| Conceptual Reference Database for Building Envelope Research |
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| Essay:stresss and strain of building envelope"The strains and deformations in building elements from stresses produced by loading are well recognized and are regularly dealt with in structural design. But other small dimensional changes take place in materials due to causes other than loading. These may give rise to deformations, loads and stresses that are not always adequately taken into account. Creep, a deformation with time under load, and thermal expansion and contraction are usually considered in the analysis of the structural frame, but they may be neglected in the design of the wall itself. Dimensional changes may also result from changes in moisture content in certain materials and from aging and degrading effects produced by environment.When these expansions or contractions are not restrained, the element merely changes dimension. This in itself may often be a problem. For example, a temperature differential of 100 deg. in steel or concrete gives rise to dimensional changes of about 0.06 per cent. When these are restrained by adjacent material or by an adjoining element, strains and corresponding stresses can develop. A strain of only 0.01 per cent is required to fail normal plain concrete in tension. A dimensional change of this magnitude can be produced by a temperature change of about 20 deg. Normal concrete blocks will shrink as much as 0.04 per cent when dried from a saturated state. Cracking, crazing or buckling of exposed surfaces or of surface coatings often occur as a result of temperature or moisture changes that promote either shrinkage or expansion. Once cracks have developed, severe wetting of the wall will usually follow. This results not only from direct penetration of rain but also from the condensation of water vapour carried in air leaking outward through the cracks. Obviously this whole subject of dimensional changes in materials and the induced loads and failures that can result merits more extended treatment. There is some discussion of the subject in CBD 30 and this will be extended in future Digests. It has been introduced here because of its obvious relationship with other requirements. Dimensional stability, although a highly desirable characteristic of all materials, elements and components, and having particular implications in exterior wall design, bas not been listed as a requirement. It is implicit in the requirements for strength and durability and in the barrier requirements, because these influence temperature, moisture and other conditions that may produce dimensional changes. Discussion of it has served to emphasize further the inter-relationships and the inter-dependence of the various requirements that have been set out. It has also demonstrated the importance of a knowledge of the dimensional stability characteristics of elements and components as well as of materials in any rational approach to wall design." | ||||||||||