STABILITY OF ELASTOMERIC SEISMIC ISOLATION SYSTEMS

Elastomeric bearings with and without a lead core are commonly used as seismic isolators for protecting buildings, bridges and industrial plant from earthquakes. However these devices have relatively low critical loads and those with slender geometry are believed to become unstable when subject to heavy axial load and simultaneously deformed to a high shear strain. But little experimental evidence exists to confirm the critical loads of these bearings and even less is known about the stability of a system of elastomeric bearings. In this paper an experimental investigation into the stability of a large-scale, curved, 3-span bridge model isolated by 12 lead-rubber bearings is described. The bridge was tested using the shake table array at the University of Nevada Reno. It was found that the horizontal curvature had little effect on the response of the isolators, but it did cause asymmetry in the seismic response resulting in higher lateral displacements in the abutment isolators. Adequate vertical load capacity was maintained even at displacements exceeding the isolator diameter, when, by first-order theory, they should be unstable. Isolator instability at an abutment and a pier did occur at three times the design earthquake where the isolator shear strain was up to 400%. However, this instability did not cause the bridge to collapse because the isolators at other supports remained stable due to smaller imposed displacements. It has been shown that a system of isolators may remain stable even as individual isolators in that system become unstable.