Stability of Elastomeric Isolation Bearings

Elastomeric isolation bearings are required to be stable at high shear strains, which occur during strong earthquakes. Hence, rigorous determination of the critical axial load during design is important. Currently the critical load is determined using small displacement Haringx theory and modified to account for large shear strains by an approximate correction factor. The objective of this study is to experimentally determine the effect of horizontal displacement or shear strain on critical load and to study the validity of the approximate correction factor. Experiments were conducted on a series of elastomeric bearings with low shape factors. Test procedure and test results are presented in detail. It is shown that the critical load decreases with increasing horizontal displacement or shear strain. It is also shown that substantial critical load capacity exists at a horizontal displacement equal to the width of the bearing and is not zero, as predicted by the correction factor. It is further shown that the approximate formula is not conservative at smaller displacements and overly conservative at larger displacements. The critical loads obtained from experiments are compared with results from finite element analyses and nonlinear analytical solutions; the comparisons indicate that the effect of large horizontal displacements on the critical load can be reliably predicted.