Investigation on vertical stiffness reduction of thick rubber bearings under lateral displacement

Abstract

Elastomeric bearings with thick rubber layers, called Thick Rubber Bearings (TRBs), can be used as three-dimensional (3D) isolators. During their design, the vertical stiffness is a critical parameter. This study focuses on exploring the parameters that influence the vertical stiffness reduction of TRBs and on developing a simplified method for its prediction. Four full-scale TRBs with various first and second shape factors (S1 and S2) were designed and subjected to axial tests under lateral displacement. Their vertical stiffness was determined. The experimental results showed that for bearings with a lower S1, the vertical stiffness under small lateral displacement increases and then decreases as the lateral displacement increases. Then, tests at the material and isolator levels were used to calibrate a finite element (FE) model of TRBs. A parametric FE study was carried out to investigate the effects of various parameters on the normalized vertical stiffness of TRBs over a wide range of lateral displacements. The study quantified the vertical stiffness reduction for different values of pressure, S1, and S2. It also evaluated code recommendations for the steel shims used in TRBs. Then, existing formulations were compared to numerical results and it was shown that they cannot accurately predict the reduction of the vertical stiffness of TRBs. Finally, an empirical formulation considering the variation in S1 and S2 of TRBs was proposed. The predicted results showed good agreement with the numerical data.