A methodology for soil-structure interaction analysis accounting for spatially incoherent seismic free field motion

This paper deals with soil-structure interaction (SSI) analysis including spatial variability of the free field ground motion. Recordings of strong motion arrays on test sites have indicated that seismic free field exhibit spatial variability over even short distances. When considering large foundations, spatial variability may reduce translational structural responses. Concurrently, it may induce differential displacements between two points leading to rocking, or amplify nonlinear behavior such as lift off. It is therefore desirable to have at our disposition numerical methods to predict response when spatial incoherency of seismic ground motion is introduced. When carrying out soil-structure interaction in the frequency domain, then linear filtering is the simplest and appropriate approach for determining structural response. In this context, we present a methodology, based on the spectral decomposition of the coherency matrix, that allows to introduce free field ground motion incoherency in the common SSI analysis. The proposed methodology has been implemented in open source finite element software Code Aster and enables the analyst to determine time domain response as well as frequency domain power spectral densities and transfer functions. Frequency dependent impedance matrices are calculated with PROMISS3D, a subdomain boundary element code coupled to Code Aster. The methodology is very close to the one implemented in SASSI (EPRI version SASSI-INCOH) as will be detailed in the paper. The accuracy and the performance of the proposed method will be demonstrated by means of two study cases: a simple test case proposed by Mita& Luco and a more complex industrial building.