Seismic Collapse Fragility Assessment Of Non-ductile Reinforced Concrete Bridges

Non-ductile bridges with light and inadequately detailed transverse reinforcement are vulnerable to flexure-shear failure or shear failure during earthquakes due to the poor shear capacity.The collapse of the bridge due to shear failure has aroused many engineering scholars'interest in recent years.Focus on regular highway girder bridges,the method of incremental dynamic analysis is used to establish the seismic collapse vulnerability model,and the seismic risk assessment is carried out.The main research works are as follows:(1)The damage mechanism of non-ductile piers is analyzed and the failure mode of non-ductile piers is predicted in advance according to the ratio of shear capacity and shear demand.The finite element model of pier is established by using OpenSees.Taking a bridge column subjected to flexure-shear failure as an example,the influence of the longitudinal bar slip and shear failure of the pier is taken into account.The finite element models of flexure failure,flexure-shear failure and shear failure are established.In order to verify the accuracy of the finite element model,the analysis results of the finite element models are compared with the hysteretic curves from four static cyclic-loading experiments of single columns.(2)According to the existing bridge design codes in China as well as survey results,the design parameters of bridge column samples are determined.Non-ductile pier and ductile pier samples are designed by orthogonal design method.The axial failure and structural instability are elected as the judgment index of bridge piers collapse through statistic analysis.The near-fault and far-fault ground motions are downloaded from the PEER database,and the seismic response of piers is obtained by nonlinear dynamic transient analysis.The parameters of the vulnerability model for different pier types are obtained by maximum likelihood estimation method,which is also compared with under different ground motion.The sensitivity of different pier parameters,design specifications and ground motion types on collapse vulnerability model are discussed.(3)A Three-span simply supported girder bridges located in the Wenchuan is introduced as a reference bridge.Considering the uncertainty of material parameters and model parameters,the ductile and non-ductile bridge samples are designed by Latin hypercube sampling method.The seismic collapse vulnerability curves of bridges are obtained by using IDA method,and the influence of different collapse criteria on the bridge vulnerability model is compared considering the piers may collapse and the girders may fall down.The simply supported bridge system is assumed to be a series system and the effects of different design specifications and ground motion types on the probability of bridge collapse are discussed.According to the seismic hazard curve of the region,the seismic risk assessment of the bridge is carried out combining with the seismic vulnerability model of the bridge.