Abstract Summary
In important facilities such as nuclear power plants or plant facility data centers, the safety of both the structure as well as the facilities and equipment installed in the structure must be ensured. This is because when the equipment installed in the structure is damaged, nuclear power plants can cause fatal damages, such as core damage or severe social damage caused by failure to perform the main functions of the facility. To reduce the intensity of the seismic response of the installed equipment effectively, the entire structure should be used as a seismic isolation structure. In this context, the floor response spectrum (FRS) must be analyzed to evaluate the seismic response on the equipment installed on each floor. The FRS has a peak value at the frequency corresponding to the structural vibration mode, but the frequency and amplitude of the peak can vary because of several uncertainties such as site characteristics, soil structure interaction, and structural vibration characteristics. Therefore, the broadening of the peak frequency in FRS has been introduced in order to evaluate the design FRS conservatively. The criteria for FRS peak broadening for fixed base structures was suggested, but there are no suggested criteria for seismically isolated structures. In terms of seismic isolation structures, whether the criteria of a fixed structure can be applied must be verified, since they are obtained via analysis, the isolator’s initial stiffness, and the uncertainty of ground motion due to nonlinearity. In this study, the vibration of FRS due to the initial stiffness of bi linear behavior of isolators and the intensity and duration of ground motions was evaluated that have not been reviewed in detail when designing isolators and generating FRS of isolated structures. By analyzing a simplified structural model for base isolated structure, it appears that the natural period of the fixed base structure has shifted owing to the seismic isolation system, and this phenomenon was confirmed by solving the equation of motion of the two degree-of-freedom seismic isolation system. It was found that the uncertainty of initial stiffness of isolators also affects significantly to the FRS. The FRS of earthquake waves weaker than the design earthquake intensity was almost overlapped by that of the existing design earthquake intensity. Also, the FRS changed significantly depending on the strong motion duration of the earthquake, and the shorter the strong motion duration of the earthquake, the higher was the peak value only in certain periods, such as the seismic isolation period. In the case of seismic isolation structures, the FRS increases nonlinearly as the earthquake intensity increases from a low earthquake intensity owing to the nonlinear behavior of the isolator. This phenomenon is particularly important when evaluating the seismic intensity of equipment exceeding the design earthquake intensity, and it can be applied by expanding the characteristics of the FRS based on the earthquake intensity discussed herein. From these results, the several considerations for generating design FRS for seismically isolated structures were suggested.
