Soil-Structure Interaction Analyses for Seismic Response

Abstract

In this reply to issues raised by discussers, the author attempts to present some of the current views with regard to this subject. The most critical aspect of soil-structure interaction analysis is the specification of the seismic design forces. This involves determination of acceleration level, frequency content, and spatial variation of the motion at the site prior to construction. Analysis with waves at bedrock level are discussed and it is suggested that a practical approach would be to assume a spatial variation corresponding to vertically propagating S-or P-waves. The approach is used in most methods including impedance methods which assume that all points on the surface have the same motion. The method implies the assumption of a horizontally layered site. The assumption of vertically propagating waves leads to the conclusion that surface motions are site dependent. The existance of natural site periods and significant site amplification predicted by theories of vertical propagation are discussed, and it is noted that observed discrepancies between spectral shapes can be entirely explained in terms of expected variations in the response to the bedrock level in different directions and during different earthquakes. Comments are made regarding surface waves, and vertical wave propagation. Strain-dependent soil properties and radiation dampling versus hysteretic damping are also discussed.