Numerical Study on Buckling of Elastomeric Seismic Isolation Bearings

摘要

Seismic isolation allows the engineer to control damage in moderate and large earthquakes for both a building and its contents using low-cost structural systems. At the present time, there are several types of isolation system in use, many variants of existing systems are being developed, and new systems are being proposed and investigated. The most widely adopted system uses elastomeric bearings that decouple the building or structure from the horizontal components of the ground motion by interposing the structural elements with low horizontal stiffness between the structure and the foundation. This soft layer gives the structure a fundamental frequency that is much lower than both its fixed-base frequency and the predominant frequencies of the ground motion. One of the factors limiting further seismic application of bearings has been a general lack of knowledge regarding the buckling behavior of this type of bearing. The buckling of multi-layer elastomeric bearings under compression loads is a reasonably well-understood phenomenon. What is not well known is that theoretical buckling analysis for compression predicts that the isolator can buckle in tension at a load close to that for buckling in compression. The explanation of this rather counter-intuitive behavior is that the deformation in tension or compression is mainly shear and shear is inherently symmetric. The linear elastic model that leads to both compression and tension buckling is an extremely simple one and it might be argued that the tensile buckling may be an artifact of the model itself and not of the isolator. For this reason, the results of this simple model have been verified by a numerical simulation, using a finite element model of a multi-layer elastomeric bearing. The paper presents the major findings of the numerical analysis for the buckling of elastomeric bearings. The theoretical results are compared with a finite element analysis conducted on numerical models with various shape factors. The rubber material in the numerical models is assumed incompressible or almost incompressible and can undergo large deformations when the stress-strain relationship becomes significantly non-linear. The study shows that the prediction of tensile buckling by the simple linear elastic theory is, in fact, accurate and not an artifact of the model. The numerical study not only confirms the theoretical prediction but also shows an excellent correlation between theoretical and numerical buckling loads in both compression and tension and with other aspects of the behavior of multi-layer elastomeric bearings in tension and compression.