Probabilistic Evaluation Of Progressive Collapse And Seismic Fragility Analyses Of Steel Truss Bridges

In recent years,several progressive collapse events of bridges have happened due to earthquakes,vehicle or ship collision,overloads and other factors,which has attracted scholarly attention.Up to now,limiting research has been done on seismic fragility of steel truss bridges;only sparse studies on progressive collapse of bridges were published,especially the publications about the progressive collapse of steel truss bridges considering structural uncertainty have not been found yet.Considering the uncertainty of structural material and seismic effects,the progressive collapse and seismic fragility of steel truss bridges were studied in this dissertation.A framework of equivalent nonlinear static analysis for steel truss bridges' progressive collapse was proposed,and a rating system based seismic fragility method united with a model updating method was established.The progressive collapse and the seismic fragility analyses of the 64 m standard railway steel truss bridge and the Hengqin Second Bridge(a steel truss arch bridge with 420 m main span)were carried out.The main contents and conclusions of the dissertation are listed as follows:1.Taking into account the structural material uncertainty,a framework for equivalent nonlinear static analysis of steel truss bridges' progressive collapse was proposed.The structural responses of the 64 m standard railway steel truss bridge and the Hengqin Second Bridge were obtained.Both the 2D and the 3D finite element(FE)models of the railway steel truss bridge with different types of joint model(rigid joints,combination of rigid and pinned joints,and semi-rigid joints)were established;and the nonlinear static and dynamic responses of these models under different collapse events were compared with respectively.Factors that influence the responses,such as the member removing time,the stress-based or displacement-based results,were investigated respectively.Finally,the dynamic increase factor(DIF)and demand capacity ratio(DCR)were extracted under different progressive collapse scenarios,and some recommendations to prevent the two case studying bridges from progressively collapsing were given.The DIF of steel truss bridges are between 1.1?1.4.2.A seismic fragility analysis method based on a member-component rating system for steel truss bridges was proposed and the damage rating for bridge components were introduced in detail.Considering seismic and material uncertainty,the nonlinear dynamic seismic responses of the 64 m standard railway steel truss bridge were calculated with Open Sees.By obtaining the structural responses under unidirectional and multidirectional seismic waves under vehicular and non-vehicular operating conditions,the seismic fragility curves were obtained by the incremental dynamic analysis(IDA)method and the proposed method.The results of the two methods are similar to each other,and the transverse component of the seismic waves is the dominant factor to control structure's fragility.If there is no train operating on the truss bridge when seismic events happen,the probability that the steel truss bridge experiences severe and absolute damage is low.3.An improved approach for the artificial bee colony(ABC)algorithm was developed for structural model updating.The novel ABC algorithm included the Tabu search method and chaotic search techniques to enhance the exploration and exploitation ability,and its global searching ability was improved by the tournament selection strategy.The accuracy and effectiveness of the proposed method were verified by several explicit test functions and an implicit numerical example.Moreover,influences of some site conditions such as measuring point arrangement and measurement error were studied.Finally,the finite element model of the Hengqin Second Bridge was optimized by both the novel ABC algorithm and the Bayesian model updating method.The results show that the updated parameters of the bridge agreed well with each other.And the updated model was used for seismic fragility analyses.4.A system fragility analysis approach unifying the statistical meaning between component fragility and system fragility of bridges was presented;and the univariate conditional approximation method was employed to calculate the system failure probability with high efficiency.The component and system fragility curves of the piers and bearings of the Hengqin Second Bridge were obtained.The numerical results showed that the failure probability of the side bearing is high.On the contrary,the failure probability of the main truss bar is low.The calculated system fragility curves for severe and absolute damage with present method are close to their corresponding failure probability with upper bound method.