Wind Spectra and Dynamic Alongwind Response

Current methods for determining dynamic alongwind response are based on the assumption that turbulence spectra are independent of height. The adequacy of this assumption is assessed in the light of recent theoretical and experimental results of boundary layer meteorology. Expressions for the alongwind response, including deflections and accelerations, are proposed. It is shown that, in the case of tall, flexible, lightly damped structures (i.e., of structures for which the resonant response, as defined by Davenport and Vickery, is high), expressions of the spectrum which reflect its dependence upon height yield lower values of the alongwind fluctuating response than the expression currently used in building codes and standards in which this dependence is ignored. Results of numerical calculations show that the values of the fluctuating deflections and of the gust factors obtained if the latter expression is used may be higher by as much as 50 and 25%, respectively, than those obtained if the variation of spectra with height is taken into account.