Experimental Studies on Seismic Response of Skew Bridges with Seat-Type Abutments. I: Shake Table Experiments

Girder unseating in skewed bridges with seat-type abutments has been frequently observed in past earthquakes. This has been attributed to excessive in-plane rotation of the superstructure, and a handful of numerical studies have been conducted to investigate the cause of this rotation. The most common explanation has been pounding between the superstructure and abutment back wall, but this phenomenon has not been verified by experimental testing. Accordingly, this paper describes an experimental investigation into the behavior of skewed bridges, with special attention being given to the interaction between the superstructure and abutment. Shake table experiments are described on a family of four single-span simply supported bridge models with skew angles of 0 degrees, 30 degrees, 45 degrees, and 60 degrees and five different expansion gaps. These models were subjected to a suite of ground motions that varied by type (near field and far field), intensity, and input direction. In total, 876 experiments were conducted. Data collected included superstructure displacements and accelerations, and impact forces at the abutments. The results confirm that a skew bridge experiences large in-plane rotation when the obtuse corner of the span impacts the adjacent back wall with or without slippage along the face of the wall. As a consequence, these bridges can experience large in-plane displacements normal to the back wall at their acute corners, and larger support lengths are required to prevent unseating than for straight bridges. The comprehensive data set obtained in these experiments may be used in future studies to validate numerical models for skew bridges and improve the empirically based minimum support length requirements that are specified for skew bridges in many bridge design codes.