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The strength of a building refers to its capacity to carry loads without failure of the construction method; stability refers to the ability of a building to resist collapse, distortion, localized damage and movement. ---1 Building loads: dead loads. ---2 Building loads: imposed loads. ---3 Building loads: wind loads. Dead, live and wind loads A building is required to resist loads imposed by gravity as well as other externally and internally applied forces: loads and forces from roofs, floors and walls must be transferred by load-carrying mechanisms to the supporting ground. The loads and forces acting on a building are shown; they comprise the following: (a) The weight of all the materials from which it’s made (bricks, mortar, concrete, timber, plaster, glass, nails, screws, etc.). These weights are more or less constant during the life of a building and are called dead loads; they can be calculated from tables for weights of materials, etc. (b) The weight of people using a building, and their furniture, goods, storage, etc. These weights are called live loads or imposed loads and, as they will vary, an average maximum load can be assumed from tables giving values applicable to the particular use of a building. (c) Various forces may be applied to a building during its life such as those resulting from wind, physical impact by people, machines, or explosion, and ground movements caused by changes in soil characteristics, earthquakes, mining subsidence, etc. The calculation of these forces is much more problematic and relies on adequate research and experience. Maximum wind loads (gusts) for various locations in the country have been tabulated and should be consulted before finalizing the structural requirements for a building. These loads and forces must be resisted by the supporting soil so that a building remains in equilibrium. A building can be visualized, therefore, as being 'squeezed' between the downward applied loading and the upward reactions of the supporting soil. Structural organization There are four basic methods of structural organization which can be employed in a building to ensure loads, forces and soil reactions act together in providing equilibrium to a building. The choice of method for a particular building is initially dictated by strength and characteristics of the soil providing support and analysis of the precise nature of all the structural influences. Structural influences are closely related to the function of the building and involve consideration of such factors as whether long or short spans are required, the height of the building, and the weight of materials necessary to fulfill other performance requirements, etc. Often, the juxtaposition of existing buildings provides positive guidelines or maybe limitations on the selection of structural form for a new building. The structural organization of a building forms one of the most important aspects which influence appearance and other functions. Since technological solutions are now available which make almost every structural organization possible, an increasing burden of responsibility is being placed on designers to make rational decisions. The rigorous limitations imposed by simpler construction methods (materials and technology) no longer exist, and it’s now feasible to develop a building with volumetric spaces (plan and height) greater than ever before; smaller, to complicated configuration; or to the same size using much less material. Nevertheless, although perhaps interpreted with greater understanding, certain basic structural principles still remain. Expressed simply, the four basic methods involved in the use of construction methods resist the combined building loads by compression, tension or a combination of the two. These methods are defined by various terms, but here will be called: continuous structures, framed structures, panel structures and membrane structures. Continuous structures These are continuous supporting walls which transfer the combined loads and forces through their construction, mainly by direct compression. Materials commonly used for this purpose are stone, horizontal timber logs, brick, block and concrete. In this respect, continuous supporting walls may be the oldest form of structural organization. Walls constructed from fairly small units such as bricks, blocks or stones rely on their strength by being laid in horizontal courses so that their vertical joints are staggered or bonded across the face of the wall. In this way the compression loads which may initially affect individual, or a series of, bricks, blocks or stones can be successfully distributed through a greater volume of the wall. The units are held together by an adhesive mixture known as mortar, thereby completing the structural (and environmental) enclosure. This mortar also serves the function of taking up any dimensional variations in the bricks or blocks so that they can be laid in more or less horizontal and vertical alignment. Mortars usually consist of water-activated binding mediums of cement and lime, and a fine aggregate filler such as sand, in the proportion of one part binder to three parts aggregate. Lime aids workability, but is now rarely used in mortar as it may be substituted with a proprietary liquid plasticizer. Masonry cement may be preferred, as this has an integral plasticizer. Masonry cement is not suitable for producing concrete. Laterally applied forces which could create tension in the wall are resisted by its preloaded condition. The weight of materials forming the wall, together with any loads they carry from floors or roof, combine in counteracting the tendency for horizontal movement or overturning as a result of horizontally applied forces, e.g. wind. Alternatively, where preloading is insufficient, the action of lateral forces can be resisted by the provision of buttresses at predetermined centers to resist the tendency for over turning. In this way a system of buttresses or piers can be used to provide stability to a long length of wall. Walls which are serrated or curved in plan will also be stronger than straight walls because they are more able to resist laterally applied forces. Additional stability can also be provided by the floor(s) and roof of a building, provided there is adequate connection at the junction between horizontal and vertical elements. A wall which is laterally braced in this manner has the advantage of using considerably less material to support the same load as a