Seismic soil-structure interaction effects on probabilistic floor response spectra

For the purpose of performing a Seismic Probabilistic Risk Assessment (SPRA) for the Individual Plant Examination of External of Events (IPEEE) program, probabilistic soil-structure interaction (SSI) analyses for the major nuclear power plant (NPP) structures on the site were performed. The paper describes the probabilistic seismic SSI methodology and the probabilistic models used for the idealization of seismic excitation and surrounding soil deposit. To illustrate the effects of randomness in the input parameters, simulated and/or probabilistic Floor Response Spectra (FRS) at selected locations inside the Reactor Building (RB) and Auxiliary Building (AB) are plotted. The computed probabilistic FRS show that the randomness coming from the soil stiffness affects significantly the FRS of the RB and less significantly the FRS of AB. The coefficients of variation (or the standard deviations in the log normal format) of the FRS of RB due to soil stiffness randomness are considerably larger at upper elevations than at the basemat level showing that the random SSI effects are primarily manifested through the rocking motions and less through the horizontal translation of the base. The use of the current accepted rule for SPRA median response for median input may lead to unrealistic peaks in the median FRSmore » especially when the SSI effects are significant as shown herein. This is due to the strong nonlinear relationship between the spectral amplitudes and soil stiffness at the SSI resonant frequencies.« less