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The dimensional suitability of a construction method involves the consideration of two areas: ++ The manner in which movement of materials causes dimensional variations in a building, or parts of a building during its life. ++ Appropriate sizes for the parts of a building which suit the materials available to fulfill specific design functions, cost ratios, manufacturing processes and assembly techniques. Movement A building never remains inert; this is because changes in the environment and/or changes in loading cause dimensional changes in the building materials. Variations in moisture content and temperature produce movements in a building which tend to occur in relation to the stronger 'fixed points' in the building - between foundations and first floor, between top floor and roof, between partitions and main structure, or between panels and supporting frames. ---Typical movements likely to occur in a building. --- Moisture and thermal movements in calcium silicate and clay brick walls. Irreversible and reversible movement The moisture content of porous building materials can cause irreversible movement or reversible movement. Irreversible movement is generally associated with establishing a 'normal' or atmospheric moisture level in the materials of a component which have recently been manufactured. For example, clay bricks leaving a kiln will be very dry and will immediately begin to absorb moisture from the air which causes expansion. Conversely, calcium silicate bricks will be more saturated than normal bricks because they are cured by autoclave processes and will immediately shrink after manufacture as their moisture content moves towards an equilibrium with that of the atmosphere. For this reason, newly manufactured bricks should not be immediately used for building walls as cracking will inevitably occur. Reversible moisture movement occurs in materials which are in use and generally involves expansion on wetting and shrinkage on drying. These movements have both immediate and long-term effects on the fabric of a building and thoughtful detailing is essential if damage is to be avoided. Care must be taken to ensure movements are reduced to accept able amounts by limiting the uninterrupted heights and lengths of components and elements. This can be achieved through a precise knowledge of the characteristics of the materials involved and the incorporation of movement joints at centers beyond which excessive movement is likely to occur. These movement joints should be positioned so as to take account of their visual effect on a building. It’s important not to confuse changes in the size of materials due to absorption of moisture with the problems associated with moisture movement through materials. Movement through materials will be dealt with later. Most building materials also expand to a greater or lesser extent with rises in temperature, and if they are restrained could induce considerable stress, producing cracking, bowing, buckling or other forms of deformation. Severe damage to walls can be caused by attempting to restrain beams and slabs - particularly where temperature ranges are likely to be great. Fortunately, dramatic failures of this nature are not very common. But daily (diurnal) temperature ranges are a frequent cause of damage to a building; this can occur immediately in the form of buckling metal sills, cracking glass, etc., or over a period by causing gaps in a weather-impermeable construction which allows the free penetration of moisture. The same care needed in constructional detailing and the provision of movement joints, which is required to limit moisture movements, is also necessary when considering thermal movements. In fact, both problems are often interrelated. In general terms, irreversible moisture movement in porous building materials is greater than reversible movement, and reversible moisture movement is usually less than movement due to temperature changes. Softening and freezing Detrimental effects resulting from temperature changes are also caused as a result of softening and of freezing. The majority of materials used for a building won’t become softened by normal climatic temperature. However, those containing bituminous or coal tar pitch (e.g. asphalt and bituminous felts used for roofs, floor finishes, damp-proof courses, etc.) are liable to become more and more plastic as temperatures rise. This can produce indentation and perforation under load, or result in elongation causing their displacement. A bituminous damp-proof course can soften sufficiently for the load of a wall above to squeeze it outwards from its bedding and even upset the stability of the wall. This problem is most likely to occur on exposed south-facing walls. --- Moisture and thermal movements in brick walls. Freezing causes a rather special form of thermal movement and, in this respect, is not dissimilar from the chemical attack. Sometimes water may penetrate into a structure and freeze along a junction between two materials. In forming ice lenses, the water expands by about 10 per cent and causes considerable damage. Water freezing in air pockets or other fissures in the mortar of brickwork can cause spalling of the joint and the adjoining arises of brickwork. If the water freezes within the body of a material, a similar disintegration process will occur. A clear understanding is required about the ratio between amount of water absorbed and the volume and distribution of pore space available if this phenomenon is to be avoided. The presence of moisture in certain materials, even in very minute quantities, can produce chemical changes that lead to movement. The corrosion processes of iron and steel form a porous layer of rust, which becomes liable to expansion. This action can cause spalling of the concrete cover when the metal bars of rein forced concrete beams become rusted. Shrinkage By way of contrast, the chemical action for setting Port land cement produces a volumetric shrinkage. Constructional detailing should take this into account by allowing adequate gaps or tolerances between in situ concrete components and other materials. This is particularly true when detailing the junction between an in situ concrete frame and a brick infill panel, which is liable to expansion due to moisture absorption and climatic temperature increases. --- Movement joints to panel walls. Loading As most materials used for buildings are elastic to some degree, a certain amount of plastic flow or creep will occur over a period of many years, depending upon the type and amount of load or force to which they are subjected. Regardless of this long-term movement, however, structural elements such as beams and columns of any material are likely to be subject to initial deflection. This will occur even as their superimposed loads accumulate during construction and will continue until full loading conditions are achieved. Construction methods should take into account the possibility of these movements by ensuring adequate tolerances between structural and non-structural parts of a building to avoid the effects of crushing and cracking. For example, the details for an internal partition of a framed building should ensure that the