Energy-based seismic design method for seismically isolated structures with friction pendulum systems

Abstract

Friction pendulum systems (FPSs) serve as effective isolators that can help ensure adequate seismic performance of structures against significant aftershocks. A commonly used method for FPS design is the equivalent static analysis method outlined in ASCE 7-22. However, this method risks underestimating the seismic performance of FPSs due to its inability to account for energy dissipation from multiple hysteresis loops exceeding two cycles. Moreover, it is not well-suited for designing FPSs with conical friction surfaces, which exhibit nonlinear elastic behavior and hysteresis. A straightforward solution to address these issues is to apply an energy-based seismic design method. In this study, we propose and analytically validate one such method tailored for seismically isolated structures employing FPSs. Our analysis considers FPSs with both spherical (ISF) and conical (ICF) friction surfaces. For ISFs, the design follows the energy-based seismic design method proposed by Akiyama, whereas, for ICFs, it is based on the energy input to the device and its geometric approximation to an ISF. The conclusions of this study are summarized as follows: (1) the proposed energy-based design method effectively predicts the maximum FPS deformation, facilitating a rough estimation of FPS size; (2) because the design assumes symmetrical seismic response of the isolation layer in both the positive and negative directions and the friction surfaces of the ISF and ICF are geometrically similar, in the proposed method, the maximum deformation of an isolation layer with a natural period greater than 2 s depends solely on the friction coefficient, regardless of the friction surface type; (3) to satisfy the relevant conditions, the appropriate FPS design range is determined through nonlinear analysis. The results of this study demonstrate that the proposed energy-based seismic design method provides a reliable and effective means of designing FPSs for enhanced seismic performance.