The response of a 344 m long bridge to non-uniform earthquake ground motions

This paper investigates the influence of wave velocity and the dispersion of waves associated with variations in seismic ground motions, on the inelastic responses of four 344 m long bridges. The bridges were 9 span continuous prestressed concrete box girders supported on sliding bearings, which eventually permitted movement in the longitudinal direction of the bridges, with shear keys that prevented transverse movement. The sub-structure consisted of reinforced concrete circular piers with cross-heads and rigid beams at the abutments. Earthquake motions were applied in the transverse direction for two bridges and for two others, with two expansion joints, the motions were applied longitudinally. The analyses of the latter were carried out to examine expansion gap movements and all analyses were carried out to produce time-history responses of pier drift, pier shear forces and pier curvature demands. The reinforcement in the piers was modelled so that either one base hinge can form or one base hinge with a hinge at the top of the pier. Pier heights varied between 5 m and 11 m or, for one bridge, were of constant height of 11 m. The non-uniform earthquake inputs at supports were generated by using the conditional simulation method with a natural earthquake record specified at one abutment. The response to wave velocities from 100 to 2000 m/s and infinity were studied both without dispersion and with various degrees of dispersion. The El Centro 1940 N–S and Sylmar Northridge 1994 N–S were used as the transverse earthquakes and the E–W components were used longitudinally. The results of these analyses show that non-uniform earthquake ground motions significantly influence the response of long bridges both with and without expansion joints. The responses change significantly with travelling wave velocity and the degree of dispersion and these can be more critical than for uniform inputs. Significant dispersion can generate rotational inertia of the deck and, with the torsional stiffness of the deck, can lead to the formation of top and bottom pier hinges and significantly larger shear forces compared to the normal cantilever design of these types of bridge piers.