Generate floor response spectra: Part 1. Direct spectra-to-spectra method

Abstract

A direct spectra-to-spectra method is developed for generating floor response spectra (FRS), which are used as input for seismic design and assessment of secondary systems in nuclear facilities. Only ground response spectra (GRS) and basic modal information of primary structures, including natural frequencies, modal damping, participation factors, and mode shapes, which can be readily obtained from a modal analysis, are needed. In tuning cases, when equipment is resonant with the primary structure, the concept of t-response spectrum (tRS) is proposed, and statistical relationships between tRS and GRS developed in the companion paper are employed to determine FRS. Furthermore, a new modal combination rule (called FRS-CQC), which considers the correlation between modal responses of a structure and correlation between responses of equipment and its supporting structure, is derived based on random vibration theory. Numerical examples of a typical service building in nuclear power plants indicate that results from the proposed method agree extremely well with the numerically “exact” FRS obtained from a large number of sets of time history analyses. It is observed that FRS from time history analyses have large variabilities, particularly in tuning cases. Numerical examples demonstrate that the modal combination rules have a significantly effect on the accuracy of FRS by the direct spectra-to-spectra method. The proposed direct spectra-to-spectra method, which avoids the deficiencies of time history methods, is of excellent accuracy, efficiency, and simplicity; it is convenient to implement in practice for generating FRS.