Stability of Elastomeric Isolators: Critical Load Tests and Computations

By Satish Nagarajaiah and Ian Buckle One of the most common seismic isolators in use today is the elastomeric bearing. The combination of rubber layers and reinforcing steel shims gives a device that is axially very stiff but soft laterally. Flexibility may be increased, and large period shifts achieved, simply by increasing the number and/or thickness of the rubber layers. But increasing the shear flexibility of these short columns can lead to relatively low buckling loads, which may be further reduced when high shear strains are simultaneously imposed. As a consequence, many design procedures require the axial load rating of a bearing to be reduced as the shear displacement increases (e.g., AASHTO 1999). These reductions are based on engineering judgment and very little science. For example, for a rectangular bearing of width B, the critical load P