The influence of damping on floor spectra in seismic isolated nuclear structures

Abstract

Summary Seismic isolation is now being considered for nuclear power plant structures and has been applied to many nuclear structures including reactor buildings in France. A draft report entitled ‘Technical Considerations for Seismic Isolation Nuclear Facilities’ has been prepared by the USA Nuclear Regulatory Commission in 2010. It sets out to describe different types of isolators and recommends that lead plug isolators and FP isolators are acceptable choices for nuclear isolation systems. For the design of base isolation systems for nuclear structures, the design engineer is faced with very large design displacements for the isolators and supplementary dampers or highly damped isolators are prescribed to reduce them. These dampers reduce displacements but at the expense of significant increases in interstory drifts and floor accelerations in the superstructure. In this paper, an elementary analysis based on a simple model of an isolated structure is used to demonstrate this dilemma. The model is linear and is based on modal analysis, but includes the modal coupling terms caused by high levels of damping in the isolation system. Estimates of the floor response quantities are obtained by the response spectrum method. It is shown that as the damping in the isolation system increases, the contribution of the modal coupling terms becomes the dominant term. The results show that the use of damping in seismic isolation when the purpose of the isolators is to protect sensitive internal equipment is a misplaced effort, and alternative strategies to solve the problem are suggested. Copyright © 2014 John Wiley & Sons, Ltd.