Floor response spectra for beyond design basis seismic demand

Abstract

Vulnerability assessment of Structures Systems and Components (SSCs) of nuclear facilities for earthquake ground motion exceeding the design basis ground motion has become a key issue to ensure safety in case of highly improbable but possible extreme earthquake event beyond design basis. In order to assess seismic safety of SSCs located at different floors of the building, floor response spectra (FRS) for such beyond design basis seismic demand, taking into account possible nonlinear behavior of structure, is required. Currently used methods (e.g. IAEA Safety Series 28) do not capture softening of structure at increased demand. A performance based approach would be ideal under such situations. FRS for higher seismic demand may be developed through nonlinear time history analysis of the structure. However, this approach is complicated and cumbersome for structural configurations such as that of NPPs and requires deep insight of solution algorithm and large resources for numerical solution. In this work a simplified methodology for FRS generation for higher level of seismic demand using linear time history analysis of an equivalent linear structural model is proposed. Nonlinear behavior of structure and associated damping are estimated from Nonlinear Static Pushover Analysis. Through this approach realistic estimation of stiffness degradation of the structure and increase in damping at different levels of seismic demands can be made. Subsequently, linear time history analysis is performed on degraded structure, accounting for stiffness reduction and hysteretic damping to obtain the floor time histories which are then converted to FRS. Application of this method is shown through case studies.