Experimental Evaluation of Seismic Pounding at Seat- Type Abutments of Horizontally Curved Bridges J.D. Wieser, E. Maragakis & I.G. Buckle Center for Civil Engineering Earthquake Research, University of Nevada, Reno, U.S.A. A.E. Zaghi University of Connecticut, U.S.A. SUMMARY: The expansion gap closure in seat-type bridge abutments during strong earthquakes results in seismic pounding. This pounding significantly affects the behaviour of the bridge and yet there are limited studies focusing on this impact. As a part of the Federal Highway Administration (FHWA) funded project, a 2/5 scale curved bridge model was constructed to be tested on the four shake tables in the University of Nevada, Reno Large Scale Structures Laboratory. One of the six configurations of the bridge model was designed to study the seismic pounding at the abutments with an equivalent nonlinear backfill soil. An abutment configuration was designed for the experiment to investigate the abutment impact accounting for the nonlinearity of the backfill soil. In this configuration, the superstructure was forced to impact a backwall supported by nonlinear springs with initial stiffness similar to that of typical embankment soil. The preliminary experimental results presented in this paper demonstrate that the closure of the expansion gap significantly influences the global response of the bridge system. Experimental measurements of the impact forces is planned to be used to calibrate numerical impact models. Keywords: Curved Bridges, Large Scale, Experimental Testing 1. INTRODUCTION Seat-type abutments consist of a footing, stemwalls, seat, and backwall. The superstructure is supported by bearings on the abutment seat. The backwall retains the backfill above the abutment seat so that the backfill of the approach embankment is not in contact with the superstructure. A gap between the abutment backwall and the superstructure provides a stress relief during thermal loadings. This type of abutment is commonly used for long span, highly skewed, or highly curved bridges to avoid large or unbalanced stresses in the superstructure and embankment backfill soil under temperature loads. However, during significant seismic events the expansion gap is forced to close resulting in seismic pounding between the bridge superstructure and abutment backwall. Large inertia forces generated in the superstructure mobilize the active pressure in the backfill soil behind the abutment backwall which can result in nonlinear soil behaviour. Damage to seat type abutments caused by seismic pounding has occurred in several recent earthquakes. Severe damage to highway bridges in the 1971 San Fernando earthquake occurred due to expansion gaps closing (Jennings, 1971). In the 1994 Northridge earthquake, considerable impact damage was observed at the expansion joints of the I5/SR14 interchange located near the epicenter (EERI, 1995a). During the 1995 Kobe earthquake, seismic pounding was cited as a major cause of bearing support damage and may have contributed to the collapse of bridge superstructures (EERI, 1995b). Significant damage to shear keys, bearings, and anchor bolts in the 1999 Chi Chi earthquake was the result of pounding of the expansion joints (EERI, 2001). Inspired by the damage to bridge abutments observed during recent earthquakes, several researchers