Response of Seismically Isolated Bridges due to Property Modification in Friction Pendulum Bearings Arising from Environmental and Construction Irregularities

Friction pendulum bearings (FPBs) are widely employed in bridge design for seismic isolation. Water infiltration into FPBs has been documented in various cases, presenting a potential risk of ice contamination in cold environments. Both types of contamination may alter the seismic response of FPBs. Despite its practical relevance, there persisted a knowledge gap on the effects of water and ice contaminations on FPBs. To address this gap, experiments were previously conducted at the Earthquake Engineering Laboratory at University of Nevada, Reno, during which the responses of FPBs under water and ice contamination were evaluated relative to the clean, dry condition. The key findings from the experiments were that water contamination reduces the dynamic coefficient of friction in FPBs, and that single-surface sliding (SSS) occurs routinely under irregularities in loading and support conditions. SSS refers to sliding being engaged only on one surface of a double pendulum bearing. In this paper, to assess the implications of these phenomena on bridge seismic performance, analytical models are developed for multispan continuous RC box-girder bridges (hypothetical) and a steel I-girder bridge (representative of the Susitna River Bridge in Alaska) with both single and double pendulum bearings. Nonlinear time history analyses are performed for each bridge, considering various contamination scenarios. SSS is found to notably increase peak shear demands in the piers of the concrete box girder bridges, while water contamination modestly decreases these demands. The steel I-girder bridge is less sensitive than the RC box-girder bridges to the variations in bearing response. Additionally, this study establishes a correlation between relative isolator and pier force demands and the substructure-to-superstructure mass ratio. As the mass ratio approaches zero, pier shear force is very sensitive to variations in the isolator response, whereas as the mass ratio increases, the pier shear force becomes insensitive to variations in the isolator response.