Spatial Effects On The Seismic Responses Of Long-span Bridges

Long-span bridges are highly relevant to public safety and economic development. Therefore, their aseismatic capabilities have been concerned and much research has gone into establishing practical seismic analysis and design methods for them.Spatial effects of the ground motion on responses are very significant to the long-span bridges. Three methods of the aseismic analysis, i.e. the response spectrum method, the time history method and the random vibration method are introduced firstly. Based on the random vibration theory, the pseudo excitation method (PEM) is extended to make further investigation to the multi-supported seismic analysis of long-span bridges. Two simplified models are extensively investigated to examine the spatial effects on the structural seismic responses. It shows that spatial effects including wave passage effect, incoherence effect and site response effect have significant influence on the responses of structures and neglecting those effects may cause insecure factors to the structural design. More over, the three effects may weaken or strengthen one another. Therefore, seismic analysis of bridge structures is quite necessary from the view of safety and investment.Presently, a seismic analysis can be performed by the response spectrum method, the time history method or by the random vibration method. Another important work of the paper is that seismic response analysis of a cross-sea cable-stayed bridge is fulfilled by all the three ways. The overall length of the bridge is 1150 m, with a central span of 720 m between its two towers. Comparisons of these three methods and spatial effects are investigated based on the projects: National Natural Science Foundation "theoretical and experimental study of seismic reliability of long-span bridge using multiple shaking table and the Ministry of Communications' subject "earthquake resistant design code for highway bridges". It can be seen that for the uniform ground motion, results got from the three methods are almost identical. While three kinds of ground motion variation effects have significant influence on the responses. The current "earthquake resistant design code for highway bridges" only has definition on medium or small bridges with main spans within 150 meters, but no definition on the effect of multiple seismic excitations for longer span bridges. While, late corresponding modifications have been made for "earthquake resistant design code for highway bridges (detail principles)", and will be implemented. It suggests that for long-span bridges, multiple seismic excitations should be considered. And the random vibration method has been adopted in it. But because the random vibration method is different from the response spectrum method and the time history method, which are more familiar to engineers, more research and introductions should be done for the inconsistence to impulse the implement of the new code. In this paper, some detailed research and comparisons are demonstrated. And those will help the bridge engineers understand the new content better.