Canyon Topography Effects On Seismic Response Of Multi-support Bridges

Induced by the topographic irregularity, the seismic wave propagation, reflections and refractions become quite complex. The propagation of seismic waves on canyon site distinctly differs from that on flat surface. The distribution characteristics of ground motions are influenced by small-scale topographic features, which cause non-uniform ground motions. So, the extended structures, such as long-span bridges, are influenced significantly by the non-uniform ground motions considering the canyon topography effects. And it is necessary to evaluate canyon topography effects on long-span multi-support bridges during earthquake.The survey of the state of the art in topographic effects on seismic wave propagation and seismic response of multi-support bridges has been given to demonstrate the significance of this thesis. The task of this dissertation is introduced.Based on the seismic damage investigations and strong ground motion data from Xishan-Park Array in Zigong city, the indirect and direct evidences are presented to demonstrate that the local topographic effects on seismic wave propagations. The typical mountain topography effects and canyon topography effects during Wenchuan earthquake are summarized.The two-dimensional site finite element model is formed, and the seismic analysis of a canyon has been carried out on visco-elastic half-space respectively under SV waves and SH waves using the assumptions of vertical incidence and oblique incidence. It indicates that the distribution of ground motions is affected by small-scale canyon topographic features and the incidence angle of the waves. In the case of vertical incident SV/SH waves, the peak ground accelerations increase greatly at the upper corners of canyon; whereas the peak ground accelerations decrease at the bottom corners of canyon. In the case of oblique incident waves, the shaking of the slope perpendicular to the incidence direction is more severe, however on the opposite side, the peak ground acceleration values decrease. The relative displacements of ground motions mainly depend on the incident direction of waves. Generally, the relative displacement increases obviously under oblique incident wave, compared with those under vertical incident wave. And the high frequency content of incident waves has been amplified obviously.The analytical methodologies for multiple-supported seismic excitation are reviewed, which include the Relative Motion Method (RMM), the Large Mass Method, the Displacement Method and Large Stiffness Method (LSM). With the assumption of Rayleigh damping, the error origins and applicability of the LMM, the DM and LSM are discussed. It is found that The LMM and LSM can cause distinct errors when the Rayleigh damping is adopted .And the improved LMM and improved LSM are presented and validated respectively. It has been proved that the improved LMM and improved LSM can be applicable to non-uniform seismic excitation analysis with the assumption of Rayleigh damping. And the improved methods can yield results that are identical to theoretical method with a high precision.Based on the ground accelerations obtained from seismic response of a canyon, the seismic responses of two bridges are performed considering the canyon topography effects using the improved LMM presented in this dissertation. The response characteristics of bridge are summarized. It indicates the canyon topography influences on the response of bridges greatly and complexly. In terms of the incidence direction,frequency content of incident waves and structural styles,the characteristics of seismic responses have become complex. The responses of structure members considering the canyon topography effects may be enlarged or decreased compared with the results of uniform seismic excitation case. The canyon topography effects under SV incident waves on structure are much more obvious than that under SH incident waves. In general, in the case of oblique incident SV waves, the rigid frame bridges suffer most critical responses.Based on the analysis of wave passage effects on rigid frame bridge with assumption of different apparent velocity, the response characteristics were compared with those in the case of canyon topographic effects considered.It indicates that the low apparent velocity brings critical responses to the bridge; however the topographic effects are still not neglected.Finally, an acquisition technique for quasi-static response components and dynamic response components in multi-support seismic excitation analysis is presented and validated based on the improved LMM and DM. The quasi-static response components and dynamic response components of a long-span rigid frame bridge subjected to non-uniform seismic excitations considering the canyon topography effects under incident SV waves are successfully obtained, and the characteristics of quasi-static response components and dynamic response components are compared. It indicates that the quasi-static response components mainly rely on the relative displacements between bridge supports and the dynamic response components depend on the frequency content of incident seismic ground motions. And in the case of vertical incident waves on the canyon, the quasi-static force components is much smaller than dynamic force components, however it become quite bigger and can not be ignored in the case of oblique incident waves.It also indicates that in the case of oblique incident SV waves the quasi static components and dynamic components resonate with a higher probability.The total response of structures subjected to non-uniform seismic excitations consists of quasi-static components and dynamic components, which contribute to the complexity of structural responses. The canyon topography effects under oblique incident SV waves should be considered seriously in seismic response analysis of multi-support bridges.