Seismic margin review procedures for nuclear power plant civil structures

Depending on the level of the hypothetical Seismic Margin Earthquake (SME), some reevaluation of one or more civil structures may be required. The reevaluation may consist of generation of new in-structure (floor) response spectra or new structural seismic loads or both. The goal of a Seismic Margin Assessment (SMA) is to provide rational, moderately conservative predictions of seismic response of the buildings. In order to generate this seismic response, consideration of a number of parameters which enter the analysis process is required. It is not necessary that conservatism be introduced at every step of the reevaluation process with subsequent compounding of the final level of conservatism as has normally been the case of seismic design procedures. In the recommendations presented in this paper, the approach taken is to concentrate the conservatism in the parameters which have the largest uncertainty, and utilize mean or median-centered or slightly conservative values for the remaining parameters. Thus, it is recommended that the SME for the site be described in terms of the median plus one standard deviation response spectra, and that a relatively broad range of soil parameters be used in soil-structure interaction analysis in order to assure an above expected response is computed and, hence, assure an adequate overall level of conservatism. For other areas of the SMA, it is recommended that median-centered approaches be used.