Dynamic Response Analysis Of Bridges On Deep Overburden Layer

Bridges are mostly located on the rock and soil foundation, researches on the problems such as the damage mechanism and the disaster forms of bridges in earthquake are inseparable from the study of embedded solid foundation. Generally, seismic design calculation just regards the bridge as an independent structure to study, neglecting the influence of site effect on bridges, which will make the calculation of dynamic characteristics and response distort. So it is of great importance to study the dynamic response of bridges on the thick overburden layer.This thesis makes a seismic analysis of the bridge in a nuclear power plant. Directing at the foundation conditions of thick overburden layer in this project, the author establishes overburden foundation-finite element model, uses the equivalent load of ground motion input to analyze the dynamic response of the bridge structure under three-directional seismic action, and obtains distribution regularities of the pier columns'internal forces. Moreover, comparing with the results of bridges under rigid foundation conditions, the variation of the most unfavorable position of internal forces is worked out. Finally, a comparative analysis of the two conditions, with and without collar beams on the overburden foundation, is made to show the effect of setting collar beams on the dynamic response of the bridges. The thesis mainly carries out the following work:(1)Create a three-dimensional model of the free wave field:taking a soil column as high as the foundation, only applying Viscous-spring artificial boundary (VSB) to the bottom of the model, using shear lateral boundary (When P wave is input, lateral boundary is horizontally constrained; S wave is input, lateral boundary vertically constrained). The equivalent node load is applied to the bottom of the model to obtain the stress and the load of the border.(2)Create the thick overburden layer-bridge model and rigid foundation-bridge model, viscous-spring artificial boundary is added to the whole model. Making the stress and the load obtained above as an equivalent earthquake load input, analyze the influence of thick overburden layer on the internal forces of the piers, acceleration time history response and the displacement of the deck.(3)In order to meet the requirement of static force and stability, set collar beams between adjacent piers in the transverse direction of bridge. However how the bridges react with collar beams under seismic action is unknown, so in this thesis, the dynamic responses of bridge with and without collar beams on soil foundation are analyzed.The following conclusions are drawn, providing certain reference for engineering practice.(1)Soft soil has a poor bearing capacity. However, there is big axial force in the piles, so an obvious mutation appears at the junction of hard and soft soil.(2)Layer foundation has an obvious filtering effect for high frequency component of the response, but an amplification effect for the low frequency component. Thus, bridges' transverse is relatively soft, and earthquake response is obvious in long cycle; the longitudinal stiffness of the bridge is bigger, and it is sensitive to low frequency component.(3)Collar beam mainly affects the transverse seismic response of the bridge, redistributing the internal force across the bridge, and improving the peak acceleration and vibration frequency across bridge. However, the collar beam has little impact on the response of the longitudinal and vertical direction of the bridge.