Chapter 8 Elements Of Construction - FAO

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149Chapter 8Elements of constructionIntroductionWhen designing a building, an architect plans forspatial, environmental and visual requirements. Oncethese requirements are satisfied, it is necessary to detailthe fabric of the building. The choice of materials andthe manner in which they are put together to formbuilding elements, such as the foundation, walls, floorand roof, depend largely upon their properties relativeto environmental requirements and their strength.The process of building construction thus involvesan understanding of: the nature and characteristics ofa number of materials; the methods to process themand form them into building units and components;structural principles; stability and behaviour underload; building production operations; and buildingeconomics.The limited number of materials available in therural areas of Africa has resulted in a limited number ofstructural forms and methods of construction. Differentsocio-economic conditions and cultural beliefs arereflected in varying local building traditions. Whileknowledge of the indigenous building technologyis widespread, farmers and their families normallycan erect a building using traditional materials andmethods without the assistance of skilled or specializedcraftsmen. However, population growth and externalinfluences are gradually changing people’s lives andagricultural practices, while some traditional materialsare becoming scarce.Hence, a better understanding of traditional materialsand methods is needed to allow them to be used moreefficiently and effectively. While complete understandingof the indigenous technology will enable architectsto design and detail good but cheap buildings, newmaterials with differing properties may need to beintroduced to complement the older ones and allow fornew structural forms to develop.Loads on building componentsmass of these loads can be calculated readily, the factthat the number or amount of components may varyconsiderably from time to time makes live loads moredifficult to estimate than dead loads. Live loads alsoinclude the forces resulting from natural phenomena,such as wind, earthquakes and snow.Where wind velocities have been recorded, thefollowing equation can be used to determine the expectedpressures on building walls:q 0.0127 V2kwhere:q basic velocity pressure (Pa)V wind velocity (m/s)k (h/6.1)2/7h design height of building, in metres (eave height forlow and medium roof pitches)6.1 height at which wind velocities were often recordedfor Table 8.1.While the use of local wind velocity data allowsthe most accurate calculation of wind pressures onbuildings, in the absence of such data, estimates can bemade using the Beaufort wind scale given in Table 8.1.Table 8.1Beaufort wind scaleVelocity in m/sat a height of6.1 m abovegroundLarge branches in motion;whistling in telephonewires; umbrellas usedwith difficultyModerate galeWhole trees in motion;difficult to walk againstwindFresh galeTwigs break off trees; verydifficult to walk againstwind21Strong galeSome structural damageto buildings24Whole galeTrees uprooted:considerable structuraldamage to buildings28StormWidespread destruction33Loads are usually divided into the following categories:Dead loads, which result from the mass of all theelements of the building, including footings, foundation,walls, suspended floors, frame and roof. These loads arepermanent, fixed and relatively easy to calculate.Live loads, which result from the mass of animals,people, equipment and stored products. Although the11–14Strong breezeFrom the United States Weather Bureauup to 17