thick unbraced wall. When it’s required to provide openings (e.g. doors and windows) in a building using continuously supporting walls, it’s necessary to use a beam or lintel made from a material or combination of materials capable of resisting both compression and tension forces resulting from the loads above. These loads must be transferred to the sides of the opening or jambs. Stone can be used for a lintel, but it provides only a limited resistance to tensile forces relative to its depth and therefore permits only small openings. Timber, steel, a combination of steel bars and concrete (reinforced concrete), or steel angles and bricks, can be suitable materials for lintel construction, although care must be taken to ensure their durability corresponds with the intended life of the building. Arches formed over openings use materials in direct compression only, in a similar manner to the wall itself. Therefore, arched openings are often considered by design purists to be more compatible with the aesthetic of this form of wall construction. ---4 The transfer of loads in continuous structures. ---5 Effects of bonding in small building units used for walling. ---6 Lateral bracing of walls. ---7 Reinforced concrete lintels and block arches. ---8 Transfer of loads in framed structures. ---9 Position of structural frame (column) and its effect on a building's appearance. Framed structures These consist of a framework of timber, steel or reinforced concrete consisting of a regular system of horizontal beams and vertical columns. The beams resist both compressive and tensile forces and transmit loads from the floors, roof and walls to the columns. The columns are required to resist mainly compressive forces; they transfer the beam loads (and the self-weight of beam and column) to the foundation and finally to the supporting soil. This obviously results in more concentrated loads being sup ported by the soil than for a similar weight of building using continuous supporting walls, unless special forms of foundations are used. The infill panels between the frame work used to provide the external wall can be constructed of any suitable durable material which fulfils performance requirements satisfactorily. If the wall material is positioned away from the framework so as to be externally or internally free of the columns and beams, it’s known as cladding. Both panel and cladding walls are generally non-load-bearing, although in practice they must carry their own weight (unless suspended from above), resist the wind forces acting on their external face, perhaps provide support for internal fixtures, shelves, etc., and resist localized impact forces. However, depending upon the form of construction adopted, the resulting loads are usually transferred back to the supporting columns and beams by their fixing method. A structural framework and panels, or cladding walling, is an example of composite construction. Here the use of different materials to provide independent functions requires careful constructional detailing and skilled work to ensure an entirely successful enclosure. The external appearance of a framed structure will vary according to the location of the beams and columns relative to the external wall. ---9 indicates the basic permutations. When the structural frame is not located within an enclosing panel wall, additional precautions may be necessary to protect the frame against possible detrimental effects resulting from an outbreak of fire, and also from the effects of weathering when the frame is external to the wall. Although this may pre sent no special problems for reinforced concrete - other than a slight increase in cross-sectional area - the use of timber and exposed steel frames requires special consideration. Table__ provides a brief checklist of the structural materials used for a framed building. (Google this: use of in situ concrete framework with clay brick infill panel wall construction.) ==== ___Comparison between timber, steel and reinforced concrete as structural framing__ Criteria, Availability, Conversion, Site operations, Site progress, Site progress, Fire protection, Adaptability, Maintenance, Reinforced concrete supplies of aggregates and cement, Steel is mostly imported, Factories and steelworks convert into basic use materials (aggregates, cement, bar reinforcement), Precast concrete components manufactured to fine tolerances under factory conditions In situ structures erected by semi skilled and unskilled operatives, Various shapes possible dependent upon potential of formwork material (up to 40% of total cost), May need crane although in situ materials can be pumped to higher levels; Precast components erected by skilled operatives Slow progress when necessary to wait for hardening before commencing next sequence or trade Multi-storey structures can be formed near ground level and raised into position Slow sealing against weather; design can help by using repeat formwork Steel reinforcement insulated by concrete cover High inherent degree of fire protection and nil spread of flame Fire resistance periods can be improved by selecting aggregate to reduce spalling Damaged reinforced concrete can be repaired Can be adapted Difficult to provide extension of existing structure (steel reinforcement must be exposed) and separate new structure required Self-finish quality depends on site skills and formwork quality, Steel Most iron ore imported Steelworks produce bulk sections and components manufactured in factory-controlled conditions; Components manufactured to fine tolerances under factory conditions Site erected by skilled operatives Very accurate within small tolerances Heavy sections used need crane Heavy components but can be erected quickly; Mistakes difficult to correct Progress dependent upon weather conditions unless protection provided; Once erected floor/roof components can be placed and building sealed against weather Not considered to be good fire risk and generally must be insulated for all but very small structures; Possible to expose sections providing design permits shielding isolation or cooling (see text) Spread of flame characteristic depends on type of insulation provided Damaged steel irreparable Difficult to adapt on site owing to precut lengths or difficulty in cutting Extension can be achieved providing access to original sections available through fire protection Needs regular maintenance if not encased or weathering steel lntumescent paint can be decorative and provides fire protection Material Timber Mostly