Damping implementation issues for in‐structure response estimation of seismically isolated nuclear structures

Abstract

AbstractSeismic (base) isolation of a nuclear power plant (NPP) is a viable strategy to reduce seismic demands in structural elements and nonstructural systems and components. Accurate estimation of in‐structure response of base‐isolated NPPs is critical to the analysis and design of safety‐related structural components. Damping models and their implementation in contemporary analysis platforms play a significant role in the numerical response estimation of a nonlinear system. Most of the recent damping studies cater to moment frame buildings for which nonlinearities are modeled using massless rotational springs. There are unique challenges associated with the implementation of damping to numerical models of the base‐isolated NPPs due to the broad frequency range of interest and the markedly different damping characteristics of the isolators and the superstructure. In‐structure response spectra of a base‐isolated NPP are sensitive to the chosen numerical and modeling techniques, both of which are investigated in this paper. Three representations of base‐isolated NPP are considered: (1) a two‐node macro model, (2) a lumped‐mass stick model, and (3) a detailed finite element model. Rayleigh damping, modal damping, uniform frequency damping, and several combinations thereof were used with the three NPP models to obtain the median response of the base‐isolated NPP subject to 30 sets of three‐dimensional ground excitations. The results allow informed decisions regarding the selection of a numerical model and damping implementation to different analysis platforms needed to obtain a reliable in‐structure response of base‐isolated NPPs subject to earthquake shaking.