Shear and Friction Response of Nonseismic Laminated Elastomeric Bridge Bearings Subject to Seismic Demands

Laminated elastomeric bridge bearings are commonly used in areas with low-to-moderate seismicity, although the applications are typically intended for service-level considerations such as thermal movements of the bridge superstructure. These components provide a potential source of displacement capacity frequently neglected in seismic design. An experimental program was carried out to evaluate the behavioral characteristics and performance of steel-reinforced, laminated elastomeric bearings, which had not been designed for seismic demands, as the primary quasi-isolation components for seismic events by permitting slip at the interface of the bearing and substructure. The rubber at the top of the bearing is vulcanized to a steel plate, which is bolted to the test frame to simulate a connection to the superstructure. At the base of the bearing, the elastomer directly contacts concrete representing the substructure, with no restraint of horizontal motion other than friction. The elastomeric bearings investigated during the experimental program displayed an approximately linear elastic response before sliding, with an initial friction coefficient in the range of 0.25-0.5 (at a shear strain between 125 and 250%) depending on combinations of the contact surface roughness, applied load, and bearing velocity. The friction coefficient decreased as a nonlinear function of the imposed vertical load. The maximum elastomer shear strain prior to sliding exhibited nonlinear increases with vertical load, resulting from the influence of the variable friction coefficient. Linear shear moduli were primarily influenced by the maximum shear strain imposed on the bearing, and showed shear stiffness reductions of approximately 40-50% following multiple, large displacement slip cycles, compared with 15-25% after reaching 50% shear strain. Multiple cycles of large displacement demands resulted in noticeable degradation in the friction coefficient over the duration of the tests. However, the bearings possessed a high degree of resiliency, considering that the specimens retained load-carrying capacity through total cumulative slip travel demands in the range of 3.5-4.5 m (140-180 in.).