Rural structures in the tropics: design and development150Table 8.2Wind-pressure coefficients for gable-roof farm buildingsH:W Windwardwall coefficientWindward roof coefficientRoof slopeCompletely closedLeeward roofcoefficientLeeward wallcoefficient15 30 pen on both sides 30 30 Windward slope 0.6 0.8Leeward slope-0.6-0.8H height to eaves, W width of buildingSome idea of the worst conditions to be expected canbe formed by talking to long-time residents of the area.The effect of wind pressure on a building isinfluenced by the shape of the roof and whether thebuilding is open or completely closed. Table 8.2 givescoefficients used to determine expected pressures forlow-pitch and high-pitch gable roofs and open andclosed buildings. Note that there are several negativecoefficients, indicating that strong anchors and jointfasteners are just as critical as strong structural members.Data on earthquake forces is very limited. The bestrecommendation for areas prone to earthquakes is touse building materials that have better-than-averagetensile characteristics, to design joint fasteners with anextra factor of safety, and to include a ring beam at thetop of the building wall.Table 8.4Table 8.3Table 8.5Mass of building materialsLoads on suspended floorskPaCattleTie stalls3.4Loose housing3.9Young stock (180 kg)2.5Sheep1.5Horses4.9PigsPoultry(90 kg) Slatted floor2.5(180 kg) Slatted floor3.2Deep litter1.9CagesVariableRepair shop (allowance)3.5Machinery storage (allowance)8Mass of farm productsMaterialkg/m3kg/m2Concrete2 400ProductSteel7 850Maize (shelled)Angle of reposeEmptyingFillingMasskg/m³2716720Dense woods (19 mm)90017.0Maize (ear)--450Softwoods (19 mm)58011.0Wheat2716770Plywood (12 mm)7.3Rice (paddy)3620577Galvanized roofing3.9Soybeans2916770Concrete hollowblock wall100 mm200 mm300 mmBrick walls100 mm200 mm145Dry Groundnuts (unshelled)385Hay (loose)65–80Hay (baled)190–240Snow loads are a factor only in very limited areasat high altitudes in east and southeast Africa. Localinformation on the mass of snow loads should be used.Table 8.3 provides information useful in determiningdead loads and Tables 8.4 and 8.5 give informationrelevant to live loads.218Footings and foundationsA foundation is necessary to support the building andthe loads within or on the building. The combinationof footing and foundation distributes the load on thebearing surface, keeps the building level and plumb,

Chapter 8 – Elements of construction151and reduces settling to a minimum. When properlydesigned, there should be little or no cracking in thefoundation, and no water leaks.The footing and foundation should be made of amaterial that will not fail in the presence of ground orsurface water. Before the footing for the foundationcan be designed, it is necessary to determine the totalload to be supported. If, for some reason, the load isconcentrated in one or more areas, this will need to betaken into consideration. Once the load is determined,the soil-bearing characteristics of the site must be studied.Soil bearingThe top layer of soil is seldom suitable for a footing.The soil is likely to be loose, unstable and containorganic material. Consequently, the topsoil should beremoved and the footing trench deepened to providea level, undisturbed surface for the entire buildingfoundation. If this is not feasible because of a slopingsite, the footing will need to be stepped. This procedureis described later and illustrated in Figure 8.5.The footing should never be placed on a filled areaunless there has been sufficient time for consolidation.This usually takes at least one year with a normalamount of rainfall. The bearing capacity of soil isrelated to the soil type and the expected moisture level.Table 8.6 provides typical allowable soil-bearing values.If a building site with poor natural drainage mustbe used, it can be improved by the use of contourinterceptor drains or subsurface drains in order to cutoff the flow of surface water or to lower the level of thewater table. Apart from protecting the building againstdamage from moisture, drainage will also improve thestability of the ground and lower the humidity of thesite. Figures 8.1 and 8.2 illustrate these methods.Subsurface drains are usually laid 0.6 metres to1.5 metres deep and the pipe layout arranged to followthe slope of the land. The spacing between drains willvary between 10 metres for clay soils to 50 metres forsand. Subsurface drains are usually formed from buttjoined clay pipes laid in narrow trenches. In cases whereit is desirable to catch water running on the surface, thetrench is backfilled almost to the top with rubble, eithercontinuously along the trench or in pockets.A trench filled with rubble or broken stone willprovide passage for water and is effective in dealing withflows on the surface. Pipes and trenches belonging tothe main site drainage system may cause uneven settlingif allowed to pass close to, or under, buildings. Whereneeded, a separate drain surrounding the building,installed no deeper than the footing, is used to drain thefoundation trench.Table 8.6Soil-bearing capacitieskPaSoft, wet, pasty or muddy soil27–35Alluvial soil, loam, sandy loam(clay 40–70 percent sand)80–160Sandy clay loam (clay 30 percent sand), moist clay215–270Compact clay, nearly dry215–270Solid clay with very fine sand–430Dry compact clay (thick layer)320–540Loose sand160–270Compact sand215–320Red earth–320Murram–430Compact gravelRockFigure 8.1 Contour interceptor drain750–970Subsurfaceinterceptor drain–1700An extensive investigation of the soil is not usuallynecessary for small-scale buildings. Foundation and pierfootings can easily be designed to keep within the safebearing capacity of the soil found on the building site.Footing drainsRubble andgravel fillcontinuouslyor pocketsSite drainageIt is desirable to site any building on well drained land.However, other considerations such as access roads,water supply, existing services or a shortage of land maydictate the use of a poorly drained area.150 soil backfill150 soil backfill600 to 1 500Soil type100 200400Figure 8.2 Subsurface site drains400

