Experimental Investigation of Elastomeric Isolation Bearings with Flexible Supporting Columns

In an effort to mitigate major costs associated with isolation systems, especially in retrofit applications, isolation bearings may be placed on the tops of columns. In this configuration, a seismic gap and additional rigid diaphragm are no longer necessary, and excavation and foundation work may potentially be avoided in retrofit applications; however, columns under the isolation layer may not provide rigid boundary conditions, resulting in bearing end plates not remaining parallel. To investigate the effects of flexible columns in a column-top isolation system with elastomeric bearings, quasi-static cyclic testing of bearing-column subassemblies were conducted. Findings show that flexible end conditions can significantly reduce the lateral stiffness of elastomeric bearings. A simple analytical stiffness matrix is derived for bearings consisting of translational and rotational degrees of freedom at both end plates and is compared against experimental results. The simple model compares well for initial behavior but diverges from experimental results with larger end-plate rotations; however, when calibrated nonlinear material properties are included, the model performs well at capturing the behavior of elastomeric bearings when subjected to partially restrained boundary conditions.