Probability Model Of Dynamic Responses And Reliability Assessment For Stiffening Girders Of Suspension Bridges Under Random Traffic Flow

A large number of vehicles repeatedly travel on the highway bridges during the opetational period. With the increasing of traffic flow and load capacity, assessing the safety level of these bridges is a burning assignment. The uncertainty of traffic flow, structural parameters, and failure modes bring out the probability issue of safety assessment for the existing bridges under the vehicle loads. Failure of transfinite displacement and cumulative fatigue damage of structural details are pron to occur. Thus, the safety level of these bridges under the two failure modes can be assessed based on the structural dynamic reliability and the fatigue reliability theory, represently.This dissertation was supported by the "973" project named "Study on basic theory of safety design and assessment of long-span bridges(No.2015CB057700)" and the Natival Science Foundation of China named "Time-variant fatigue reliability analysis of reinforced concrete bridges under overload and chloride penetration(No.51378081)". The random traffic flow model of a highway bridge was established based on statistical data in the weigh-in-motion system. Probability models of dynamic responses (e.g. displacement and fatigue stress) of suspension stiffness girders under random traffic flow were established. The first-passage damage standard and fatigue damage standard were adopted to be the basis of dynamic reliability theory. The first-passage dynamic reliability associated with displacement in the entirety model and the fatigue reliability in the detail model were studied. The main works of this paper are as follows:(1) The simulation method of random traffic flow was studied based on the WIM(weigh-in-mothion) system. The probabilic model of vehicle type, axle weigh, vehicle distance, vehicle speed and vehicle lane of a highway bridge were established. Random traffic flow simulation software was developed based on GUI function of MATLAB. Random traffic flow model under sparse traffic flow and heavy traffic flow were given. The increase coefficient of traffic volume was forecasted.(2) A FEA-based simplied method was proposed to analysis dynamic response of bridge under random traffic flow. The simulation capability for bridge and vehicle simplified elements and post-dispose performance of transient analysis were adequately applied. The displacement and acceleration of response of long-span bridges under vehicle traffic could be quickly and accuracyly calculated.(3) The probability model of extremum value of displacement response of suspension bridge stiffening girders under random traffic flow was established based on the crossing theory. The vehicle and traffic flow parameters effect on displacement response of suspension bridge stiffening girders were studied. The statistical data revealed that the displacement response was stationary gaussian process. The displacement extremum values were predicted based on the Rice formulation.(4) The dynamic reliability of stiffening girders of suspension under random traffic flow was studied based on displacement first-passage criterion. The Possion assumption and Markov assumption effect on boundary cross rate were discussed. The effect rule of parameters of vehicle and stiffening girder on dynamic reliability index are revealed. The parameter anslysis indicate that the degeneration of cable stiffness and heavy vehicles are the main factors for the displacement first-passage failure.(5) The typical fatigue vehicles were improved as random traffic flow model. The fatigue stress and damage of stiffening girder details of suspension bridges during operation period were established based on the Gaussian Maxed Model. In order to handle the time-consuming problems, a UD-SVR (an algorithm combined with uniform design and support vector regression) algorithm was proposed.(6) The fatigue reliability of steel box girder details was analyzed, considering the traffic flow parameters and growth factor of traffic volume and vehicle weigh. The fatigue damage functions were established, considering the random traffic flow parameters. A fatigue reliability sofeware was developed. The effect growth factors of traffic volume and vehicle weigh effect on fatigue reliability of steel box girder details on different vehicle lanes were analyzed. The fatigue performance was assessed based on the target reliability index. The results lay the foundation for assessing the fatigue safety of bridge in operating period based on traffic information.