performance requirements (particularly strength and stability) are not negated by any deflection inherent in the nature of the materials from which beams and columns are formed. If the tendency towards deflection or creep becomes too great in the external fabric of a building, secondary problems could develop from moisture penetration through cracks, etc. Similar problems may arise if a supporting or loading condition of a building is altered and the new conditions cannot be accommodated by the existing construction methods. Appropriate sizes Even the simplest form of building requires thousands of individual components for its construction. With traditional building practice, these products were completely unrelated in dimension and necessitated the use of skilled workers to scribe, cut, fill, lap and fit them together on the site. Such techniques were notoriously wasteful of both materials and labor. In order to make a building financially viable today, it’s necessary that products are manufactured to sizes which coordinate with each other so they can be assembled on site without the need for alteration. This means that the role of the manufacturers becomes much more important as they must ensure the overall dimensions between various products coordinate, and ensure jointing methods between each allow connection with other related products. Products which can be easily assembled together using simple jointing methods are theoretically likely to be less reliant on exacting site skills to fulfill their intended functions. However, in practice, the ease and success of the joint again relies on adequate interpretation by the manufacturer of all performance requirements, labor skills and actual site conditions. --- Deflection of reinforced concrete beams and the stability of non-load-bearing blockwork internal wall. --- Dimensional coordination --- Component types. Apart from providing savings in materials and labor, the use of dimensionally coordinated products for a building eases the processes of selection by allowing a greater range of similar items to be available. In addition to the availability of home-based products, a designer can select from those abroad, providing they follow the same system of dimensional coordination. --- Using dimensionally coordinated components which fit together to form a building. Many of the sizes of basic building materials used today derive through history and are directly related to the human scale (anthropometrics). The clay brick, For example, was dimensionally standardized during medieval times relative to its weight, so as to enable easy manipulation by one hand, leaving the other hand free to operate a trowel. This standard size, now translated to 215 mm × 102.5 mm × 65 mm (Specification for clay bricks), has given rise to the conventional aesthetic of a brick-built building. When used in large quantities, the bricks, together with their joints and the bonding method, determine the scale and proportion of the overall shape and thickness of the walls, as well as the size of window and door openings, etc. Provided the window and door components are manufactured to be fitted into an opening formed by the brick dimensions without adjustment, the building shell can be constructed easily and economically. Similarly, internal partition units, floor joists, cupboards, floor and wall finishes should also be available in dimensions suitable to those of brickwork. Certain manufacturing processes and some materials cannot conveniently conform with common dimensional standards. For example, walls of stone, concrete or plastics will have different dimensional characteristics to walls of brick. It’s necessary, therefore, that manufacturers should be given a guide on the likely range of dimensions for which particular components will be required, and the variations for which they should allow within this range. Accordingly, a range of overall dimensions of components related to building use, anthropometric requirements and manufacturing criteria has been recommended. ---8 illustrates the recommendations for the vertical dimensions used in the construction of a house. There are similar re commendations for the horizontal dimensions. In order not to overstretch the resources of manufacturers and to further rationalize the available range of components: Specification for modular coordination in building states that components should be manufactured in basic incremental sizes of 300 mm as first preference, or 100 mm as second preference, with 50 mm and 25 mm being allowed up to 300 mm. Modular coordination Attempts have been made to persuade manufacturers to agree to manufacture components in standardized modular increments of 100 mm, which is the dimension most common in building products at home and abroad. This system is known as modular coordination and, when adopted, means that a building is designed within a three dimensional framework of 100 mm cubes. A successful design relies on the certainty that a vast range of products will be available for the construction - a range which can be selected from almost any country adopting the dimensional system. Products requiring to have dimensions greater than the basic module are manufactured so as to be a multiple of the basic module; or, if they are required to be smaller, are manufactured so that when placed together their combined dimensions suit either the basic module or a multiple thereof. Although the idea of modular coordination has been in existence for a considerable time, many products will remain in imperial and uncoordinated dimensions until it becomes cost viable for manufacturers to replace their machinery. --- Recommended vertical controlling dimensions for housing. ---Preferred linear sizes for components used in a building. ---Using modular coordinated sizes in a building (M = standard module of 100 mm). Jointing and tolerances Whether modular or otherwise, components cannot be manufactured to precise dimensions to suit a regular three dimensional grid. Allowance must be made for jointing the component, as well as for inaccuracies in the products due to material properties (shrinkage, expansion, twisting, bowing, etc.) and manufacturing processes (changes in mould shapes, effects on materials, etc.). Furthermore, allowance must be made for positioning the components on site. This is particularly important for very large and heavy components which are maneuvered into position by cranes, etc., and final location could vary by as much as 50 mm. Allowances of this nature are known as tolerances; if manufacturers supply products without these tolerances, cumulative errors in trying to place each component on a grid line would result in not only a building of incorrect overall dimensions, but also a great deal of frustration in the fixing of secondary items such as windows, or furniture and finishes within the building. An important document covering the aspects of tolerances: Guide to accuracy in building. --- Tolerances allowed when designing components: a simplified method of arriving at the 'final dimension' of a component. More accurate but more complicated calculations are now available tolerances for building. Previous: Building/Construction
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