An alternative procedure for direct generation of seismic floor spectra

Several approaches have been developed for generation of floor spectra which do not require seismic inputs in the form of acceleration time histories but can use a prescribed set of ground response spectra directly. These approaches are often referred to as direct approaches. Most of these approaches are based on the method of mode displacement of structural dynamics. Some problems can, however, occur with these approaches if the response of a system is affected by the high frequency modes and if these modes are omitted from the analyses. Herein an alternative approach based on the method of mode acceleration is proposed wherein the effect of high frequency modes is correctly included without their explicit evaluation. The seismic inputs in this approach are required to be prescribed in terms of relative acceleration and velocity spectra. The approach is very effective for the calculation of floor spectra for structural systems which have significant contribution from high frequency modes, and also for floors close to the base which are usually affected by the higher modes. In other cases too, this approach has been shown to provide better results than the mode displacement approach for a given number of modes used in the analysis. Thus, a general use of this approach in lieu of the mode displacement approach is advocated. (orig.)