Elastomeric Bearings: Background Information and Field Study

This report deals with Phase One of TxDOT Project 1304 (Behavior of Elastomeric Bearings). The overall objectives of that project are (1) to recommend procedures for designing elastomeric bearings used by TxDOT and (2) to recommend practical guidelines and procedures for inspecting existing and future elastomeric bearings. The overall objectives of Phase One of that project are: to verify field reports; to conduct field reports; to conduct field surveys; and to document bearing and girder movements.

To accomplish the Phase One objectives, the following tasks were carried out: a comprehensive literature review was conducted; field reports were verified by site visits (Slaughter Creek, Beaumont, Paris, and Alanreed) and by coordination with the BRINSAP database; two bridges were selected for field instrumentation and study (Slaughter Creek and Alanreed) with emphasis on bearing and girder movement; and the BRINSAP database was used to identify bridges that might have bearing problems.

The results of field study can be summarized as follows:
-- Two bridges were instrumented and monitored.
-- Bearing and girder movement was monitored on both bridges. The effect of resetting the original natural rubber bearings at the Slaughter Creek bridge was compared with the effect of replacing the rubber bearing with a neoprene bearing.
-- Girder movement was measured up to 3/8 inch (1 cm) in contraction and expansion, due to temperature changes; both daily and seasonal variations were recorded. It was shown that these movements were easily predicted with simple engineering models.
-- After the variousresetting and replacement operations, the bearing movement measured at Slaughter Creek was inconsistent and not reproducible. Movement was found to be zero at one time and then unlimited at another time, with no apparent change in loading conditions. However, when it did occur, bearing movement was immediate.
-- Resetting with natural rubber bearings was ineffective. However, when the natural rubber was replaced with neoprene bearings, bearing movement stopped.

The principal conclusion were:
-- Bearing movement is primarily driven by girder thermal movement.
-- Girder thermal movement consistently agrees with simple calculations.
-- Bearing movement is inconsistent, not stopped by resetting, but stopped by replacement with neoprene.
-- Correctly designed neoprene bearings are not moving and natural rubber bearings are moving under the same loading conditions.