Design of Safe Base Isolated Structures Using Hardening of Bearing

摘要

For cities located in regions susceptible to strong earthquake shakings, seismic design is always a critical concern for structural engineers and the general public alike. Nowadays, with the development of more advanced seismic design concepts and tools, structural safety is often not the only design objective. Seismic resilience, which aims at enhanced seismic performance objectives, such as continued functionality and reduced business disruption, is of greater interest to the earthquake engineering community and is being incorporated in the design of a wide range of projects. Seismic isolation is an innovative means of enhancing safety and resilience, and has been widely studied and used. The classical idea of seismic base isolation uncouples the movement of the structure from the ground motion by supporting it on manufactured devices typically having low horizontal and high vertical stiffness. This limits the forces transferred to the super-structure and concentrates horizontal deformations in the isolation plane. A wide variety of analytical and experimental studies have demonstrated the ability of various seismic isolation systems to reduce structural and nonstructural damage during ground shaking representative of frequent to rare seismic events. However, concern has been raised recently about the possible performance of isolated structures during earthquakes larger than those considered in design. This might happen over the life of a structure, if estimates of seismic hazard at a site increase. It may also be associated with current design codes and practices where isolators are designed for the average displacement demand predicted for the MCER hazard. In such cases, nearly half of the time maximum considered ground motions might produce bearing displacement demands greater than those considered in design. Because current building codes generally do not require restraint of the maximum lateral displacement of the isolators, these displacement demands might exceed the capacity of the bearings, resulting in a risk of collapse larger than recommended by modern building codes. A straightforward solution would be to use isolation bearings having larger displacement capacities. However, this is not always feasible due to the cost or size of bearings needed. Another approach would be to rely on a moat wall or similar mechanism to provide a certain hard stop. But this may not be architecturally possible, and impact forces on the superstructure might cause unacceptable damage. The use of bearing that significantly harden under large lateral displacement is explored in this paper as an effective means of limiting isolator displacement and reducing the risk of collapse during unusually large seismic events. Since displacement hardening will increase the force demands in the bearings and superstructure, a key problem to be solved is finding a reasonable balance point where enough hardening occurs to limit displacements without introducing too much force or damage into the superstructure. While various isolation bearings, including lead plug rubber bearings (LPRB) and high damping rubber bearings (HDRB), exhibit hardening behavior at large lateral displacement, this study considers only Triple Pendulum Friction Bearings (TPFB). Their behavior is representative of many types of isolation systems. In this paper, the seismic response of a three-story base isolated concentrically braced steel frame structure supported on TPFBs is examined. Special attention is paid to inelastic responses under MCER level events where bearing hardening is introduced. Results of parametric studies are presented, highlighting key parameters such as bearing displacement, story displacement demands and so on. Based on the limited analysis results presented, preliminarily recommendations are offered regarding improved design procedures.