Seismic Fragility Analyses Of Reinforced Concrete Bridge Piers Under Chloride-induced Corrosion

The seismic performance of bridge structures affected by environment factors inevitably deteriorates during their life-cycles,especially for coastal bridges under the continuous aggression of chloride ions.Reinforced concrete(RC)bridge piers,which are the main lateral force resisting elements of bridges,are more vulnerable to be damaged under seismic loadings.Based on the literature review of the corrosion mechanism of reinforcements in concrete and seismic fragility analyses of bridge structures,numerical method is utilized here to conduct a systematic research on the evaluation approach for the time-variant seismic performance of RC bridge piers under chloride-induced corrosion.Meanwhile,the time-variant seismic demand and structural capacity are employed.This study aims to be appropriate for the lifetime seismic design of bridges,and the major contents and conclusions are listed as below:(1)The existing experiments show that the bond slip between corroded reinforcements and concrete deteriorates significantly.The formula of yield slip of anchored reinforcements is firstly derived by introducing the corrosion effects on the constitutive models of concrete and steel bars and the bond strength,and then,the bond slip model including strain penetration effect is modified for corroded reinforcements.After that,finite element models(FEMs)of corroded RC bridge piers are established based on the OpenSees platform with the consideration of deteriorated strain penetration effect caused by the chloride-induced corrosion.The results indicate that the hysteretic behaviors of corroded RC bridge piers can be simulated well by the established FEMs,and the slip deformation of longitudinal steel bars can be satisfactorily predicted with high accuracy.Further,the effects of corroded reinforcements and strain penetration effect on seismic responses of bridge piers are evaluated.It turns out that the calculation error of drift ratio is influenced remarkably by the longitudinal reinforcement ratio,aspect ratio and axial load ratio.In addition,the calculation error increases under higher corrosion level of reinforcements.(2)A prediction model is proposed to calculate the ultimate tip displacement of corroded RC bridge piers based on the discrete model,in which the bending and slip deformations are calculated separately.The calculation procedure is compiled for both the conditions of uniform and non-uniform corrosion along the pier height.Also,the local corrosion occurs in splash and tidal zone can be simulated.The bending deformation can be calculated according to the sectional moment-curvature analysis of column base and the plastic hinge model.For the condition of non-uniform corrosion along the pier height,it is necessary to take into account the compatibility of load and the synchronous unloading in various corrosion zones in the calculation process of bending deformation.As for calculating the slip deformation,the distribution pattern of strain of anchored reinforcements proposed by Lehman is adopted,meanwhile,the constitutive model of corroded steel bars is introduced to modify the bond slip model.The ability of the proposed prediction model to calculate the ultimate displacement is demonstrated by simulating the quasi-static test results of corroded RC columns.(3)The assessment framework,which is based on the seismic fragility analysis,is established for evaluating the current seismic performance of corroded RC bridge piers by considering the joint uncertainties of corrosion initiation time of reinforcements,material parameters and ground motions.Firstly,the necessity of adopting the time-variant structural capacity and seismic demand is quantified by hypothesis testing when the seismic fragility analysis is employed to evaluate the time-variant seismic performance of RC bridge piers subjected to chloride-induced corrosion.Then,the time-variant performance of RC bridge piers in condition of non-uniform corrosion along the height is investigated,and the uncertainties stemming from corrosion initiation time of reinforcements,material parameters and ground motions are included.The numerical results indicate that the structural seismic fragility will be underestimated when only the uncertainty in ground motions is considered.The structural failure probability will increase once the structural uncertainty is taken into account,and the increased degree will become more significant with the increase of service time.Moreover,the evaluated structural seismic performance is significantly affected by these uncertainties.