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Apart from daily wear and tear by its users, a building is subjected to the constant influences of climate (wind, rain, snow, hail, sleet, sunlight), perhaps attack from vandals and vermin, or even damage by fire, explosions and structural movements. Both the inside and the outside of all buildings are therefore subject to forces which can cause deterioration during their life. Durability is the measure of the rate of deterioration resulting from these and other forces. Changes in appearance When related to external climatic or environmental factors, the durability aspects of a building are known as weathering. The action of frost, temperature variations, wind and rain on the materials of a building can cause changes in appearance by gradual erosion and/or the transportation of atmospheric pollutants which cause staining. Unless the building is carefully designed and detailed, these visual changes seldom enhance appearance and therefore pro duce disfiguration. The degree to which a building will suffer from erosion and/or staining depends upon many interacting variables. One involves the relationship between type and amount of atmospheric pollution with the exposure of a building to wind, rain, frost, snow and solar radiation. Their effects will be determined by the characteristics of the materials used in a building and include their capacity or otherwise for moisture absorption, as well as their surface profiles, orientation, texture and color. ---1 Effects of south-west and north-west rain-carrying winds on a building. In an urban situation it’s normal to see dark bands of staining below most horizontal projections (moldings and sills, etc.) of a surface-permeable masonry wall. These projections provide a shield against direct rainfall or rainfall run-off from above, and dirt deposits are therefore left relatively undisturbed when compared with the lower regions of the wall (see also examples in -). It’s interesting to note that the design of projected moldings for buildings of the past generally ensured an even weathering to the surrounding vertical surfaces. Nevertheless, although today's buildings are often devoid of moldings and decorative devices, it’s possible for designers to allow water to be guided down a specific route by means of ribbed projections or recessed grooves, and therefore to create 'controlled' areas of weathering which enhance rather than detract from the appearance of the building. Porous surfaces may accumulate dirt and dust, vertical surfaces stain according to roughness and absorptivity, surface color affects visual density of staining, but carefully detailed designs can also use these properties to advantage. It’s first necessary to establish the degree of climatic exposure which is likely to affect a building. Generally, disfiguration will be slight where there is a moderate rainfall and a moderate rate of pollution, and constructional detailing needs to be far less 'bold' than for areas suffering greater amounts of rainfall and pollution. However, freak weather conditions can result from a group of tall buildings which create a weather pattern between them contrary to accepted predictions. Also, as buildings receive more direct rainfall on their upper storeys, high buildings are generally cleaner at the top and dirtier towards the lower storeys. This is caused by dust being washed down the face of the building and being retained at the point where the porosity of the surface absorbs (or constructional detailing collects and channels) a major part of the water. Low buildings, lacking exposure, will obtain an overall covering of pollution which will be unrelieved by washing. This is preferable to the random streaking to be seen on elevations which are subject to strong dust accumulation and which don’t receive direct rainfall in sufficient quantity to provide overall cleansing. The prevailing rain-carrying winds in the Pacific Northwest US and UK blow mainly from the south-west and the north-west quarters. In non-polluted areas the rain will be clean and will wash the building down. In polluted areas the rain will absorb the dust in the air and thus will be dirty when striking the surface. It will also carry dirt from the upper surface to the lower surface and cause staining. South west and north-west aspects of buildings will generally remain cleaner than the south-east and north-east aspects. North and north-east aspects are particularly liable to severe accumulation of dirt. Thus for a building to be cleaned the volume of clean rainwater must exceed the volume of dirty water passing over the surface. Snow has no beneficial cleaning effect. Physical deterioration Apart from causing the staining of a building, certain forms of weathering also result in chemical actions which cause physical deterioration by decay. From the time they are placed on a building site, all materials commence on a path of deterioration which could continue after a building has been constructed until a point is reached when they are not fulfilling a useful purpose. Again, the precise form of attack depends upon exposure conditions, and on the susceptibility of the materials used. Corrosion, erosion and disintegration of materials and construction details can follow from the effects of changes in moisture content, frost, sunlight, soil and groundwater action, atmospheric gases, electrolytic action, fungal and insect attack, or domestic and industrial wastes, etc. Critical conditions are influenced by the selection of materials appropriate for their design function, detailing for their application, and the skillfulness of their installation. These provide the 'control' on the extent to which deterioration and decay may be allowed over a given period of time. The durability factors concerned with the effects of fire, explosions and structural movements will be mentioned under the appropriate performance requirement to follow. The problems of daily wear and tear by the users, and attack from vermin, vandals and burglars, also form an important part of the design criteria for the building. All involve the careful selection of appropriate materials and functional detailing as well as thoughtful overall planning which will lessen the likelihood of their occurrence. Intended life span Ideally, a chosen design and construction method should be able to resist all detrimental effects and provide pro longed durability. As economic considerations make this impossible, the selection of materials and assembly technique for a design should be made to ensure that their rate of deterioration won’t impair the functional performance of a building, including appearance, during its intended life span. This period is difficult to quantify, so selection processes are generally influenced by an interrelationship between initial costs of materials and likely future maintenance costs. When deciding, it should be remembered that maintenance costs include not only labor and materials for renovation and for cleaning, but also sometimes financial losses resulting from the temporary curtailment of trade or business by the building owner or tenants while remedial work is being executed. Furthermore, where access for maintenance is difficult because of design detailing, consideration must be given to the cost of hiring scaffolding, special plant or even the provision of permanent gantries, mobile or otherwise. Gantries can form a dominant feature of the external or internal appearance of a building. The degree to which subsequent maintenance of a building is possible, desirable or necessary provides the 'fine tuning' which assists in achieving an optimum design in terms of initial running and future costs. A designer must be expert in the choice of materials, or carry out necessary research into their physical and chemical properties, before devising construction techniques which will take full advantage of their potentialities. Continually rising costs are forcing serious consideration of designing buildings to give a guaranteed, but limited life span (limit state design). In this way, the life of a building is predetermined by quantifiable characteristics, including predictability of certain detrimental influences. For example, it would be unnecessarily costly to incorporate an earthquake-resisting structure in a building with an intended useful life of 20 years if the earthquake 'cycle' where the building is to be erected does not fall within this period. If available, the money saved by not using elaborate structural solutions can be effectively used on materials and constructional details with guaranteed performance requirements during the 20-year life of the building. This approach can be extended to the statistical analysis of freak wind turbulences and amounts of rainfall in otherwise predictable climatic regions. However, financial savings in capital costs of construction methods (materials and technology) can only be made after thorough study and research have revealed the precise degree of durability achievable and therefore the life span of a building with minimal maintenance. Previous: Building
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