Seismic Response And Performance Analysis Of Simply Supported Bridge System In Liquefiable Ground

In China,earthquakes are characterized by high frequency,strong intensity,shallow source,wide distribution and often cause a serious disaster.The area of intensity Ⅶ accounts for more than half of China’s land area,and the liquefiable soils are often found in the bridge construction areas.Recently,the development speed of bridge engineering in China is getting faster and faster.Moreover,multi-span concrete simply pile-supported bridge system is one of the widely used types of bridges.The seismic problem of multi-span concrete simply pile-supported bridge system is still in the research area of soil-structure interaction.Earthquake damage investigation shows that the inaccurate model for soil-structure interaction invalidates the accurate bridge seismic response analysis methods.Whether the multi-span concrete simply pilesupported bridge system damage caused by the soil liquefaction during an earthquake has become an engineering problem and research hotspot in the earthquake engineering for the bridge.In the current seismic design method of bridges,the effects of dynamic soil-pilebridge structure interaction and soil liquefaction on the seismic behavior of bridge pile foundation are not considered.At present,China’s seismic design code for bridges has no useful technical details for seismic design of multi-span concrete simply pilesupported bridge system in liquefying ground,and it is extremely unsuitable for the development speed of bridge engineering.In order to ensure the scientific,systematic and safety of the seismic design of simply pile-supported bridge system,the performance-based seismic design of the bridge structure system should be developed,which has become the consensus in academic and engineering areas.In view of the above,this study takes the damage of multi-span concrete simply pile-supported bridge system which is well designed based on the current code as background and aims to solve the seismic design problem by considering the nonlinear behavior of the pile-soil-structure system.The advisory investigation,case studies,theoretical method,numerical simulation,and experiment will be adopted to investigate the seismic response of multi-span concrete simply pile-supported bridge system.The main research contents and important findings are as follows:Firstly,the numerical modeling techniques are employed to simulate a centrifuge experiment to study the dynamic soil-pile interaction in liquefiable soils.In this threedimensional finite element model,the two-phase material response of saturated sand was formulated based on a u-p formulation,and the saturated sand was simulated with a multi-yield-surface plasticity constitutive model,the pile was simulated using nonlinear beam-column elements,and the soil-pile interface was simulated by the zero-length interface element.The result of the FE analysis was found to be in reasonable agreement with the experimental data.Due to the p-y curve of sand recommended by the API does not take into account the effect of liquefaction and pile group,the numerical model is used to obtain the dynamic p-y curve of liquefiable soils.Based on the construction of the stress-strain relationship curve of soil,a new p-y curve of seismic soil-pile group interaction in liquefiable soils considering the pile group effect is proposed,which is modified by the p-y curve given by the API.Then,based on the proposed p-y model and nonlinear Winkler foundation beam theory,a simplified method for seismic soil-pile group interaction analysis in liquefying ground is developed,which is used to verify the reliability of the p-y curve model of pile group.Secondly,a simplified numerical model corresponding to a five-span simply supported concrete pile-supported bridge in liquefying ground is established.The model is established with the proposed p-y model of pile group based on the nonlinear Winkler foundation beam method.The dynamic analysis of free field is carried out by using the one dimensional nonlinear finite element program,the displacement and pore pressure ratio time histories obtained from numerical calculation were applied to the model as the external seismic excitation.On this basis,a numerical model of seismic response of soil-pile group-bridge structure system responding is established.The reliability of the method is verified by comparison with the results of the global finite element method and the substructure finite element method.At the same time,a three-dimensional finite element calculation software Mss SRA for seismic response analysis of soil-pile-multispan bridge structure system in liquefying ground is developed,and it is verified by the results of theoretical analysis and shake-table test.Thirdly,the typical near-fault pulse-like,near-fault non-pulse-like,and far-fault ground motions were selected to analyze the seismic behavior of the soil-pile groupbridge structure in liquefying ground using the Mss SRA.The effect of different type of ground motions on the seismic behavior of the bridge has been investigated.Then,the correlation between the ground motion parameters and seismic response of pile group multi-span simply supported bridge system is investigated,and it was found that PGV/PGA is an appropriate ground motion parameter to characterize the response of the multi-span simply pile-supported bridge system in liquefying ground.Finally,in order to describe the performance of the multi-span simply supported bridge system in liquefying ground,the analytic expression of the hazard curve of the performance index is derived based on the selected ground motion intensity index PGV/PGA,which takes into account the uncertainty caused by the seismic randomness and the uncertainty caused by cognition,and the uncertainty of the seismic hazard curve.The seismic response analysis of the multi-span simply pile-supported bridge system in liquefiable soils is carried out by using the selected ground motion intensity index and ground motions with different amplitudes.Based on the finite element analysis results,the maximum response is selected as the response of the performance indexes.Through the linear fitting in the log-log coordinate,the seismic demand model for the performance indexes is obtained,and then the seismic hazard curve of the performance indexes is determined.Overall,this study will contribute to understanding the dynamic characteristics of multi-span concrete simply pile-supported bridge system in liquefying ground under earthquakes,and it may provide a significant reference for the subsequent development of seismic design code of the concrete simply pile-supported bridge system,which is significant in engineering practice as well as in theory research.