1 Published by Elsevier Science Ltd. All rights reserved 12 th European Conference on Earthquake Engineering Paper Reference 423 (quote when citing this paper) INFLUENCE OF PIER STIFFNESS ON THE RESPONSE OF BRIDGE ABUTMENTS TO SEISMIC LOADING: A SIMPLIFIED MODEL M.P. Limongelli 1 , G. Benzoni 2 1 Dipartimento di Ingegneria Strutturale, Politecnico di Milano P.za L. Da Vinci 32, 20133 Milano, Italy 2 Department of Structural Engineering, University of California San Diego Mail code 0085, La Jolla,Ca 92093, U.S.A. ABSTRACT The influence of pier stiffness on the shear on the abutments of a bridge subjected to seismic load in its transversal direction is herein analyzed. The feasibility of a simplified method previ- ously proposed by the authors for the estimate of the abutments shear is investigated for dif- ferent values of pier stiffness. Results from the simplified method are compared to time history nonlinear analyses results for different values of pier stiffness. The method is based on a plastic collapse analysis of the bridge subjected to abutment shear forces, deck inertia forces and the bending strength of the piers. Notwithstanding its simplicity the proposed method is able to take into account the changes of the pier characteristics giving a very good estimate of the maximum shear on the abutments for all values of pier stiffness. Keywords: Bridge abutments, Pier stiffness, Simplified method, Transversal seismic excitation INTRODUCTION Past experience shows that bridge abutments attract a large portion of seismic forces during seismic events. It is often reported, in post earthquake investigations, the pervasive problem of insufficient shear capacity of the bridge transverse keys (i.e. Kocaeli, Turkey, Earthquake, Re- connaissance Report, Earthquake Spectra). The problem is, at the same time, related to the correct definition of design forces as well as to the design of details. However, an accurate determination of design forces, at shear key locations, is often difficult and requires the use of dynamic nonlinear analysis. In a previous paper a very simple relationship, allowing a good estimate of the shear on the abutment as a function of the abutment flexibility has been pro- posed by the authors. The proposed solution provides a reasonable estimate of the maximum shear forces transferred to the abutments, simply based on pier strength, superstructure mass and maximum expected acceleration. In order to assess the reliability of results given by the simplified method for configurations characterized by different values of the pier stiffness, time history nonlinear analysis have been carried out on 2D finite element model of the bridge. A series of the configurations characterized by different values of the stiffness of the abutments,