Research On Seismic Performance Of Offshore Isolated Bridge Based On The Pier And Friction Pendulum Bearing Degradation

In recent years,with the improvement of China’s economic development level and the strengthening of regional cooperation and exchanges,a number of offshore bridge works have been built in the coastal areas.The bridge with seismic isolation device can effectively resist the damage of seismic action,ensure the safety of the superstructure of the bridge,and also play an important role in the normal operation of the traffic after the earthquake,so offshore isolated bridge has been more and more widely used.However,the offshore bridge project always experiences a long period of complicated marine environment,with its characteristics of over long and large span,it also faces serious threat once the earthquake happen.The marine environment in the coastal area is complex and special,which may cause deterioration of the bridge structure,and adversely affect the safety performance of the bridge structure.In order to study the influence of coastal corrosive environment on the seismic performance of offshore seismic isolation bridge during the whole life service period,the degradation effects of longitudinal bars,stirrups and concrete in reinforced concrete piers are considered in this paper.A model of isolated bridges with different service periods and different deterioration factors is established to consider the deterioration effect of the friction pendulum isolation bearings in the marine environment.The seismic performance of the friction pendulum bearing offshore seismic isolation bridge is evaluated by three methods: seismic capability,seismic response and fragility curve.The main contents are as follows:1.The overall deterioration model of reinforced concrete pier column is established: The research status of deterioration of reinforced concrete at home and abroad is summarized.The corrosion of longitudinal reinforcement in reinforced concrete pier column of bridge is calculated by pitting model.According to the experimental data of stirrup corrosion rate and longitudinal reinforcement corrosion rate obtained from the relevant accelerated corrosion tests,the variation law of the corrosion rate of stirrups with the corrosion rate of longitudinal reinforcement is established.The deterioration of core concrete can be calculated according to the deterioration of stirrup performance when the stirrup is corroded.The change of mechanical properties of concrete with corrosion time can be determined by the relevant formula.In the analysis of reinforced concrete pier column performance,considering the deterioration of longitudinal reinforcement,stirrups,protective layer concrete and core concrete,the overall deterioration model of reinforced concrete pier column is established.2.The deterioration rule of friction pendulum isolation bearings is put forward: According to the previously artificial accelerated corrosion test results of friction pendulum bearings,the experimental data of atmospheric and splash zones are simulated respectively by dry and wet cycle tests and periodic salt fog tests.The variation law of the static and dynamic friction coefficients with the deterioration time of the friction pendulum isolation bearings is put forward,The fitting results of the test data are applied to the study of this paper,and the friction coefficient of the friction pendulum isolation bearings can be calculated within 120 years of the service period.3.Seismic Capacity analysis based on pier deterioration: The nonlinear analysis model of offshore seismic isolation bridge is established by using Midas/Civil software,and the pier column is simulated with elastic-plastic fiber beam column element.The deterioration model of isolated bridges is established every 15 years during the service period,considering the single and coupling degradation models of the bridge piers,longitudinal bars,stirrups and concrete.The moment curvature analysis of the bridge model with different deterioration factors and different service times is carried out,the influence of single and coupling deterioration models of reinforced concrete piers,longitudinal bars,stirrups and concrete on the seismic resistance of pier columns is obtained.4.The seismic response analysis based on the bridge pier and the friction pendulum is considered as follows: Considering the longitudinal reinforcement,stirrup and concrete pier column separately and coupling deterioration model respectively,and consider the pendulum bearing deterioration model of friction and single pier joint deterioration model of bridge deterioration model is established by 7 seismic waves,three peak acceleration(0.2g,0.4g and 0.6g)and a total of 189 analysis of model 9 deterioration time points.To the seismic response analysis of bridge in longitudinal seismic wave under the pier and bearing along the bridge,pendulum bearing deterioration on the influence of the seismic response of the bridge pier column reveals the deterioration and friction,ensure the safety of bridge in the life period.5.Based on the analysis of the fragility of the bridge pier and the friction pendulum bearing: During the time history analysis,20 seismic waves which meet the requirements of the site are selected.Considering random bearing friction coefficient,corrosion coefficient,steel yield strength and main factors of concrete compressive strength,using the Latin hypercube sampling method,obtaining the formation of 20 seismic wave-sample model corresponding with seismic wave.Considering the common degradation model of bridge pier column and friction pendulum bearing,the deterioration parameters of each model are calculated after 60 and 120 years respectively.By using IDA analysis method,the peak acceleration is increased from 0.1g to 0.8g,and the interval 0.1g is analyzed by 480 nonlinear time history analysis.The damage index of the bridge pier column adopts the two parameter index which can reflect the energy consumption and deformation,Park-Ang damage index D,and the bearing displacement reflecting deformation is used as the damage index of the friction pendulum bearing.Probabilistic seismic demand model is established between bridge structure requirement and earthquake intensity index PGA,by calculating the fragility curve of pier column and support,the change rule of exceedance probability of pier column and support in full life service period is obtained.This paper lays a foundation for the whole life analysis of offshore seismic isolation bridges,and provides a scientific basis for the investment,construction and maintenance of the bridges.