Steady and unsteady wind loading of buildings and structures

Abstract

Wind loads are conveniently divided into steady (time-average) loads and unsteady (time-dependent) loads. The latter arise from fluctuating forces due to turbulence as well as from self-excited aerodynamic instability. The relevant characteristics of atmospheric winds—speed profile and turbulence—and their dependence on the local terrain are briefly discussed. For design based on steady wind loadings the design wind speed is dependent on the acceptable degree of risk. Force and pressure coefficients may be influenced by Reynolds number, surface roughness, wind characteristics and proximity to other structures. Unsteady loadings due directly to turbulence are assessed through the concept of aerodynamic admittance. Only the vortex-shedding and galloping mechanisms leading to aerodynamic instability are reviewed here, together with some design features for avoiding such excitations. To predict the response of even conventional structures while in the design stages further information is needed on the speeds and turbulence characteristics of natural winds, especially of those over cities. Further investigations concerning the aerodynamic admittance and the aerodynamic excitation of practical structures are also needed. Research on these properties involves wind-tunnel studies in which the separate effects of intensity and scale of turbulence must be determined, and improved techniques for simulating the characteristics of natural winds in wind tunnels are required. A major difficulty in the application of these aerodynamic data to the prediction of structural response is the uncertainty in the assessment of values of the structural damping.