Uncertainty parameters of the floor response spectrum in a seismically isolated structure

Abstract

Abstract To evaluate the earthquake safety of the equipment in structures, it is essential to analyze the Floor Response Spectrum (FRS). FRS has a peak value at a frequency corresponding to the structural vibration mode; however, the frequency and amplitude at the peak can vary because of many uncertain parameters. Several seismic design criteria exist for FRS peak-broadening in fixed-base structures. However, no criteria for constructing a design FRS for seismically isolated structures have been suggested. The FRS of isolated structures may change because of the major uncertainty parameter of the isolator, which is the shear stiffness of the isolator, and several other uncertainty parameters caused by the nonlinear behavior of the isolators. This study evaluated the effects of the initial stiffness of the bilinear curve of isolators and the variation in the effective stiffness based on the input ground motion intensity and intense motion duration. An analysis of a simplified structural model for an isolated base structure confirmed that the FRS at the frequency of the structural mode was amplified and shifted. The uncertainty in the initial stiffness of the isolators was found to significantly affect the shape of the FRS. The variations caused by the intensity and duration of the input ground motion were also evaluated. These results suggest several considerations for generating FRS for seismically isolated structures.