Probabilistic Seismic Demand Models And Fragility Estimates For Multi-span Simply Supported Railway Box Girder Bridges

Bridge structures are important part of the highway and railway transportation networks,and seismic disaster risk of bridges become more and more concerned by the earthquake engineering communities and researchers today.Over the past decade,China’s high-speed railway construction has made rapid development.Design and construction of high-speed railway bridges are in accordance with the standardization requirements,and large number of prefabricated concrete box girders is used in the design and construction process,speeding up the progress of high-speed railway construction.For the proposes of seismic fragility analysis,a typical configuration of multi-span simply supported concrete box girder bridges are selected as analytical objects from the statistics of as-built high-speed railway bridges in China.This thesis mainly carried out the following research work:(1)A set of bridge samples are established,in which five uncertain parameters,i.e.the yield strength of HRB335,the compressive strength of concrete C35,the weight of superstructure,the gap size between adjacent spans,the friction coefficient of basin rubber bearings,are considered,using the trial and error method based on the Latin hypercube sampling.Then detailed three-dimensional finite element models for each bridge samples are modeled using structural analysis software CSI Bridge 2015.(2)According to soil type,magnitude,epicenter distance and shear wave velocities,a portfolio of recorded ground motions are assembled from Pacific Earthquake Engineering Research Center Strong Motion Database,and then divided into bins,and matched with those bridge samples.Nonlinear time history analyses are performed to capture the responses of structure for development of probabilistic seismic demand models.(3)Damage states of bridge components are defined,and probabilistic seismic demand models are formulated by linear regression analysis and quadratic regression analysis for the ratios of the capacity and demand respectively.The PSDMs developed using two different regressions are compared.The results show that PSDMs formulated by quadratic regression are better than by traditional linear regression.(4)Seismic fragility curves of bridge components are developed consequently,and then seismic vulnerability assessment for bridge components was carried out based on the median exceedance probability.The bridge system fragility is evaluated using the first-order upper and lower bound method and a single estimate for the second order bounds method respectively.Bridge system fragility curves are developed,and the fragility curves developed using two different methods are compared.Seismic vulnerability assessment for bridge system was also carried out based on the median exceedance probability.The results show that bridge system are fragile than any single component under seismic load.(5)The seismic fragility of multi-span simply supported concrete box girder bridges under different directions of seismic incidence,and fragility curves for longitudinal and transverse directions are generated respectively.The seismic vulnerability of bridge structures under longitudinal and transverse direction of seismic incidence is evaluated respectively.The results show that bridge structure is more vulnerable in the transverse direction than in the longitudinal direction.