Field Studies of Elastomeric Bridge Bearings

Elastomeric bearing pads have been utilized extensively in bridge structures for the past forty years. Recently, excessive movement of predominantly natural rubber, tapered pads prompted an investigation to determine the causes of bearing pad slip. Two bridge sites were instrumented and monitored over a three-year time period. Natural rubber and neoprene bearing pads were utilized at both sites under various conditions. The methodology and procedures that were implemented are discussed in detail. The field study results, in conjunction with the laboratory results, indicated that the antidegradent additives in natural rubber bearings, namely waxes, will migrate to the bearing contact surfaces. This viscous leachate reduces the coefficient of friction and, in some cases, the horizontal force is sufficient to overcome the friction force and the bearings “walk out”. Neoprene pads on these waxy surfaces may also demonstrate excessive movement. Neoprene pads on wax-free surfaces have shown no indication of slip.