l 4000 = = 13.3 b 300 l 6 300 = = = 126 r 50 Hence Table 7.4 gives Pcc = 3.47 N/mm² by interpolation. Pw = Pcc × A = 3.47 × 300 × 400 = 416.4 kN. The load of 5  kN on the cantilever causes a bending σ f moment of M = F × e = 5 kN × 0.5 m = 2.5 kNm in the Eccen+trica≤lly1 li.oea. ded timber and steel columns post below the cantilever. WhPecwre a fcwolumn is eccentrically loaded, two load effects need to be considered: The effective length of the post = L × K = 3 000 × 2.1 = The axial compressive stress caused by the load and 6 300 mm. Try a post with a diameter of 200 mm. the bending stresses caused by the eccentricity of the load. The cross-sectional properties are: axial compressive stress bending stress + ≤ 1 allowable compressive stress allowable bending stress σc f + ≤ 1 kλ ×σcw fw L = 3 000 P1 σ + cw M × ≤ σ K × cw λ A fw Z P1 σcw M + × ≤ σ Kλ × A f cw w Z P1 σ M + cw × ≤ σ Kλ × A fw Z cw P πD2 π × 2002 A= = = 31 400 mm2 1 σ + cw M × ≤ σ K c λ × A f 4 w Z w 4 πD2 π × 2002 A= = = 31 400 mm2 4132 4 Rural structures in the tropics: design and development πD2 π × 2002 A= = = 31 400 mm2 4 4 π D3 π × 2003 Z = = = 785 400 mm2 32 32 πD2 π × 2002 A= = = 31 400 mm2 4 4 P π D3 π × 2003 e Z = = = 785 400 mm2 32 32 π D3 π × 2003 Z = = = 78D5 40200m0 m2 32 32 r = = = 50 mm 4 4 π D3 π × 2003 Z = D 2=00 = 785 400 mm2 r = 32= =3520 mm 4 4 D 200 b r = = = 50 mm 4 4 l 6 300 The slenderness rPatio = = = 126 1 σ + r cw 5M× 0 ≤ σ D 2K00P r = = × c λ =A50 mfwm ZM w 1 σ + cw × ≤ σ Inter K lpol6at4 3io0n0 in4 Tλab×leA cw P 7.3 fgwivesZ kλ = 0.18 1 σ Many agricultural buildings have walls built of = + =cw M × =≤1σK 2c6 λ × A r fw 50 Z w blocks or bricks. The same design approach as that P shown for plain concrete with axial loading can be used. 1 σ + clw M ×6 30≤0σ K × w λ A = f = c w Z = 126 The maximum allowable compressive stresses must r 50 be ascertained, but the reduction ratios can be used as 30 000 l 693002.5×106 before. = =+ × = 126 = 8.17 N/mm2 ≤ 9 N/mm2 l 2 800 0.183×030104000 9 2.5×106 r +1050×78 40 = = 23.3 = 8.17 N/mm2 ≤ 9 N/mm2 Example 7.10 b 120 0.18× 31 400 10 785 400 30 000 9 2.5×106 Determine the maximum allowable load per metre of a + × = 8.17 N/mm2 ≤ 9 N/mm2 0.18× 31 400 10 785 400 120 mm thick wall, with an effective height of 2.8 metres 30 000 9 I2f .5th×e1 0p6ost has a diameter of 200  mm, it will be and made from concre l lte gr 2 2ad 808 e 00 0C 15: (a) when the load is + ab×le to carry =th8e.1 l7oaNds/,m bmut2 ≤th9e Nta/smk mw2as to determine central; (b) when the load is eccentric by 20 mm. 0.18× 31 400 10 785 400 l 2=8=00 = =232.33.3 =bb 1210=20 the diameter within 25  mm. Therefore a diameter of 23.3 b 120 175 mm should alsλo 6 300 =be tried.= 144 λ 463.37050 l 2 800 = = 144 Slenderness ratio, = = 23.3.3 b P1cw2=0 2.8 − (2.8 − 2.0)= 2.27 N/mm2 = 2.27 MPa 43.75 λ 6 300 5 = = 144 kλ = 0.13 43.75 Interpolation gives: λ 6 300 = = 144 43.75 3.3 Pcw = 2. 3.3 Pcw = 2.838.−3− (2(2.8.8−−2.20.)0=)=2.22.72N7 N/m/mm2m2 30 000 9 2.5×106 + × = 23 N/mm2 > 9 N/mm2 = 2=.227.2M7PMa Pa P 55 cw = 2.8 − (2.8 − 2.0)= 2.27 N/mm2 = 2.27 MPa 0.133×020400050 109 126.57×41806 + × = 23 N/mm2 > 9 N/mm2 5 0.13× 24050 10 167 480 30 000 9 2.5×106 3.3 e 20 P = = 0.167 cw = 2.8 − (2.8 − 2.0)= 2.27 N/mm2 + × = 23 N/mm2 > 9 N/mm2 = 2.27 MPa 0.13× 24050 10 16T7h4is8 0diameter is too small, so a diameter of 200 mm Allowable lo5ad Pw = A × Pcw = 1.0 × 0.1b2 ×1 2.027 × 106 30 000 s9hou2ld.5 b×e1 c0h6osen. It will be appreciated that the choice = 272.4 kN/m wall + of e×ffective len=g2th3 bNa/semdm o2n> e9ndN f/imxitmy2 has a great effect 0.13× 24050 1o0n th1e6 s7o4lu8t0ion. Ratio of eccentriceity ee 2020 2=0= = =0.106.1767 Plain and centrally reinforced concrete walls =bb 121=0200.167 b 120 Basically walls are designed in the same manner as columns, but there are a few differences. A wall is A double interpolaetion 2g0= ive=s:0.167 b 120 1.06 × 106 distinguished from a column by having a length that is Pw = A × Pcw = 1.0 × 0.12 × = 127.2 kN/m wall more than five times the thickness. Pcw = 1 .06 N/mm² = 1 .06 MPa 1 000 Plain concrete walls should have a minimum thickness of 100 mm. Allowable load Where the load on the wall is eccentric, the wall 1.06 × 106 Pw = A × Pcw = 1.0 × 0.12 ×1 .06 × 106 must have centrally placed reinforcement of at least Pw = A × Pcw = 1.0 × 0.12 ×1. 06 × 106 = 1=2 17.227 k.2N k/Nm/ wma lwl all Pw = A × Pcw = 1.0 × 0.12 × 1 01 0000=0 127.2 kN/m wall 0.2 percent of the cross-section area if the eccentricity 1 000 ratio e/b exceeds 0.20. This reinforcement may not be 1.06 × 106 included in the load-carrying capacity of the wall. Pw C= eAn t×r aPl crwe =in f1o.0r c×e m0.e1n2t ×is not require=d 1b2e7c.a2u k e seN /m wall 1 000 < 20 b e e e < <2020