Response of base-isolated nuclear structures for design and beyond-design basis earthquake shaking

ASCE 43-05 presents two performance objectives for the design of nuclear structures: 1) 1% probability of unacceptable performance for 100% Design Basis Earthquake (DBE) shaking, and 2) 10% probability of unacceptable performance for 150% DBE shaking. To aid in the revision of the ASCE 4-98 procedures for the analysis and design of baseisolated nuclear power plants (NPPs) and meet the intent of ASCE 43-05, a series of nonlinear response-history analyses was performed to study the impact of the variability in both earthquake ground motion and mechanical properties of isolation systems on the seismic responses of base-isolated NPPs. Computations were performed for three representative sites (rock and soil sites in the Central and Eastern United States and a rock site in the Western United States), three types of isolators (lead rubber, Friction PendulumTM and lowdamping rubber bearings), and realistic mechanical properties for the isolators. Estimates were made of 1) the ratio of the 99%-ile (90%-ile) response of isolation systems computed using a distribution of spectral demands and distributions of isolator mechanical properties to the median response of isolation systems computed using best-estimate properties and 100% (150%) spectrum-compatible DBE ground motions; 2) the number of sets of three-component ground motions to be used for response-history analysis to develop a reliable estimate of the median response of isolation systems. Only the results for the rock site in the Central and Eastern United States are presented. The results of this study provide the technical basis for the revision of ASCE Standard 4-98.