Effective seismic input through rigid foundation filtering

Abstract

This paper develops a simple yet realistic approach to account for a class of important soil-structure interaction phenomenon, namely wave scattering. The foundation is considered to act as a filter, partitioning the impinging excitation into lowpass filtered translational motion together with torsional motion (for horizontally polarized shear and Love waves) and rocking motion (for compressional, vertically polarized shear, and Rayleigh waves). These effects are evaluated for arbitrarily incident, horizontally polarized shear waves. The results are expressed in terms of filtering functions for various foundation geometries and embedment conditions and compared with ‘exact’ and other approximate solutions. Numerical results for a time history are presented in the form of translational and torsional acceleration response spectra. The formulae for these filters in the frequency domain show an interesting general relationship between the ‘effective’ translational and torsional motions of the foundation: the two motions are 90° out of phase for a given free-field harmonic wave, suggesting the possibility of computing the responses of structures to the two foundation motion components independently and combining the results by SRSS method. The future research need for adaptation of filtering concept to seismic excitations is discussed.