Study On The Fatigue Of Seel-concrete Composite Bridges Based On Vehicle-bridge Coupled Vibration

Vehicle loading is the main cause of the fatigue damage of highway steel bridges.Practice indicates that under the long time repeated action of the vehicle load,the cracks will initiate and propagate continuously in the steel components of steel bridges,which may result in fatigue fracture under some severe conditions.The fatigue life of steel bridge components is mainly affected by the value of the stress range that bridge components experience.Accurately obtaining the value of stress range that bridge components experience under the action of the vehicle loading is of great importance for fatigue design and evaluation of steel bridges.Based on the three dimentional vehicle-bridge coupled system,the following work were performed and completed in terms of the fatigue issues of steel bridges:(1)A method was proposed for determining reasonable impact factors for fatigue design(IM_FD)and number of stress cycles for fatigue design(NSC_FD)of steel I-girder bridges that can more rationally consider the effect of the deterioration of the road surface condition(RSC)during its whole life cycle.The deterioration process of the RSC was investigated under the given traffic and environment condition and the number of truck passages taken for the RSC to deteriorate from one class to the next was investigated.A three-dimensional vehicle-bridge coupled model was developed to simulate the interaction between the bridge and the vehicle,with both the bridge and fatigue load models adopted from the AASHTO LRFD code.Numerical simulations were conducted to study the influence of three important parameters,including the RSC,bridge span length and vehicle speed,on the impact factor of stress range(IM_SR)and the equivalent number of stress cycles of simply-supported steel I-girder bridges.Results show that the RSC has a great impact on the IM_SR and the ENSC and that the RSC has a greater impact on the IM_SR than on the traditional IM calculated using the maximum stress and the IM_SR is greater than the traditional IM.By considering the cumulative fatigue damage caused by the passage of each truck under different RSCs and the deterioration process of the RSC during its whole life cycle,simple and reasonable expressions were proposed for the IM_FD and NSC_FD of steel I-girder bridges under the given traffic and environmental condition.(2)A new approach for fatigue design of steel bridges was proposed which can consider the effect of dynamic vehicle loading and truck overloading on the fatigue life of steel bridges more rationally.A three-dimensional vehicle-bridge coupled model was developed to simulate the interaction between the bridge and vehicle.A simply-supported steel I-girder bridge was used as an example for illustration of the proposed approach.The fatigue vehicle model was adopted from the AASHTO LRFD code and overloading was considered by increasing the gross weight of the truck model.Numerical simulations were conducted to study the influence of three important parameters,including the RSC,vehicle speed and truck gross weight,on the fatigue damage of the bridge considered.The results show that the RSC and truck gross weight both have a significant impact on the bridge fatigue damage.By considering the cumulative fatigue damage caused by each truck passage under different RSCs and the deterioration process of the RSC during its whole life cycle,a simple and reasonable expression was proposed for bridge fatigue design.(3)A new approach was proposed for determining the truck weight limit of simply-supported steel girder bridges based on the fatigue reliability of bridge girders.According to the available traffic data from the literature,random traffic flows were firstly generated using the Monte-Carlo method and were used as the traffic loading.A simply-supported steel girder bridge designed according to AASHTO LRFD code was used as the bridge model to illustrate the presented approach.By loading the influence lines of the used bridge with the generated random traffic flows,numerical simulations were carried out to investigate the effects of three important parameters,including the fraction of traffic in the fast lane(FTFL),violation rate(VR)and truck weight limit(TWL),on the average fatigue damage(AFD)induced by each truck in the random traffic flow.Based on the limit state function deduced with the Miner's cumulative damage model and the S-N curve,the relationships between the fatigue reliability index of the steel bridge girder and the number of the average daily truck traffic(ADTT)were obtained under different FTFL,VR and TWL values.The truck weight limit for the bridge girder to achieve the target fatigue reliability after the design life of 75 years was determined based on the fatigue reliability analysis of the bridge girder.(4)The effect of the pavement maintenance cycle on the fatigue reliability of simply-supported steel I-girder bridges under dynamic vehicle loading was studied.The bridge model and the fatigue load model from the AASHTO LRFD code were adopted to establish the three-dimensional vehicle-bridge coupled model.Numerical simulations were carried out to study the influence of three important parameters,including the road surface condition,bridge span length and vehicle speed,on the fatigue damage induced by the fatigue truck.The results from this study show that the bridge fatigue reliability decreases dramatically under repeated dynamic vehicle loads when the road surface condition is very poor and that the pavement maintenance cycle has a significant influence on the bridge fatigue reliability.Finally,a procedure for determining the desired pavement maintenance cycle to achieve the target fatigue reliability of steel bridge girders was developed.(5)A new method was proposed for determining the reliability index of the ultimate load-carrying capacity of steel bridge girders.This approach can more reasonably take into account the effect of the fatigue damage accumulation induced by the dynamic vehicle loading on the reduction of the ultimate load-carrying capacity of steel bridge girders.A three-dimensional bridge-vehicle coupled system was firstly established to simulate the bridge-vehicle interaction.A simply-supported steel I-girder bridge designed according to the AASHTO LRFD code was utilized as the bridge model,while the fatigue truck model and the design truck model from the AASHTO LRFD code were adopted for vehicle loading.The fatigue damage(FD)and the maximum stress(MS)induced by the fatigue truck and the impact factor(IM)induced by the design truck were investigated under different RSCs based on the developed vehicle-bridge coupled system.The distribution pattern of the FD,MS,and IMwere checked based on the Chi-square test.By considering the effect of the fatigue damage accumulation induced by each passage of the fatigue truck,the reliability index of the ultimate load-carrying capacity of bridge girders was analyzed,and its relationship with the number of daily passage of the fatigue truck was obtained.