Rural structures in the tropics: design and development152Foundation footingsA footing is an enlarged base for a foundation designedto distribute the building load over a larger area of soiland to provide a firm, level surface for constructing thefoundation wall.A foundation wall, regardless of the material usedfor its construction, should be built on a continuousfooting of poured concrete. Although the footing willbe covered, and lean mixes of concrete are consideredsatisfactory, a footing that is strong enough to resistcracking also helps to keep the foundation from cracking.A 1:3:5 ratio of cement–sand–gravel is suggested, with31 litres of water per 50 kg sack of cement. The amountof water assumes dry aggregates. If the sand is damp, thewater should be reduced by 4 litres to 5 litres.The total area of the footing is determined bydividing the total load (including an estimated mass forthe footing itself) by the bearing, by dividing the areaby the length. In many cases the width required forlight farm buildings will be equal to, or less than, thefoundation wall planned.In that case, a footing that is somewhat wider thanthe foundation is still recommended for at least tworeasons. The footings conform to small variations inthe trench and bridge small areas of loose soil, making agood surface on which to begin a foundation wall of anykind. The footings are easily made level, and this makesit easier to install the forms for a poured-concrete wall orto start the first course of a block or brick wall.Even when not required for loading, it is commonpractice to pour a concrete footing that is as deep asthe wall is thick, and twice as wide. The foundationfootings for large, heavy buildings require reinforcing.However, this is seldom necessary for lightweightrural buildings. Once a firm footing is in place, anumber of different materials are suitable for buildinga foundation. Figure 8.3 shows footing proportions forwalls, piers and columns.aWalls and PiersFigure 8.4 The division of loads on footings. Each pierfooting must carry one-eighth of the floor load. Thewall must carry five-eighths of the floor load and theentire roof and wall load.a2apier or column. Figure 8.4 illustrates the load distributionon a building with a gable roof and a suspended floor.If wall footings are very lightly loaded, it is advisableto design any pier or column footings required for thebuilding with approximately the same load per unit ofarea. Then if any settling occurs, it should be uniformthroughout. For the same reason, if part of the footingor foundation is built on rock, the balance of thefooting should be twice as wide as usual for the soiland loading. Footings must be loaded evenly becauseeccentric loading may cause tipping and failure.If a foundation is installed on a sloping site, itmay be necessary to dig a stepped trench and install astepped footing and foundation. It is important for allsections to be level and for each horizontal section ofthe footing to be at least twice as long as the verticaldrop from the previous section. Reinforcing in the wallis desirable, as shown in Figure 8.5.1½ aColumnsFigure 8.3 Footing proportions for walls,piers and columnsReinforcingHAlthough continuous wall footings are frequentlyloaded very lightly, this is not the case for column and pierfootings. It is therefore important to estimate carefullythe proportion of the building load to be carried by eachVH 2VFigure 8.5 Stepped footing and foundation

Chapter 8 – Elements of constructionThe procedure for finding an appropriate footing isillustrated in Figure 8.4.Example 8.1Assume a building is 16 metres long and 8 metres wide.The roof framing plus the expected wind load totals130 kN. The wall above the foundation is 0.9 kN/m.The floor will be used for grain storage and will supportas much as 7.3 kPa. The floor structure is an additional0.5 kPa.The foundation wall and piers are each 1 metre highabove the footing. The wall is 200 mm thick and the piersare 300 mm square. The soil on the site is judged to becompact clay in a well-drained area. Find the size of thefoundation a

Elements of construction IntroductIon When designing a building, an architect plans for spatial, environmental and visual requirements. Once these requirements are satisfied, it is necessary to detail the fabric of the building. The choice of materials and the manner in which they are put together to form building elements, such as the foundation, walls, floor and roof, depend largely upon .File Size: 2MBPage Count: 56