Wind Loading and Building Exposure: Are We Still on A, B, C?

Abstract

Wind-induced pressures for the design of components and cladding, as well as primary structural systems of rigid buildings are calculated by using provisions of ASCE 7 [ASCE/SEI 7-05, 2006] or the National Building Code of Canada [NBCC 2005] depending on the building roof shape and location. In general, these pressures are the product of a dynamic velocity pressure (q), an exposure factor (Ce) and a gust pressure coefficient (CpCg). For simplicity, gust pressure coefficients used to originate from extreme values obtained in boundary layer wind tunnel experiments under conditions of open country upstream exposure and were reduced by directionality arguments by a factor of 0.80 or 0.85. Regardless of the actual exposure of the low building, the conservative assumption of open upstream exposure warranted very good results, at least in most, if not all, cases. The paper reviews some of the rationale behind the values provided in wind standards and codes of practice and compares experimentally measured vertical uplift and horizontal thrust coefficients on an end bay of gabled roof low buildings in suburban terrain roughness with Canadian, American and European corresponding provisions. Some recent work on the influence of exposure on wind loading of low buildings is also presented.