Seismic Risk Probability Assessment Of Offshore Bridge Based On The Material Performance Degradation

A large number of offshore or cross-sea bridges have been built in the coastal areas of china,they may be subjected to destructive earthquakes during their service period because they are located near the seismic belt of the Pacific Rim.In the marine environment,the corrosion of steel bars caused by chloride ions erosion of the bridges would lead to the seismic performance of the bridge structures appearing premature degradation,especially the stirrups corrosion will significantly affect the shear capacity of the structures,which cause the maintenance cost of the offshore bridges increasing rapidly.The existing probability risk assessment framework without including the structural reliability assessment method considering the combined action of seismic risk and structural continuous degradation risk.Therefore,it is of great theoretical significance and practical value to evaluate the effect of chloride ions corrosion on the seismic performance of the offshore bridge structures.Based on the probability theory and the Monte Carlo sampling method,the seismic performance evaluation of the bridge structures,the seismic fragility analysis and the seismic risk probability assessment for the offshore concrete continuous beam bridge considering the uncertainty of the reinforcement concrete material performance degradation caused by the chloride ions corrosion are studied in depth.The main research contents of this paper are as follows:(1)The probabilistic models of chloride ions diffusion coefficient,surface chloride ions concentration and chloride ions concentration threshold are established by summarizing the existing research results,and the probabilistic distribution models of the initial corrosion time of steel in different corrosion environments can be obtained.According the existing performance deterioration models of the reinforced concrete,the random samples in different service time including the diameter of longitudinal reinforcements,the yield strength of longitudinal reinforcements,the diameter of stirrups and the compressive strength of cover concrete under different corrosion conditions are got by Monte Carlo sampling and censored sampling,and the probability models of material degradation are obtained by probability analysis of random samples.(2)The failure mode of corroded piers under different service time are analyzed based on the existing calculation model of shear capacity and the criterion of failure mode.The finite element models of corroded pier in different service time are established by using the fiber beam-column element,the zero-length shear spring element and the zero-length rotation spring element in OpenSees software to consider the effects of the bending deformation,the bonding and shearing deformation.Pushover analysis,static reciprocating cycle analysis,modal analysis and incremental dynamic analysis are carried out for the corroded piers in the different service time,and the damage states of the corroded bridge columns are redefined.Pushover analysis and empirical model are used to evaluated the capacity of different damage states of the bridge columns,it is found that the seismic capacity of the bridge columns is deteriorating continuously in the whole life cycle,and the time-varying seismic fragility curves of the piers are got by the vulnerability analysis method.(3)A probabilistic method for seismic risk assessment of offshore concrete bridges considering the degradation of material performance in chloride environment is presented.The three-dimensional finite element model of continuous beam bridge is established by using OpenSees software,random samples of material parameters of piers are got based on the probability method,and 50 sets of ground motion-bridge samples in different service time are obtained.Incremental dynamic analysis is carried out for the finite element model of the bridge,and the seismic response results of the drift for the piers top and the relative displacement for the bearings are extracted respectively,the seismic fragility curves for the components and the whole system of the offshore bridge can be got combining with the analysis results of the damage states capacity.Considering the site type of bridge,applying the theory of equal transcendental probability and the existing seismic hazard model,the seismic hazard curves of bridge structures in different service time are obtained,and Monte Carlo sampling method is used to generate a large number of ground motion intensity in different service time of the bridge.The seismic risk probability of the components and the whole system of the offshore bridge can be evaluated on the basis of the seismic vulnerability analysis and the seismic hazard evaluation.