Low Temperature Performance of a Friction Pendulum Bearing Inundated with Ice

Friction pendulum system (FPS) bridge bearings manufactured by Earthquake Protection Systems (EPS) at several locations around the country are known to fill with and retain water in the inner cavity of the bearing. FPS bearings are delivered and installed with cover seals around the outer bearing (attached to the top and bottom plate) that prevent exposure of the sliding surfaces. However, from inspection of in-service bearings, several DOTs have reported observations of breached seals and standing water on the sliding surface. The influence of water contamination on the sliding surface friction properties appears to be absent in prior literature, but anecdotal observations have suggested that water contamination may decrease the bearing friction coefficient. In Alaska, an even bigger concern is the fact that water trapped in the bearings will freeze during the winter months. The presence of ice in the cavity of the isolator could obstruct the movement of the bearing during an earthquake. This would result in increased base shear demand, which is potentially damaging to the bridge superstructure and foundation. Thus, the consequences of water and ice contamination in the bearings must be evaluated to determine how well the bearings will function, and whether uncertainties in the bearing response can be accounted for in the design process. If consequences of water or ice contamination are severe, mitigation options should be explored that can either prevent the bearings from taking on water or help the bearings to quickly dry out.