Study On Seismic Responses Of Long-span Railway Suspension Bridge Under Non-Uniform Excitation

For long-span bridges, excitations in different points are non-uniform. When discussing effects of non-uniform excitation on seismic responses of long-span bridges by time-history analysis, waves chosen by different researchers are various and researches which achieved general rules based on numbers of waves are still very deficient. What’s more, there are two different ways to consider the non-uniform excitation: traveling seismic wave excitation and multi-support excitation which may consider the partial coherence effect. But the differences of these two methods have not been studied systematically. Considering this situation, seismic responses of a proposed long-span railway suspension bridge are analysed by three different input methods. The main research works are as follows:1. A refined3-D finite element model of the proposed long-span railway suspension bridge is established by ANSYS, then the top20orders of the natural vibration modes are listed. To verify the accuracy and reliability of the ANSYS model, modal analysis is also taken by MIDAS. The results show that periods analyzed by these two finite element softwares are very similar. Some significative conclusions are achieved through analysis of the natural vibration modes.2. Combining with the artificial seismic waves generated in this paper, seismic response analysis under uniform excitation of the suspension bridge are taken, then main control modes of the seismic responses are given by analyzing the changing process of the seismic responses. The results show that,(1) for long-span suspension bridges with flexible central buckles, displacement of the main girder in longitude axis are controlled and mainly influenced by its first two longititudinal floating modes, but the stiffness are still too small that damping measures are necessary.(2) under uniform excitation, axial forces of the pylon are mainly controlled by the first vertical bending mode of the main truss, previous-order modes of the main cable and the first mode of the pylon. Bending moments and shearings of the pylon in longitude axis are mainly controlled by the first mode of the pylon. Displacements in longitude axis in the top of pylon are mainly influenced by both previous-order modes of the main cable and the first mode of the pylon.3. Seismic responses of the suspension bridge under traveling seismic wave excitation are analysed with three different apparent wave velocities. Then influence laws of the travelling wave effect on the seismic responses of the long-span suspension bridges are summarized. Then, combined the time-hisroty curves and amplitude spectrum of the responses, differences of control modes of the responses are studied. From the research we know that, the main reason that leads to the changes of seismic responses of the bridge is the increasing of relative displacement between the two anchor points of the main cable when considering the travelling wave effect. A larger relative displacement means more violent vibrations of the first vertical bending mode of the main truss, and because sensitivities of the responses to this mode are different, changing laws of different responses are not the same.4. Seismic responses of the suspension bridge under multi-support excitation are analysed with three different apparent wave velocities. Changing laws of seismic responses of the suspension bridge under the multi-support excitation are summarized, then differences of control modes of the responses are studied. Finally, differences of responses under traveling seismic wave excitation and multi-support excitation are summarized. Responses of the bridge change slightly when considering the multi-support excitation. And vibration of the first mode of main girder is not as strong as that under traveling seismic wave excitation. That means, because of the relative displacement between different points considering by traveling seismic wave excitation and the multi-support excitation are different, it’s very important to choose a reasonable input method to analysis for structures which are sensitive to relative displacements of ground. Meanwhile, the monitor of relative displacement between different points during earthquake remains a problem to be solved.