Seismic Performance Analysis Of Pile Foundation-structure System With Viscous Damper In Liquefiable Ground

Based on the engineering background of frequent disasters of pile foundation-structure in liquefiable ground,considering pile-soil interactions,focuing on the influence law of viscous damper on the seismic response of pile foundation-structure system and the damping effect of viscous damper through the numerical simulation method.Conducting viscous dampers mechanical parameters sensitivity analysis and site characteristics effect analysis and ground motion impact analysis,to reveal the influence law of the pile foundation-structure,site characteristics and ground motions on the seismic response of pile foundation-structure system with viscous damper,expected to give some reasonable understandings and suggestions on the damping design of viscous dampers in liquefiable ground.The main research work and some understandings obtained are as follows:1.Based on the completed centrifuge test by wilson,a three-dimensional finite element model was established to accurately describe the seismic response of pile group-soil-superstructure in liquefiable ground.The reliability of the numerical model was verified by the centrifuge test results.Then,viscous dampers were placed between the superstructure and the pier.Maxwell model was used to simulate the mechanical behavior of viscous dampers,and a numerical model of pile foundation-structure system with viscous dampers in liquefiable ground was established.The results show that the Maxwell model can effectively describe the mechanical behavior of the nonlinear viscous damper.The viscous damper has an ideal damping effect in liquefiable ground,significantly reduces the seismic response of the superstructure and the pier,and improves the seismic performance of pile foundation-structure system in liquefiable ground.2.Based on the established finite element model of pile foundation-structure system with viscous damper in liquefiable ground,the sensitivity analysis was carried out for the damping coefficient C and velocity exponentαof viscous damper.The results show that whenα=0.6,C=500 k N·m^-0.6·s^0.6;whenα=0.8,C=800k N·m^-0.8·s^0.8andα=0.8,C=1000 k N·m^-0.8·s^0.8,the damping effect of viscous damper is the best in liquefiable ground.Then,the influence of the counterweight of the superstructure and the height of the column and pier on the seismic performance of the pile foundation-structure system was studied,the influence of the overlying non-liquefied soil layer on the damping design of the viscous damper in liquefiable ground was analyzed,and the similarities and differences of the damping design of the viscous damper for the non-liquefied site were compared.The results show that the overlaying non-liquefied soil layer changes the suitable parameter combination of viscous damper design in liquefiable ground.The suitable value of velocity indexαchanges from 0.5-0.9 to 0.7-0.9,and the suitable value of damping coefficient C changes from 500-1000 k N·m^-α·s^αto 500 k N·m^-α·s^α.The applicable value of velocity indexαfor the damping design of viscous damper in non-liquefied site and liquefied site is 0.7-0.9 and0.5-0.9,respectively,while the applicable value of damping coefficient C is 2500-4500 k N·m^-α·s^αand 500-1000 k N·m^-α·s^α,respectively.The damping rates of permanent displacement of the superstructure of the viscous damper in non-liquefied and liquefied sites are 55.8%and 54.4%,respectively,and the damping rates of peak shear at the bottom of the pier are 11.3%and 26.1%,respectively.3.In order to study the influence of ground motion characteristics on the seismic response of pile foundation-structure system with viscous damper in liquefiable ground and the damping effect of viscous damper,24 ground motion with different characteristics were selected as input,and nonlinear time-history analysis was carried out for pile foundation-structure system with viscous damper in liquefiable ground.The results show that,compared with the far field motion and the near field non-pulsed ground motion,the damping structure system under the near field pulsed ground motion has greater seismic response and higher seismic demand.Under the action of near-field pulsed ground motion,the damping effect of velocity-type viscous damper can be better played.Peak ground motion velocity（PGV）has the most significant correlation with the seismic response of pile group-structure system,which can be used as a ground motion parameter to estimate and characterize the potential damage capacity of pile foundation-structure system in liquefiable ground.The results show that the viscous damper has a good damping effect in liquefiable ground,and significantly improves the seismic performance of pile foundation-structure system.Objective to know the effects of pile foundation-structure,overlying the non-liquefaction soil,liquefaction and non-liquefaction on the damper damping design,reveals the effects of far field motion,near field non-pulsed ground motion and near field pulsed ground motion on the seismic response of pile foundation-structure system with viscous damper in liquefiable ground and the damping effect of viscous damper.Peak velocity of ground motion（PGV）is proposed as a ground motion parameter to estimate and characterize the potential damage ability of ground motion to the damping structure system in liquefianbl ground.The research results have important theoretical and practical significance for the damping design of viscous dampers for pile foundation-structure system in liquefiable ground.