Experimental Investigation of the Seismic Response of Bridge Bearings

The Illinois Department of Transportation (IDOT) commonly uses elastomeric bearings to accommodate thermal deformations in bridges. These bearings also present an opportunity to achieve a structural response similar to isolation during seismic events. IDOT has been developing an earthquake resisting system (ERS) to leverage the displacement capacity available at typical bearings in order to provide seismic protection to substructures of typical bridges. The research program described in this report was conducted to validate and calibrate IDOT’s current implementation of design practice for the ERS, based on experiments conducted on typical full-size bearing specimens, as well as computational models capturing full bridge response. The overall final report is divided into two volumes. This first volume describes the experimental program and presents results and conclusions obtained from the bearing and retainer tests. The experiments described in this volume provide data to characterize force-displacement relationships for common bearing types used in Illinois. The testing program comprised approximately 60 individual tests on some 26 bearing assemblies and components (i.e., retainers). The testing program included (1) Type I elastomeric bearings, consisting of a steel-reinforced elastomeric block vulcanized to a thick top plate; (2) Type II elastomeric bearings, distinct from Type I bearings with a steel bottom plate vulcanized to the bottom of the elastomeric block, and a flat sliding layer with polytetrafluoroethylene (PTFE) and stainless steel mating surfaces between the elastomer and the superstructure; and (3) low-profile fixed bearings. Tests conducted to simulate transverse bridge motion also included stiffened L-shaped retainers, consistent with standard IDOT practice. Tests were conducted using monotonic and cyclic displacement protocols, at compression loads corresponding to a range of elastomer compression stresses from 200 to 800 psi. Peak displacements from initial position ranged from 7-1/2 in. to 12-1/2 in., depending on bearing size. Test rates were generally quasi-static, but increased velocities up to 4 in./sec were used for bearings with PTFE and for a subset of other elastomeric bearings. On the basis of all of the experimental findings, bearing fuse force capacities have been determined, and appropriate shear stiffness and friction coefficient values for seismic response have been characterized and bracketed.Illinois Department of Transportation R27-70published or submitted for publicationnot peer reviewe