Unsteady wind induced forces in structural members

This paper describes an analysis of full-scale unsteady surface pressure coefficients and member strains measured on a duopitch portal frame structure. On the roof of the structure it is shown that there are high-frequency fluctuations that are not fully resolved at the normal sampling rate of 5 samples/s. The technique of proper orthogonal decomposition is used to convert measured point pressure coefficient time series to loading time series at specific frame loading points. A comparison of the spectra of the loads with the spectra of the pressure coefficients shows slight attenuation or amplification in the higher frequency range above 0.5 Hz. These loads are then used within a finite element package to calculate strain spectra within the frame. These spectra show that the high-frequency loads are significantly attenuated at frequencies of above 0.1 Hz, as they are transmitted through the cladding and frame. However, a comparison with measured strain spectra shows that the degree of high-frequency attenuation is somewhat over-predicted by the finite element package. The measured pressures and strains are then used to investigate the adequacy of the load response correlation method, to predict the loading pattern that results in maximum member strains. It is shown that proper application of this method requires the specification of gust factors that vary between 3.5 and 6.5 over the surface of the structure.