Seismic Impact Analysis For The Concrete Bridges Based On The Effects Of Deteriorating Process

A large number of concrete bridges in China are located in marine zones and deicing salt-use zones,referred to as typical chloride environments.Corrosion of reinforcement in chloride environment would gradually reduce the structural stiffness and capacity of concrete bridges with time.Thus,it is important to investigate whether deteriorating concrete bridges satisfy the seismic requirements.Focusing on concrete bridges in chloride environment,this dissertation establishes a deterioration model for reinforced concrete members based on existing experimental and theoretical studies.This dissertation considers the uncertainties in ground motions and the parameters related to bridge modeling to perform seismic impact analysis for the concrete bridges,based on the effects of deteriorating process in the chloride environment.The main research focuses on the following:(1)According to the characteristics of typical chloride environment in China,the prediction models of initial corrosion time of reinforcement in marine zone and deicing salt-used zone have been improved.The major factors that influence the corrosion rate of reinforcement were obtained by analyzing the existing experiment results,while a three-stage model of corrosion rate and function of computing the residual area of reinforcement were proposed,considering the acceleration of corrosion rate after concrete cracking.Besides,the cracking time and cracking width of concrete were determined by studying the experiment of cracking mechanism.Finally,a deterioration model that is applicable to the concrete bridges in chloride environment was constructed,taking into account the degradation of strength and ductility of corroded reinforcement.(2)Based on the structural characteristics and seismic responses of concrete bridges,the selections of elements and material constitute relations and the determination of load and boundary conditions were discussed by combining the proposed deterioration model.On the basis of database of natural ground motions from PEER,the selection principle and method were determined by considering the uncertainty nature of ground motions.Then,the dynamic analysis method and software were selected to lay a foundation for the later seismic impactanalysis for the concrete bridges in the chloride environment.(3)Based on the proposed deterioration model,uniform design was used to improve the efficiency of analyzing the uncertainties of material properties,structural geometry and ground motions,then a probabilistic seismic demand analysis method of deteriorating bridges was proposed.According to the performance requirements of concrete bridges,the resistance indexes and their threshold of damage states were obtained,subsequently a probabilistic seismic capacity analysis method was also proposed by integrating with BP neutral network,uniform design and Monte Carlo method.(4)On the basis of theory and method of seismic fragility,incorporating the probabilistic models of seismic demand and capacity,the time-dependent seismic fragility model of deteriorating concrete bridges was proposed,while the time-dependent seismic fragility curves of different bridge members were obtained.The obtained results shows that,the seismic fragility of different bridge members shows different time-varying laws after considering the effect of deterioration.Meanwhile,a relatively robust member at the beginning may become the most vulnerable member with time,resulting from the the influence of deterioration.(5)Based on the intensity parameters of ground motions under specific probabilistic levels provided by “Guidelines for Seismic Design of Highway Bridges”,a method to obtain the seismic hazard curve that is applicable for the site condition in China was proposed using the nonlinear regression analysis.Besides,the main contents of seismic loss evaluation of concrete bridges was determined through studying the existing evaluation models.Then,the generalized total probability formula was utilized to establish the seismic loss evaluation of concrete bridges in chloride environment by integrating with the seismic hazard curve and the time-dependent seismic fragility curves of bridge members.In the application of bridge practice,the relativeness of each components in seismic loss evaluation model was studied,and the primary parameters in the model were analyzed quantitatively,which could provide some references for the seismic loss evaluation of similar bridges.