EFFECT OF SEISMIC WAVE INCOHERENCE ON SSI RESPONSE OF US EPR EPGB

Spatial incoherence of seismic waves has the effect of lowering responses of structural foundations. The effect is more significant at higher frequencies (generally exceeding 10 Hz) and larger building footprints. The purpose of this paper is to present the results of a seismic SSI analysis of the US EPR Emergency Power Generating Building (EPGB), a surface-founded concrete structure, for a hard-rock soil case using both coherent and incoherent ground motion input. For this study both a detailed FE model of the building, and a corresponding tuned lumped-mass stick model on a rigid foundation were used. These models were subjected to horizontal and vertical components of input motion and incorporated the coherency model for hard rock. The hard rock reference motions consisted of high-frequency rock motions. The incoherent structural responses were obtained using the Response Simulation method in MTR/SASSI. The SSI results were compared against each other to evaluate the effects of ground motion incoherency on the in-structure response spectra and to study the effects of model refinement (stick versus detailed FE model) on the calculated responses. This paper concludes with observations and recommendations regarding the treatment of incoherent input motions.