Pounding Responses And Control Of Isolated Elevated Bridges During Earthquake

Bridges are one of the important transportation infrastructures in urbans, and its earthquake resistance and hazrad mitigation have been paid more attention by researchers and engineers. The bridges excited by strong earthquake occur pounding easily, which may cause some damages such as crashing at ends of girders, collision at expansion joints, and failure of bearing, even serious hazard as unseating of girders. Until now, a few studies have been carried out on the mechanism, simulation and prevention of pounding of bridges under strong earthquake action. In this dissertation, taking the urban elevated bridges as objects, a systematic theoretical analysis and shaking-table test on the pounding responses of isolated elevated bridges are performed under strong earthquake action. This provides a reliable theoretical basis for risk prediction and prevention design on seismic pounding of the urban elevated bridges. The following innovative work and achievements are included:(1) An equivalent Kelvin impact model and determination method on parameters of the model is proposed for seismic pounding analysis of bridges. Based on the Hertz contact theory and considering the wave-motion effect, the analytical expression on impact stiffness and damping of the equivalent Kelvin impact model is derived. The changing law of the impact stiffness and damping of the Kelvin impact model are simulated, and its rational value ranges are obtained for the practical engineering. The results show that the impact stiffness of the equivalent Kelvin impact model increases with the increment of the Hertz contact stiffness, the impact velocity, and the length ratio of the short to long girders. The wave-motion effect has remarkable influence on the impact stiffness and can not be neglected. The restitution coefficient of the damping of the equivalent Kelvin impact model decreases with the increment of the impact velocity, but with the decrement of the length ratio of the short to long girders.(2) The pounding responses of the isolated elevated bridges under strong earthquake action are systematically investigated. Based on the proposed equivalent Kelvin impact model, and considering non-linearity of piers and soil-structure interaction, nonlinear finite element models for isolated simply-supported and continuous bridges are built up. An artificial seismic wave is generated to fit the normalized response spectrum and consider spatial variation of earthquake motion. The nonlinear pounding responses of the isolated bridge under the uniform excitation and the non-uniform excitations considering the traveling wave effect only, the partial coherence effect only,or both of the traveling effect and the partial coherence effect, of the strong earthquake motion is simulated. The results indicate that the influence of the spatial variation of earthquake motion on the pounding responses of the isolated bridges is significant, especially for the isolated continuous bridge. Taking the spatial variation of erathquake motion into account, the impact forces during the pounding is clearly increased, especially when the traveling wave effect and the partial coherence effect are considered simultaneously. This influence may be attenuated gradually with the increasing of the apparent velocity.(3) A calculation algorithm for the critical gap length of the isolated bridges is proposed. Considering nonlinear property of restoring force of the isolation bearings, a simplified system with two 2-DOFs is built for pounding analysis of the adjacent girders (or segments) with different natural periods. Based on random vibration theory, nonlinear equations of motion of the system and its equivalently linearized equations of motion are set up, and a calculation expression of the critical gap length of pounding is derived. The critical gap length of pounding of the isolated bridges considering the spatial variation of earthquake motion and the soil-foundation interaction is simulated. The results indicate that the critical gap length of pounding increases with increments of the ratio of the natural periods of the adjacent girders (or segments), the ratio of masses of superstructure to substructure, and the yield deformation of the isolation bearing, but decreases with increment of the ratio of the yield force of the isolation bearing to the weight of the superstructure. However, the ratio of post-yield to pre-yield stiffness of the isolation bearing has little influence on critical gap length. The influence of the spatial variation of earthquake motion on the critical gap length of pounding is significant, especially of the partial coherence effect in which the the partial coherence effect stronger, the critical gap length of pounding larger. When the velocity of shear wave of soil is smaller than 200m/s, there is a certain influence on the critical gap length of pounding by considering the soil-foundation interaction.(4) Passive control on the seismic pounding responses of the isolated bridges s performed using visco-elastic damper. Based on the random vibration theory and considering the nonlinear property of restoring force of isolation bearing, a method for analyzing the seismic pounding responses of the adjacent girders (or segments) with installation of the visco-elastic dampers between girders or between girder and pier are established. The parametric design method for the nonlinear viscous damper to pounding prevention is proposed. The influences of the ratios of natural period and mass of the adjacent girders, the performance parameters of the damper, and characteristic frequency of the ground on the control effect of pounding responses of the isolated bridges are investigated. The results show that the relative displacement between the adjacent girders can be reduced significantly by installing the viscous damper between the adjacent girders, so to prevent the occourence of pounding effectively. The control effect increases with the increment of the ratio of natural periods of the adjacent girders.(5) Semi-active control on the seismic pounding responses of the isolated bridges is performed using magneto-rheological (MR) damper. A simplified 4-DOFs system for pounding response analysis of the isolated bridges is built by installing the MR damper between adjacent girders or between girder and pier. Employing the fuzzy control theory, and taking the minimization of relative displacement between adjacent girders as the objective function, a MR damper based fuzzy logic controller (FLC) is established. The offline genetic optimization and online self-tuning are proceded for the membership function, the quantification factor and the scaling factor of the FLC. The control effect on the seismic pounding responses of an isolated multi-span continuous girder bridge is simulated under different control strategies. The results indicate that the relative displacement between the adjacent girders and the lateral deformation of the isolation bearing may be effectively suppressed by employing the MR damper based semi-active control strategy, and the control effect is better by installing the MR dampers between the superstructure and the pier. Comparing with the conventional FLC and the FLC optimized by GA only, the control effect and robustness of the FLC with the offline genetic optimization and the online self-tuning are better.(6) Shaking-table test on seismic pounding responses of the isolated bridge is performed. A model of two-span isolated simply-supported bridge is made. Through different seismic wave inputs by the shaking-table, the seismic pounding responses of the model bridge with different structural parameters and the control effect on the relative displacement between adjacent girders by employing MR damper are tested and compared with the calculated results. The results show that the proposed equivalent Kelvin impact model may make a relatively accurate simulation on the pounding responses, including the impact force between the adjacent girders and the longitudinal displacement of the girders, of the isolated bridges under strong earthquake action. The influence of the gap length between the adjacent girders, the ratio of the natural periods of the adjacent girders, and the properties of the isolation bearing on the pounding responses of the isolated bridge is significant. The pounding responses between the adjacent girders may be reduced remarkably by increasing the gap length or decreasing the ration of the natural period of the adjacent girders. Installing the MR damper between the adjacent girders may decrease the relative displacement between the adjacent girders, while has little influence on the drift at the top of the pier.