SEISMIC RESPONSE ANALYSIS OF A MULTI-TOWER SUSPENSION BRIDGE SUBJECTED TO NEAR-FAULT GROUND MOTIONS

The characteristics of seismic responses of a multi-tower suspension bridge subjected to near-fault ground motions are investigated with regard to the prototype of Taizhou bridge. Furthermore, the influences of velocity pulse effect and site effect on the seismic responses of the bridge towers and girders are investigated based on the amplification coefficient which is the ratio of seismic response amplitudes under the near-fault ground motions to seismic response amplitudes under the normal ground motions. The analysis results reveal that: 1) the distribution characteristics of displacements and internal forces in the multi-tower suspension bridge under near-fault ground motions are basically the same as those under normal ground motions; 2) under longitudinal plus vertical seismic input conditions, the vertical displacement of the girder is significantly affected by near-fault ground motions on a hard site. The vertical moment of the girder is medium, and the longitudinal displacement and shearing force of the bridge towers are relatively small. The increase amplitudes of seismic responses decrease in turn in the side-tower, in the middle-tower, and in the girder, when site condition turns from hard to soft; 3) under transverse plus vertical seismic input conditions, the transverse displacement of the middle-tower is significantly affected by near-fault ground motions on a hard site, the vertical displacement of the girder is medium, and the transverse shear of the tower as well as the transverse moment of the girder are relatively small. The increase amplitudes of seismic responses decrease in turn in girder’s transverse moment, tower’s response, and girder’s vertical displacement, when the site condition turns from hard to soft; 4) there are significant differences in the seismic responses of the side-tower, of the middle-tower, and of the girder, in multi-tower suspension bridges under the combined actions of velocity pulse effect and site effect, which should be paid an enough attention in the seismic design.