近断层地震动作用下多塔悬索桥的地震反应分析

以泰州大桥为原型研究了多塔悬索桥近断层地震动作用下的地震反应特点,在此基础上以近断层地震动反应幅值与普通地震动反应幅值的比值作为放大系数,进一步研究了速度脉冲效应和场地土效应对桥塔和主梁地震反应的影响规律。分析结果表明:1) 近断层地震动作用下多塔悬索桥位移和内力的分布规律与普通地震动基本相同;2) 纵向+竖向地震输入下坚硬场地上近断层地震动对主梁竖向位移的影响最大,对主梁竖向弯矩的影响次之,对桥塔纵向位移和纵向剪力的影响相对较小。当场地土由硬变软时,边塔的地震反应增幅明显,中塔次之,主梁相对较小;3) 横向+竖向地震输入下坚硬场地条件上近断层地震动对中塔横向位移的影响最大,对主梁竖向位移的影响次之,对桥塔横向剪力和主梁横向弯矩的影响相对较小。当场地土由硬变软时,主梁的横向弯矩增幅明显,桥塔次之,主梁竖向位移相对较小;4) 在近断层地震动速度脉冲特性以及场地土固有周期特性的共同作用下多塔悬索桥边塔、中塔以及主梁的地震反应存在明显差异,在抗震设计时需要引起重视。

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.