Evaluation of Site‐Dependent Inelastic Seismic Design Spectra

Abstract

The effect of site conditions on the response of single‐degree‐of‐freedom inelastic systems is investigated. The study is based on 124 earthquake ground motions recorded on rock, alluvium, and soft soil sites. Special emphasis is given to the effects of the level of inelastic deformation on strength and displacement demands. For each soil group, inelastic strength demand spectra corresponding to mean and mean‐plus‐one standard deviation ordinates are presented. The use of elastic analysis to estimate inelastic displacements is evaluated through mean values of the ratio of maximum inelastic to maximum elastic displacements. Spectra are presented for different levels of inelastic deformation and for different soil conditions. Results indicate that inelastic demands are strongly dependent on site conditions, period of vibration, and level of inelastic deformation. Results are compared with design forces specified by current seismic codes. It is shown that design force and displacement demands that are based on inelastic response spectra together with estimates of the overstrength of a structure can lead to a more rational and transparent approach than that of current U.S. design codes.