Exposure Categories and Transitions for Design Wind Loads

One of the greatest sources of uncertainty in the calculation of wind loads occurs in the selection of the wind exposure. This paper compares the traditional power-law exposure-coefficient curves used in North American codes and standards such as ASCE 7-02, 2002, with curves derived from modern models of the planetary boundary. It is concluded that the traditional exposure coefficients are reasonably consistent with modern boundary layer theory for heights below about 300 m. Above that height the traditional exposure coefficients increasingly depart from modern theory. The paper also examines the relationship between the dimensions and density of ground roughness obstacles and the exposure coefficient. This relationship enables a more objective assessment to be made than by the typical method of judging exposure "by eye," at least where the ground roughness is fairly uniform. However, many sites are affected by upwind changes in ground roughness. Simple expressions, suitable for use in a building code or standard, are proposed for calculating the effect of an upwind change in roughness on the exposure coefficient.