Modeling of Lead Rubber Bearings under Large Cyclic Material Strains

Abstract

Lead rubber bearings (LRBs) have been widely used in seismic isolation systems for buildings and other structures to effectively mitigate the damaging effects of horizontal earthquake ground shaking. The horizontal flexibility of the LRBs, typically placed at the base of the structure, result in a concentration of displacements in the bearings while limiting deformations in the structure. Strong earthquake shaking can produce large displacement demands on the isolation system and are typically limited by a clearance or a moat around the perimeter of the structure. Recent research has evaluated the displacement capacity of isolated structures and the significant risk of failure from increased seismic demands on the isolators or from pounding to moat walls. This paper proposes a parallel nonlinear model to capture the complex behavior of LRBs at large strains in an effort to better predict isolator displacements and the potential for exceeding a limit state. The model captures strength degradation from lead core heating, stain hardening of the rubber, and unloading effects. Allowing the bearings to sustain large strains to induce hardening is explored as a means for reducing the impact velocity in the case that the clearance to the stop is exceeded. Numerical time history analyses are used to demonstrate the sensitivity of model selection for LRB especially when considering beyond design ground motions.