Effect of Design Methodology on Collapse of Friction Pendulum Isolated Moment-Resisting and Concentrically Braced Frames

Base isolation is an effective method for mitigating seismic hazards and improving seismic performance under design-level ground motions. Previous studies have typically focused on comparing the performance of isolated and fixed-base buildings under design level or maximum considered level earthquakes without modeling the failure of the isolation bearings components. However, it is also important to include the performance of the isolation system under extreme conditions. A numerical model which explicitly includes both impact and uplift behavior of the sliding bearings as well as degrading behavior of the superstructure is used to investigate the collapse risk of the various friction pendulum isolated moment-resisting and concentrically braced frame designs. It is found that the stiffness of the superstructure has a large influence on the overall collapse risk. For flexible moment-resisting frames, increasing the strength is beneficial to improving the safety margin between impact and system-level failure. However, for stiff concentrically braced frames, because of their high stiffness, impact with the sliding bearing rim imposes a large ductility demand on the superstructure regardless of its strength, resulting in an unacceptable probability of collapse.