Static and Dynamic Stability of Elastomeric Bearings for Seismic Protection of Structures

Abstract

Bearings used in the seismic isolation of buildings can be subjected to large horizontal deformations combined with high axial loads from overturning forces during strong shaking. Elastomeric bearing design requires an evaluation of the critical load capacity under this combined loading to ensure stability. To better understand and estimate the capacity of elastomeric bearings, a comprehensive experimental program was carried out to examine bearing stability using two quasi-static and one dynamic loading procedures. The first method to evaluate the bearing critical load follows previous experimental research, where a single elastomeric bearing is held at prespecified horizontal displacements while increasing the axial load until the critical load is achieved. In a second method implemented here, a similar setup is used to apply a constant axial load followed by increased horizontal displacement until the stability limit is observed. This second method is shown to be an accurate and more direct approach to obtaining the critical load directly from the test data. The third method evaluates the dynamic stability of a rigid frame on four elastomeric bearings subjected to earthquake motions on a shake table. These tests are able to quantify the response of elastomeric bearings at and beyond their stability limit under more realistic loading conditions. Results from the three experimental procedures are compared, and the reduced-area formulation commonly used to predict the critical load of elastomeric bearings is evaluated, resulting in new recommendations for defining the parameters used in this formula to improve its accuracy.