| The long-span triple-tower suspension bridge is a new type of structural form in suspension bridges. Compared with the double-tower suspension bridge, the addition of a mid-tower makes both the structural static and dynamic characteristics more complicated. And the anti-seismic performance of this bridge has attracted special attentions in the field of bridge engineering. Taking a triple-tower suspension bridge as the engineering background, the seismic analyses are fully conducted in this dissertation, in which the seismic control with three kinds of dampers, including the elastic cable, the viscous damper and the mild-steel damper, is emphatically studied. The analytical results are expected to provide references for anti-seismic design and analyses of long-span multi-tower suspension bridges. The main research contents mainly contain,(1) Establishment of the finite element model for the triple-tower suspension bridge and its dynamic characteristics analysis. Based on the structural design parameters, a spatial finite element model of the triple-tower suspension bridge is established in ANSYS. And the modal parameters are extracted with the subspace iteration method. Accordingly, the influence of setting large-mass elements at the bottom of the towers in the large-mass method to the structural dynamic characteristics is analyzed. The results indicate that the influence of setting large-mass elements at the bottom of the towers to the structural dynamic characteristics is so small that can be neglected.(2) Seismic analysis of the triple-tower suspension bridge. Based on several typical seismic waves, the structural seismic analyses under uniform excitation are conducted with both the acceleration method and the large-mass method. And the feasibility of the large-mass method in structural seismic analysis is verified. Accordingly, the apparent wave effect on the long-span triple-tower suspension bridge is further analyzed, and the results are compared with those from the uniform excitation. The analytical results indicate that the structural responses (e.g., comparative displacement between the main girder and the tower, shear force and bending moment at the bottom of the towers, the cable force) fluctuate with the apparent wave velocity in the low-velocity range. And in the high-velocity range, the structural responses approach the results from uniform excitation.(3) Seismic mitigation analysis of the triple-tower suspension bridge based on common dampers. The elastic cable and the viscous damper are installed at the intersections of the main girder and the towers, respectively. Then the seismic mitigation analysis of the triple-tower suspension bridge with and without considering the apparent wave effect is conducted. The analytical results show that both the elastic cable and the viscous damper can effectively control the comparative displacement between the main girder and the tower, but the shear force at the bottom of the towers is obviously increased when the elastic cable is utilized. When considering the apparent wave effect, the structural seismic responses change rapidly with the increment of the apparent wave velocity, and the oscillation phenomenon is the most prominent in the low-velocity range.(4) Seismic mitigation analysis of the triple-tower suspension bridge based on the mild-steel damper. A seismic mitigation analysis of the triple-tower suspension bridge based on the parameters of the existing mild-steel damper is firstly conducted. And then the sensitivity of the damping effect to each parameter in the mechanical model of the mild-steel damper is investigated, and reasonable mechanical parameters of this damper are thus determined for the seismic mitigation of this long-span triple-tower suspension bridge. On this basis, the damping effect of the mild-steel damper is further analyzed when considering the apparent wave effect in the seismic analysis. The analytical results indicate that the mild-steel damper can more and more effectively control the comparative displacement between the main girder and the tower with the increment of the elastic stiffness, but the shear force at the bottom of the mid-tower is sharply increased.(5) Comparison of the damping effects of the three categories of dampers. In the view of structural inner forces and displacement responses, the damping effects of the three categories of dampers are compared accordingly. The comparative results indicate that the comparative displacement between the main girder and the tower can be effectively controlled when the elastic cable or the mild-steel damper is adopted, but the shear forces of the main towers are obviously increased. The control effect of the viscous damper is a little smaller than the elastic cable or the mild-steel damper, but its influence on the shear forces at the bottom of the towers is far less than the other two cases. That means the viscous damper can effectively control the comparative displacement between the main girder and the tower on the basis that the shear forces of the towers are not prominently increased. |
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