Effect of Soil-Structure Interaction (SSI) on the Surface Accelerations in Liquefiable Soils

Abstract

The real response of a structure during earthquakes depends on the foundation input motion. The foundation input motion is usually considered as free field surface accelerations based on the site-specific ground response analysis. In this study, the effect of soil-structure interaction (SSI) on the foundation input motion and base accelerations was investigated using a typical 4-5 story residential building in liquefiable soils. The foundation input motion (FIM) was evaluated in finite element geotechnical software using a constitutive model representing the liquefaction behavior. The effect of the existence of the building was examined considering the massless boundary of the foundation (kinematic effect) and the building itself (inertial effect). The results demonstrated that the foundation input motion remarkably differs from free field surface motions in the existence of the building. The spectral base accelerations were obtained with higher damping and with a wider range of predominant periods in liquefiable soils when compared to the base acceleration spectra without SSI. The modal analysis was performed for the building in a structural finite element software incorporating the SSI. The structural 1^st mode natural period significantly increased in liquefiable soils. Therefore, the actual accelerations that the building is exposed to were estimated to be higher or lower than the computed accelerations without the effect of SSI, depending on the structural natural period and thickness of the liquefiable layer.