Selection and Scaling of Ground Motions for Generating In-structure Floor Spectra in Nuclear Power Plants
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Response-history analysis has become a widely used tool for design and performance assessment of structures, systems and components. Prior to such analysis, an engineer is required to select and scale earthquake ground motions for input to a numerical model. Nowadays, best practice is to select and scale ground motions with a spectral shape similar to that of the target spectrum. Spectral matching is used for design and performance assessment of nuclear facilities in the Central and Eastern United States, because the number of available seed motions is small. Spectral matching eliminates the inherent variability in the two orthogonal components of a ground motion. As a result, the distribution of structural response, which is important for design and performance assessment, is underestimated. Variability in spectrally matched motions can be reintroduced using Max-Min approach, which uses amplitude scale factors to address the difference in the spectral accelerations of two orthogonal horizontal components. The scale factors are determined based on the past studies on the ratio of maximum to geometric mean spectral acceleration for recorded ground motions. In regions where sufficient number of ground motions are available, the Greedy algorithm can be used to select a set of recorded motions that match a target spectrum. Motions selected using Greedy algorithm will retain the inherent variability between the two horizontal components.This study compares distributions of floor spectral responses for three sets of ground motions that are selected and scaled using 1) spectral matching, 2) the Max-Min approach, and 3) the Greedy algorithm. The three sets of motions are generated to be consistent with a target spectrum for horizontal shaking. Response-history analysis of a sample nuclear reactor building is performed using the three sets of ground motions. Floor spectra are calculated at specific nodes in the structure to compare the results obtained using the three sets of ground motions. The distribution in spectral acceleration was smallest if spectrally matched motions are used. The second and the third set provide similar distributions of floor spectral acceleration. Accordingly, the Max-Min strategy could be used for generating ground motions for seismic analysis in regions of low-to-moderate seismic hazard where the number of seed motions for scaling by spectral shape is too small.