Understanding and Design of Seismically Isolated Struture Using Hardening of Bearing

Abstract eng:
For cities located in regions susceptible to strong earthquake shakings, seismic design of buildings and other facility is always a critical concern for structural engineers as well as general public. Nowadays, with the development of seismic design concepts and tools, structural safety is not the only design objective. The new concept of seismically resilient design, which aims at a higher seismic performance objective such as continually functional and shorter business downtime, is more and more accepted and implemented by earthquake engineering community. As an innovative seismic response control technology to achieve resilient design concept, seismic isolation has been widely studied and used. Classical idea of seismic isolation uncouples the upper structure movement from ground motion by concentrating displacement demand in isolation plane. However, when displacement demand is larger than the deformation capacity of isolator bearings, a failure of the isolated system will be expected, either due to the failure of bearings themselves or the large impact force introduced to the upper structure when the horizontal movements of isolator bearings are rigidly stopped. An unacceptable large risk of failure for the isolated system may be expected if the displacement capacity of isolation plane is not large enough. However, it is not economic or even not possible to provide large enough displacement capacity with a bearing designed to have constant low horizontal stiffness. Instead of using traditional isolator bearing with bi-linear hysteresis behavior, the study uses hardening bearing with increasing stiffness under large displacement and seeks an effective solution to provide enough deformation capacity and safety level for base isolated structure under beyond design seismic scenarios. However, hardening of bearing will introduce large force demand into the superstructure, therefore, the key problem to be solved in this study is how to find the balance point where enough hardening occurs to provide required displacement capacity without introducing too much damage into superstructure. The study takes Triple Friction Pendulum Bearing (TFPB) as an example of general hardening bearings. Seismic responses of a three story base isolated braced frame prototype structure with TFPB are evaluated numerically. Special attentions are paid to responses under beyond design seismic events when bearing starts to stiffening. Parametric studies have been conducted on key parameters characterizing the hardening of bearing. Based on comparison of numerical analysis results for different cases, recommendation on selection of these parameters are made through a proposed preliminary design approach.

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