Research On Rocking Isolated Mechanism Of The Railway Bridge Pier With Pile Foundation And Its Seismic Design Method

In recent years,the base seismic isolation with rocking method is widely concerned by the researchers all of the world.This seismic isolation separated the superstructure and the base,and prevented the upward earthquake force.It has been proved that the base rocking seismic isolation method has good isolation effect and advantage of self-centering after earthquakes.In this paper,the mechanism and seismic design method of the rocking isolation bridge pier with pile-foundation are investigated by model test and numerical simulation,so as to provide a basis for the application of the rocking isolation pier in railway bridges with pile foundation.(1)Based on the previous research on the base seismic isolation method of high railway bridge piers,the design of the existing railway piers with pile foundation is improved.A seismic isolation method of rocking piers constrained by steel bars for limited displacement and energy dissipation is proposed,and compared with the method for free rocking isolation and constrained rocking isolation by unbonded prestressed anti-overturning steel bars.The principles of seismic isolation for the above three types of rocking piers are described.(2)The initial stiffness,lifting load and ultimate load of three kinds of roking-isolated seismic pier are obtained through the quasi-static test.The hysteretic characteristics and energy dissipation mechanism of rocking-isolated seismic pier are analyzed.The restraint effect of the steel bars for the displacement,energy dissipation and the anti-overturning capacity of the prestressed steel bars are studied.The results show that the free rocking pier has the problem of insufficient anti-overturning capacity in the process of rocking,the rocking pier constrained by steel bars has large stiffness and enough energy dissipation capacity at the normal service stage,and the rocking-isolated pier restrained by prestressing anti-overturning steel bars has superior self-centering performance.(3)Seismic responses of rocking-isolated piers under different input conditions of seismic waves are obtained through shaking table test,including horizontal acceleration of pier top,horizontal displacement,vertical lifting acceleration and lifting displacement of platform.The effects of limit bars and prestressed anti-overturning bars on the lift and sway of pier are analyzed.The results show that the shaking response of the isolation pier with free rocking is significant,and the pounding effect is obvious,which is not benificial for the seismic performance of the bridge.The restraint effect of the steel bar is the remarkable,the rocking amplitude is declined.While the prestressed anti-overturning reinforcement can not only control the rocking amplitude,but also has the function of self-centering.It is proved that both the rocking piers with steel bar and prestressed anti-overturning reinforcement have good seismic isolation performance.(4)Combined with the test results,two springs,three springs and four springs models were used to simulate three kinds of rocking-isolated bridge piers,respectively.The numerical models for Pushover and dynamic response analysis are established.The skeleton curves obtained by Pushover analysis are in good agreement with the quasi-static test results.The results of dynamic response analysis are in good agreement with the shaking table test results,especially for top acceleration and displacement.The comparison of the results shows that the spring models adopted in this paper can simulate the mechanical properties of the rocking-isolated bridge pier.(5)Based on the results of model test and numerical analysis,the seismic design principles of the key components(platform,limited displacement reinforcement and prestressed anti-overturning reinforcement)for the three types of rocking-isolated bridge piers are proposed.It is verified that the single spring analysis model(rotation spring model)can be used for the simplified calculation of rocking isolation bridge pier.A calculation method is presented for the strengthened steel plate which is placed at the bottom of the platform for preventing concrete crush during rocking.Then,the calculation model of the pile foundation of the rocking pier is presented.The seismic response of the bridge piers and pile foundation with and without seismic isolation are analyzed and reinforcement ratio of the pile foundation is investigated.The results show that the pile foundation with relatively low reinforcement ratio can meet the seismic requirement for the isolated bridge piers.