Seismic Fragility Assessment And Seismic Retrofit Scheme Of Bridges Considering Chloride Attack

With the deepening of people's awareness of earthquake damage and disaster prevention,the seismic design theory and calculation methods of bridges have been improved.However,the current seismic analysis is mainly focused on the as-built bridges.In fact,most bridges are located in the poor service environments,crossing valleys or rivers,and exposed to the air,soil and water.It's hard to avoid being affected by these environmental factors.Chloride ion erosion is one of the most important environmental factors affecting the performance of the reinforced concrete structure.The seismic capacity and dynamic characteristics of the bridge suffering the action of chloride ion erosion are different from that of the as-built bridges.Obviously,the traditional seismic analysis method cannot be applied to these bridges.With the development of China's infrastructure construction,the aging of bridge is becoming more and more serious.Therefore,it is urgent to develop a seismic assessment method for the deteriorating bridges.On the other hand,seismic performance is an important design factor for the bridges in earthquake-prone areas,and it may decide the safety of the whole or part of the bridge.With the reduction of the seismic performance of deteriorating bridge,the seismic reliability may also reduce.In order to ensure that the seismic performance of the deteriorating bridges keeps the original design target,it is necessary to perform seismic retrofit for the deteriorating bridge in time.Obviously,the unreasonable seismic retrofit schemes will cause the waste of funds.And more importantly,it may lead to a huge potential seismic risk of the degenerated bridge.In this case,it is very important to make reasonable and scientific seismic retrofit scheme,specify the optimal retrofit measures and the proper retrofit time.For the above reasons,the Performance-based Seismic Design(PBSD)is taken as a guide,and the seismic fragility analysis method of the bridge suffering to the action of chloride ion erosion is developed.Then,the variation law of seismic reliability of deteriorating bridges is revealed.Moreover,an optimal method of seismic retrofit scheme for the deteriorating RC bridge is proposed.The detailed research work of the study is summarized as follows:(1)The methodology of seismic fragility analysis is presented.The finite element model of a common used medium-span RC continuous girder bridge is built by considering different nonlinear factors.The nonlinear time history analysis and logarithmic regression analysis are used to determine the probabilistic relationship between 8 kinds of ground motion intensity measures(IMs)and the bridge structural response.Based on different evaluation criteria,the optimal IM is investigated.Then,the bridge component and system seismic fragility curves are developed to evaluate the seismic damage probability of the bridge.After that,a seismic reliability evaluation method based on the seismic fragility functions and the seismic design code of bridge in China is summarized.(2)Based on the Duracrete model and the previous experimental results,the distribution types and the statistical characteristics of different parameters for chloride ion diffusion and steel corrosion are determined.The Monte-Carlo simulation method is applied to obtain the corrosion initiation time distribution of reinforcement,and the reasonable distribution model of the corrosion initiation time of reinforcement is determined.Based on the distribution characteristics of the corrosion rate of reinforcement,a hybrid model used to describe the distribution of the corrosion rate of reinforcement is proposed.The sensitivity analysis is applied to investigate the effects imposed by various different parameters on the corrosion initiation time and corrosion rate of reinforcement.Then,the degradation model of the material is determined by combining the degradation mechanism of the material and the corrosion law of the reinforcement.(3)The seismic capacity and seismic response of piers in different service time under the non-uniform chloride environment are investigated.The evolutionary mechanism of plastic hinges in the piers is elaborated,and the calculate method of the evolutionary probability of plastic hinges in the piers is proposed.Then,by considering the degradation of piers,the time-dependent fragility analysis framework is proposed.The time-dependent seismic fragility curves of bridge components and system are developed and damage characteristics of the deteriorating bridge is investigated.In addition,a simple method that extending the discrete time-dependent fragility curves into time-dependent fragility surfaces is proposed.(4)The optimization theory of seismic retrofit scheme for the deteriorating RC bridge is expounded,and the calculate method of seismic reliability degradation law of bridge based on the seismic reliability model and the equivalent cross-section parameters model is proposed.Then,the analytical framework and procedure of the equivalent cross-section parameters model is introduced.Based on the response surface method,the seismic reliability model is developed.Then,a detailed process of optimization of seismic retrofit scheme for the deteriorating RC bridge using the NSGA-II multi-objective optimization algorithm is proposed.And the optimization analysis of seismic retrofit scheme of the case-study bridge is carried out.(5)Based on the PEER framework,a time-dependent seismic loss assessment method is established.Bridge seismic hazard curves are developed by combining seismic hazard analysis(SHA)with seismic design code of bridge in China.Based on the previous research,the seismic loss model of the bridge is determined.In addition,based on the Life-cycle seismic design method,by considering the seismic loss in the whole service life of bridge,the optimization process of seismic retrofit scheme for the deteriorating RC bridge is presented.Finally,the seismic loss of the case-study bridge is evaluated,and the optimization analysis of seismic retrofit scheme for the case-study bridge is performed.Based on the above research,the time-dependent seismic performance of the bridge under the chloride environment is revealed,and the seismic fragility assessment framework for the deteriorating RC bridge is present.Moreover,the proposed optimization method of seismic retrofit scheme can help engineers make rational and reasonable seismic retrofit decision for the bridge during the whole service life.Meanwhile,the analysis results can provide some reference for the similar bridges.