Experimental study of the XY-friction pendulum bearing for bridge applications

The XY-friction pendulum (XY-FP) bearing is a modified Friction Pendulum (Earthquake Protection Systems, Inc., Vallejo, Calif.) bearing that consists of two perpendicular steel rails with opposing concave surfaces and a connector. The connector resists tensile forces, provides for independent sliding in the two principal directions of the isolators, and ideally, permits unhindered rotation about its vertical axis. Experimental studies on an XY-FP seismically isolated truss-bridge model were undertaken to study response under tridirectional excitations and to evaluate the use of XY-FP bearings for bridges. A truss bridge model was tested on a pair of earthquake simulators using acceleration orbits and near-field earthquake histories. The experimental results demonstrated the effectiveness of the XY-FP bearings as an uplift-prevention isolation system: the XY-FP bearings simultaneously resisted significant tensile loads and functioned as seismic isolators. The bidirectional horizontal response of the small-scale XY-FP isolation system was coupled due to the internal construction of the small-scale connectors that joined the two rails of each XY-FP bearing and the limited free-to-rotate capacity of the XY-FP bearings due to misalignment of the isolators during installation.