BACKGROUND

Many types of bearing devices are used to accommodate bridge movement: single rollers, groups of rollers, rockers, elastomeric pads, sliding plates, sliding tetrafluorethylene (TFE), etc. In general, bridge bearings can be classed in two categories: elastomeric and mechanical<.!.>· According to a recent National Cooperative Highway Research Program synthesis on the design, fabrication, construction, and maintenance of bridge bearings (2), the elastomeric bearing pad is perhaps the-best expansion bearing because it is unaffected by weather (eg, it has no moving parts to freeze), has nothing to corrode, is low in cost, and requires almost no maintenance. However, elastomeric bearing pads are limited to 700 psi for vertical load capacity and 3 in for horizontal movement and their success depends on the quality of the material. On the other hand, for mechanical bearings the movements and rotations are accommodated by rolling, rocking, or sliding actions, usually on metal parts that can accommodate much larger bearing pressures. Furthermore, mechanical bearing devices can be designed for virtually unlimited horizontal motion (2). One of the simplest type; of mechanical bearing is the roller or" pipe roller", simply a piece of steel pipe with a stiffener as shown in Figure la. The load-carrying capacity of the roller is a function of its radius and can be found from the following formula (}.): For diameters up to 25 in,