Designing for extreme events: Behavior of isolated structures subjected to impact

While it is well known that isolators provide enhanced seismic performance to structures under design basis ground motions, excessive displacements in large earthquakes may induce detrimental pounding of base-isolated structures against the moat wall or the restraining rim of friction pendulum bearings, resulting in yielding or even collapse of the superstructure. While codes have focused on designing typical non-isolated structures to ensure low likelihoods of collapse in extreme events, this is a critical missing piece for the design of isolated structures. For isolated structures, the design philosophy is different not only around the world, but also design-by-design. Using experimentally validated bearing models which can capture bearing impact and failure, this paper summarizes studies of multiple superstructure and isolation design methodologies. Worryingly, braced-frame structures designed to North American code provisions, which are recognized to provide excellent performance under design-level earthquake hazards, are shown to have high probabilities of collapse under maximum considered earthquake levels primarily because of pounding. This phenomenon is further evaluated using a simplified two degree-of-freedom model to investigate the inelastic response of isolated structures subjected to impact. It is found that the stiffness of the superstructure largely dictates its response to the transient impact force.