Performance of Nonseismic PTFE Sliding Bearings When Subjected to Seismic Demands1

Abstract

Laminated elastomeric bridge bearings incorporating a flat sliding layer of polytetrafluoroethylene (PTFE) provide service-level thermal displacement capacity to accommodate movements of a bridge superstructure in regions of low to moderate seismicity. These components also provide a potential source of displacement capacity, which is frequently neglected in seismic design. An experimental program has been carried out to evaluate behavioral characteristics and performance of steel-reinforced, laminated elastomeric bearings with PTFE at a flat sliding layer (which had not been designed for seismic demands) as primary quasi-isolation components for earthquakes by permitting large sliding displacements at a stainless steel-on-PTFE interface. The PTFE surfaces tolerated stress concentrations associated with eccentric loading stress distributions at moderate top plate displacements, with up to approximately 25% of the PTFE exposed at peak displacements. Larger displacements resulted in delamination and tearing of the PTFE, potentially leading to steel-on-steel sliding between the independent components. Bearings with moderately tall elastomer blocks were also susceptible to unstable rotated configurations with increasing displacements of the sole plate. Unseating of the sole plate from the elastomeric component occurred at displacements corresponding to the elastomer plan dimension in the direction of horizontal motion.