Seismic Response of Standard Thermal Expansion Bridge Bearings

Elastomeric bridge bearings have a history of use in the USA of close to 50 years. They are capable of carrying large vertical loads and allowing horizontal movement with low lateral force due to the extreme flexibility of the elastomeric material. They require little maintenance, have no moving parts, have good longevity and are very economical. The elastomer in bridge bearings is usually either polyisoprene (Natural Rubber) or polychloroprene (Neoprene). Thermal expansion bearings are designed for a variety of deformations produced by thermal expansion of the bridge superstructure, shrinkage due to aging or prestress, misalignment or beam rotation through a process of shear deformation, the material being essentially incompressible. Under currently accepted design practice, the maximum shear strain developed in the elastomer due to all sources of deformation is not more than about 50%. On the other hand, seismic isolation bearings for bridges and buildings are designed for much larger shear strains, and it is known that under shear loading the shear strain at failure can exceed 400%. This raises the question that bridge-bearing specifications may be excessively conservative. It is the purpose of this report to describe a series of experimental studies of the seismic behavior of three types of standard thermal expansion bearings for bridges. The experimental program has used the CALTRANS Seismic Response Modification Device (SRMD) Testing System at the University of California at San Diego. The three types covered by the test program are 1) Steel-Reinforced Elastomeric Bearings, 2) PTFE-Elastomeric Bearings, and 3) PTFE-Spherical Bearings. The test program includes wide-range dynamic tests at velocities and displacements that could be experienced by these bearings but for which they have not been designed. Only the results of the test program on the elastomeric pads are described in this report. INTRODUCTION This report will describe the testing of a set of six elastomeric bridge bearing pads manufactured to Caltrans requirements by Scougal Rubber Corp. These pads are designed and used solely as thermal expansion bearings but the purpose of the research work is to determine the response of such pads if they are subjected to seismic loading. Many bridge structures are provided with seismic isolation bearings specifically designed for that purpose and considerably more expensive