The Design Analyses Of The Displacement Constraint And The Seismic Mitigation For Elastic-plastic Seismic Isolator On Railway Bridges

With the rapidly development of the high-speed railway in China,bridge structures bave been applied in many constructions due to their good structural characteristics;meanwhile,the seismic performance of the bridge in the high seismic intensity area has been concerned by many researchers.The isolation technologies have been applied in many railway bridges and been proved to be effective during earthquakes.However,the most isolation studies only focus on the mitigation of the seismic force responses,and the damage of the seismic displacement responses did not get enough attentions.The standards also do not have clear statement of the displacement restraint design,while the damage caused by the large seismic displacement responses can always be found during earthquakes.In this paper,to overcome the deficiencies of the traditional restrainers,a new type of elastic-plastic steel restrainer bar is proposed for the restraint design of the railway bridge.Firstly,the hysteretic behavior and the restraint capability of the steel restrainer bar are studied through the theoretical,experimental and numerical analyses;then,the design method and the influence factors of the steel restrainer bar are also analyzed by the numerical simulation.Due to the good restraint and energy dissipating capabilities of the steel restrainer bar,an innovative steel damping bearing composed by the sliding bearing and the steel restrainer bars is also proposed.Shaking table tests are conducted to confirm the seismic mitigating capability of the steel damping bearing composed by steel restrainer bars and the sliding bearing.The energy-based design method of the isolated bridge is proposed to complement seismic design of the isolated railway bridges.The details of this paper are presented as follow:(1)The mechanical characteristics analysis and quasi-static experiments of the elstic-plastic steel restrainer barThe working mechanism and the installment method of the steel restrainer bar are firstly introduced;and based on the working mechanism,the mechanical parameters are calculated through the equivalent stiffness method.Then,the quasi-static experiments of two groups with four specimens are conducted to study the hysteretic behavior,skeleton curves,energy dissipating capability,equivalent stiffness and hysteretic stability of the steel restrainer bar.During the experiments,the connecting components are also studied to decide the better way for the installment of the steel restrainer bar.Meanwhile,the numerical simulation of the experiment is also applied through the ANSYS software;and the accuracy of the numerical model and the mechanical parameter calculation method are verified by the comparison with the test results.(2)Elastic-plastic design method and the restraint effectiveness analyses of the steel restrainer barThe two-degrees-of-freedom model is proposed based on the characteristics of the simply supported railway bridge.The elastic-plastic design method,considering the spatially varying ground motions and the plasticity of the pier,are derived based on the calculation formula of the relative displacement for the travelling effect and the equivalent linearization method for the elastic-plastic bebavior of the steel restrainer bar.A five-span simply supported railway bridge is simulated to verify the accuracy of the design method;and the effectiveness of the steel restrainer bar and the cable restrainer are also compared through the numerical simulation.The numerical results reveal that the design method of the steel restrainer bar performs well in restraint design,and the steel restrainer bar is more effective in displacement restraint than the cable restrainer for the simply supported railway bridge.(3)The analyses of the influence factors for the steel restrainer barBased on the practical situations,the restraint influence of the pier height,the mechanical parameters and the rail are analyzed through the numerical models.The results reveal that the relative displacement seismic responses increases as the pier becomes higher;the effectiveness of the cable restrainer and the steel restrainer bar firstly increase and then become non-sensitive with the pier height.The increasing of the initial stiffness and the yield displacement will also increase the restraint effectiveness;thus,overall consideration of both initial stiffness and yield displacement can lead to a better design result of the steel restrainer bar.The analyses of the bridge with rail reveal that the rail will lead to larger displacement and pounding responses;stringent design criterion should be applied to ensure the safety of the bridge.(4)The experimental analyses of the steel damping bearing composed by the steel restrainer bar and sliding bearingBased on the good restraint and energy dissipating performance of the steel restrainer bar,an innovative steel damping bearing is proposed for the isolation of the railway bridge.The quasi-static experiments are firstly conducted to compare energy dissipating and hysteretic behavior of two different types of the steel damping bearings:series connecting and parallel connecting steel damping bearings.The experimental results reveal that the series connecting steel damping bearing performs better than the parallel connecting bearing in energy dissipating and hysteretic stability.The 1:7 scaled shaking table tests are also conducted to analyze the seismic mitigation of the series connecting steel damping bearing;and the numerical simulation of the shaking table tests is also applied.The results indicate that the series connecting steel damping bearing not only can mitigate the seimic pier force and displacement,but also can restrain the relative displacement between the pier and girder at a reasonable range.The comparison between the test and the numerical results reveals that the numerical model can simulate the experiment and the real seismic performance very well.(5)The analyses of the design methods for the steel damping bearing composed by the steel restrainer bar and sliding bearing in simply-supported railway bridgesThe accurancy and the applicability of the force based design(FBD)method and the displacement based design(DBD)method are analyzed through comparison between the design results and the shaking table tests or the numerical results.The comparison results indicates that the FBD and DBD methods can calculate the bearing displacement and the pier force accurately,and the DBD method can also accurately calculate the pier displacement for short piers,while the FBD method cannot perform well in pier displacement due to its assumption.For the bridge with high pier,both FBD and DBD methods cannot perform well in pier displacement calculation.To overcome the deficiency of the traditional design methods,an energy based design(EBD)method is derived in this paper,and its accurancy is verified by the test and the numerical results.Based on the real design situation,the EBD method with the limitation of the relative displacement is also derived.