| Comparing to the RC bridges,small and medium-span steel-concrete composite bridges have many advantages such as low girder height,small dead load,convenient and rapid construction,and remarkable comprehensive benefits.This type of bridge has widespread applications in European and American highway systems,and has received more and more consideration in China when selecting this type of a new bridge.However,seismic design for short-to-medium span highway bridges is usually depending on the construction requirements rather than the seismic risk.This leads to severe damage of structures under earthquake loads in areas with lower earthquake risk.Therefore,it is necessary to provide a more reliable evaluation of the seismic risk of concrete composite bridges with medium and small spans.Basing on a number of quantitative performance objectives,the multi-probability based seismic risk decision-making method promoted by the Pacific Earthquake Engineering Research Center can estimate the seismic risk of a specific structure.In this Performance-Based Earthquake Engineering(PBEE)framework of seismic risk decision-making method,seismic fragility is the most sensitive component of seismic risk assessment.It is necessary to select the appropriate method,taking into account the economy and computational accuracy,to obtain the seismic fragility curve of structures.The hybrid fragility curve combines the advantages of experimental fragility,empirical fragility,and analytical fragility,and is an effective method for structural damage assessment.Meanwhile,considering both the traffic carrying capacity of bridges and the social significance of the post-earthquake functionality recovery capacity,it is necessary to introduce the functional loss and the seismic resilience considering the functional loss into the decision-making model.Under the new generation of PBEE decision framework,the seismic risk and seismic resilience of medium-small span steel-concrete composite bridges are evaluated in this paper.The main contents and conclusions are as follows:(1)Comparison of the traditional seismic risk theory and the second generation Performance-Based Earthquake Engineering(PBEE)method,the two seismic risk assessment methods are unified in a theoretical framework by expounding the meaning of seismic vulnerability and the definition of forward and backward analysis of PBEE.Based on the hybrid analysis method of collapse fragility,the hybrid fragility approach is proposed for continuous damage states under PBEE framework and the theoretical framework is applied to seismic risk assessment of composite bridges.This paper also proposes the seismic functionality loss estimation model for bridges based on vertical carrring capacity and further proposes a theoretical framework for seismic risk assessment based on functionality loss.At the same time,considering the ability of functionality recovery for bridges after earthquake events,a method for evaluating the seismic resilience of composite girder bridges considering functional loss is proposed.(2)According to the Eurocodes,a two-span continuous H-section steel-concrete composite bridge is designed as a prototype,and in addition,a bridge with improved cross-beam is designed for comparison.The seismic performance of the example bridges is analyzed and the results show that,under the earthquake intensity specified by the Eurocodes,the capacity and deformation of the bridge can meet the requirements of the design codes with a relatively large safety margin.Within the OpenSees finite element platform,a full-bridge finite element model is established by considering the constructional details such as the shear stud connecting the girder and the cross-beam.Thus,the deterministic seismic response of the bridge is obtained,which provides the reference for designing loads of the test specimen.(3)Scaled test pieces of the steel girder,the concrete deck,the concrete cross-beam and the pier are designed and tested through both the static monotonic testing method and the quasi-static testing methods.During the tests,the continuous damage states of the test specimens with different construction forms were identified by observing the actual damage of the test components.Then,the relationship between the control displacement and the damage state of the components as well as that between the displacement and the damage state of the entire bridge was determined.In addition,the vertical capacity of the specimen was analyzed by time-history analysis of the vertical force.The failure mode of the composite joints connecting the girder and the cross-beam is revealed as the cracks first occur in the concrete bridge deck,then the concrete cross-beam,finally failures in the cross-beam.While there is no severe damage existing in the concrete deck nor piers when the cross-beam fails.Furthermore,the specimen with improved cross-beam can efficiently avoid the phenomenon that the steel girder is pulled out from the cross-beam.This improves the repairability after the earthquake and is expected to reduce earthquake losses.(4)In order to fully evaluate the seismic performance of steel-concrete composite girder bridges,the near-fault pulse type ground motion record was selected as the seismic input,and the far-field seismic record was selected as the comparison.A probability demand model is established based on the cloud map method,the parameters of the capability model were calculated using the maximum likelihood estimation based on the experimental damage data,and the hybrid fragility curve of components,as well as the system,was established using the hybrid method.Analysis results show that the probability of serious damage to the three types of bridges is extremely low under the earthquake loads with a PGA of 0.3g which is the value specified by the Eurocodes.The seismic demand corresponding to the complete damage state is high,which indicates that the seismic performances of the three types of bridges are very good.Furthermore,the bridge with construction improved cross-beam is superior to the other two types of bridges for both the near-fault and far-field ground motions.(5)The seismic hazard curve of the bridge site is calculated.The performance groups are defined considering the post-earthquake repair methods,and the direct economic loss of the performance groups is estimated.Based on the global economic loss of the bridge,the seismic risk of the three bridges is evaluated.Based on the estimation approach of functionality loss proposed in this paper,the loss model parameters have been estimated while the seismic risk has been assessed.There is a large difference between the risk results based on economic loss and based on functionality loss.Quantification the effect of functionality loss on the functionality recovery function,the functionality recovery functions for three bridges are given and the seismic resilience has been assessed.For each bridge in this study,the seismic risk corresponding to resilience is smaller than the risk corresponding to functionality loss.Therefore,the different performance parameters should be selected to build the decision models to obtain the thorough and comprehensive assessment results. |
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