Wind loads in urban areas

Decisions in wind engineering usually are made on the basis of a load L—a wind pressure or force, or a concentration at a critical point. The load L, multiplied by a suitable safety factor, yields the design load Ldes. A more refined approach uses probabilities: it is demanded that the probability P{L>Ldes} of the load L to exceed Ldes is equal to or smaller than PEdes, i.e.:PL⩾Ldes⩽PEdes.This equation is the basis of stochastic design for wind loads. It is the purpose of this paper to discuss wind loads L as random variables, with particular consideration given to urban areas. Wind loads are seen as part of a design chain. Due regard is given to the fact that, in urban areas, standard wind tunnel results obtained for buildings or building complexes in the boundary layer along a rough or smooth surface cannot be used. As a step towards a general theory of wind effects in urban areas, wind forces and diffusion are considered for wind normal to the front of bluff bodies, for the three-dimensional case of cubical buildings in urban complexes, and for the two-dimensional case of street canyons. For these cases, the loads are expressed as functions of random variables, consisting of the product of the load for the reference case multiplied by gust and exposure factors. A survey of the literature on these factors is given.