imported as dimensionally coordinated sections Factory-converted sections and components Preformed components manufactured to fine tolerances under factory conditions Erected by skilled and semi skilled operatives Components relatively lightweight and to fine tolerances Components relatively lightweight and can be quickly erected Foundation work less Progress dependent upon weather conditions unless protection provided Once erected floor/roof components can be placed and building sealed against weather Not considered to be good fire risk although designs can take account of 'sacrificial' sections to provide insulation (see text) Poor spread of flame characteristics, but can be improved by chemical treatment Damaged timber irreparable Easily adapted during construction Extensions easy to provide Finished with preservative stain, varnish or paint system Intumescent paint can be decorative and provides fire resistance Designs must provide protection against insect and fungal attack ===== ---10 Transfer of loads in panel structures. ---11 Using tension cables. ---12 Using membrane structures. ---13 Slenderness ratio and lateral bracing in walls. Panel structures These include preformed load-bearing panel construction for the walls, floors and roof which carry and transfer loads without the use of columns and, sometimes, beams. This is similar to continuous supporting wall construction, but each panel is designed to resist its own imposed loads, as well as other performance requirements. They are generally more slender than most other forms of construction and are dimensionally coordinated so as to be interchangeable within their specific functional requirement. The main structural material of a panel is generally of steel or timber, and this can be faced with a suitable material (plywood, flat or profiled metal sheet) and incorporate thermal insulation to form a sandwich construction. (Certain forms of sandwich construction are also used for non-load-bearing panel cladding for a framed building.) Panels can incorporate window and door openings. The combined loads which this form of construction collects can be transferred to the supporting soil by continuous distribution or by concentrating them in a similar manner to that adopted for framed buildings. Membrane structures Thin non-structural membranes forming walls and roof (often combined in one place) are supported by tension and/or compression members. A typical ex ample of this is a tent where the walls and roof are formed of canvas and the main structural support of timber or steel. Most permanent structures can be formed by columns, compression members, from which cables are suspended, tension members, which support a plastic membrane. Alternatively, a reinforced plastic or canvas membrane can be supported by air, as in inflatable structures. In this case the membrane is in tension because of the compression forces exerted by the air under pressure. Both these examples are suitable for a building where certain of the performance requirements discussed don’t form an essential part of a proposed building enclosure. Slenderness ratio Whenever the structural organization of a building involves the resistance of vertical loads by a wall or column in compression, adequate thickness of sufficiently strong material must be employed in their construction to avoid crushing. A short wall or column can ultimately fail by crushing. But as height increases, ultimate failure is more likely to occur under decreasing loads by buckling. This form of failure results from lack of stiffness in a wall or column which causes bending to occur because, in practice, it’s impossible to ensure that vertical loads act through the vertical centerline of their support. Very tall, thin walls or columns will buckle before crushing, short squat walls crush before buckling, and walls of intermediate proportions may fail by either method. Obviously, the greater the height of a wall or column and the tendency towards buckling, the more critical becomes the relationship between thickness and height. This relationship is known as the slenderness ratio; as this ratio increases, so the load-carrying capacity of the wall or column decreases. Because the stiffness of a wall or column can be increased by lateral bracing as described for calculation purposes the dimensions used for the effective height and thickness can vary from the actual height and thickness in order to obtain a realistic slenderness ratio. The amount of variation depends on the degree and effectiveness of connection provided between the horizontal and vertical components. For example, where a wall is loaded by a floor construction which provides continuous lateral support (reinforced concrete slab or adequately connected timber joists), the height of the wall can be taken for calculation purposes to have effective height (h) equivalent to three-quarters of the actual height (H). This will give a slenderness ratio which either permits theoretically slightly less strong materials to be used than if no concession had been given, or permits the wall to be thinner and occupy less plan area. However, if the floor gives no lateral support whatsoever, the rules of calculation will double the actual height of the wall and vastly increase the slenderness ratio. A further correction factor may be applied to the slenderness ratio used for calculation purposes when applied loads are resolved eccentrically to the centre of the wall. The design of columns is also subject to similar requirements regarding slenderness ratios and correction factors for eccentric loadings. ---14 Correction factors applied to walls according to the eccentricity of loads. ---15 Slenderness ratio and lateral bracing in columns. ---16 Use of diagonal bracing in frame structures. (Adapted from Multi-storey Buildings in Steel) Diagonal bracing For frame buildings, the provision of effective lateral bracing will vary according to how well the materials employed will permit a rigid joint to be created between horizontal and vertical components. Very rigid joints can easily be created between beams and columns made of reinforced concrete, but it’s more difficult for beams and columns made of steel and very hard when they are made of timber. When jointing techniques cannot provide sufficient lateral restraint, the structural frame can be made more rigid by inserting diagonal bracing in various locations around a building, or by using shear panels. Previous: Effective
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