Study On Safety Performance Evaluation Method Of Large-Span Simply-Supported Steel Truss Bridge Under High Speed Train

Simply-supported steel truss bridge is a common bridge structure of largr-span bridges in China.With the speed increase of railway trains in China,it is difficult to replace the large batch of simply-supported steel truss bridge in short term.It is an urgent problem to solve the requirement of its structural safety performance to meet the requirements of the operation stage.This paper,taking the Xiangtan Xiangjiang bridge of Shanghai Kunming line as the project background,studied the safety performance evaluation method of the lower bearing type simply-supported steel truss bridge under the action of high speed passenger train.In this paper,the finite element software LUSAS is used to establish the calculation model of vehicle bridge coupling vibration.Based on the time variant reliability theory,the structural reliability of the simply-supported steel truss bridge is analyzed by the first order second moment method(JC method),and the safety performance of the structure is evaluated by comparison with the standard target reliability index.In order to provide vascular basis for management units.The main contents and conclusions of this paper are as follows:(1)Based on reliability theory,a bridge structure safety performance evaluation method is proposed.Considering the structure of the simply-supported steel truss bridge is not complex,and the function has a clear expression form,the calculation of the reliability index adopts the first order second moment method(JC method).Meanwhile,based on the randomness of load and resistance in the actual operation environment of bridges,the theory of time-varying reliability is applied to the reliability of the target during the reference period.(2)Based on the modal analysis of the initial state of simply-supported steel truss bridge,the stiffness and vibration characteristics of the simply-supported steel truss bridge are studied.The modal analysis shows that the lateral vibration frequency of the bridge appears earlier than the vertical vibration frequency,which indicates that the lateral vibration stiffness is higher than the vertical vibration stiffness,and the earlier torsion vibration mode also shows that the torsion stiffness of the bridge is smaller.The vertical stiffness and lateral rigidity meet the requirements of line driving by comparing with the limit of natural vibration frequency prescribed by codes.(3)The coupling vibration model of vehicle bridge is established by LUSAS finite element software.Considering the influence of train operation speed,the vertical vibration changes of the bridge are calculated and analyzed from the one aspect of the time domain.When the speed of the train is at 150km/h~300 km/h,the vertical displacement and acceleration of the bridge span increase uniformly.When the train speed is raised to 300km/h,the vertical dynamic response of the bridge span is all reached the peak.When the train speed is at 300km/h~400 km/h,the vertical displacement and acceleration of the bridge will decrease rapidly.The analysis shows that the change of the dynamic response is mainly influenced by the speed of the train.When the train operation speed is resonant,the dynamic response of the train is maximum,and the train resonance speed is mainly related to the vertical vibration frequency of the bridge structure.(4)Based on the finite element model of the initial state of the bridge,the calculation program of the reliability index of all bridge components is compiled by MATLAB programming language.The reliability index of each component is calculated and the change law of the reliability index of the whole bridge is studied.The reliability index of the inclined rod is related to the position of the bridge.At the end of the bridge,the reliability index of the bridge is the lowest,and the reliability index of the span is the highest,showing a big change trend in the middle of the two ends.The reliability index of the bottom chord at the end of the bridge is the largest,and the reliability index of the middle span is the smallest,showing a trend of small middle and large ends.The results show that the reliability index of the full bridge is larger than the target reliability index when the structure is designed in the limit state.It shows that the bridge reliability index in the initial state can meet the requirement of the structure bearing capacity under the existing operating speed.(5)Based on the research results of the reliability index of the whole bridge,the reliability index of all components of the whole bridge during the target service life is studied by using the theory of time-varying reliability.The analysis results show that the reliability index of the inclined rod decays to the reliable target value of the bridge target when it is in service for 60 years ~70 years.When the vertical bar is in service for 70 years ~80 years,its reliability index is reduced to the reliable target value of the bridge target.The bottom chord in service for 80 years ~90 years,to bridge the objective reliability index of the reliability index attenuation.As a whole,the reliability index of the members of the simply-supported steel truss bridge gradually decreases with the service year,but the reliability index of the lower chord bar is the highest in the target period,the vertical bar is the second and the slanted bar is the weakest.The initial state of the article and the results of the reliability analysis in the period of use of the target can provide the management basis for the management units,and can also provide reference for the maintenance and reinforcement of the same type of bridge.