Experimental and analytical study of the XY-friction pendulum (XY-FP) bearing for bridge applications

The XY-FP bearing is a modified Friction Pendulum TM (FP) bearing that consists of two perpendicular steel rails with opposing concave surfaces and a connector. The connector resists tensile forces, slides to accommodate translation and provides rotation capacity about a vertical axis. Numerical and experimental studies on an isolated truss-bridge model were conducted to study both the behavior of an XY-FP isolated system under three-directional excitation and the potential uses of XY-FP bearings for the seismic isolation of bridges. Two of the key features of these bearings for the isolation of bridges are their resistance to tensile axial loads and the opportunity to provide a different period of isolation in each principal direction of the isolated structure. A 1/4-length-scale truss-bridge model with a clear span of 10.67 m and a total weight of 399 kN supported on XY-FP bearings was tested on a pair of earthquake simulators. Testing included 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 construction detail of the small-scale connector of the XY-FP bearings and misalignment of the isolators on the test fixture did not permit fully uncoupled orthogonal responses. Numerical analyses on an XY-FP isolated bridge with different isolation periods in the principal directions subjected to near field ground motions demonstrated the effectiveness of the XY-FP bearings to limit displacements in either the longitudinal or the transverse direction. Numerical analyses that investigated the sensitivity of the XY-FP isolation system response to differences in the coefficients of friction of the bearings demonstrated that bounding analysis using uniform upper and lower estimates of the coefficient of friction will generally provide conservative estimates of displacements and shear forces for isolation systems with non-uniform isolator properties.