Fatigue Study Of Steel-Concrete Composite Girder Bridges Considering The Performance Degradation

Steel-concrete composite girder bridges,which take advantages of high tensile strength of steel and excellent compressive strength of concrete,are becoming more and more popular in application.During the service period,bridges are affected by vehicle loads which cause fatigue damage threaten the bridge safety.In addition to the effect of vehicle loads,the degradation of structural performance,caused by the concrete crack or environmental corrosion,can also increase the fatigue damage of bridges.Therefore,this thesis studies the variation characteristic of bridge fatigue life considering performance degradation under random traffic loads,and the main research contents and achievements are summarized as follows:1.A steel-concrete composite girder bridge is taken as an example,to calculate the dynamic response of the bridge through model simulation and field test.The rain-flow counting method is used to process the stresses to get the fatigue analysis stress spectrum.2.The influence of the road roughness and vehicle speed on the impact coefficient is analyzed,and the values of impact coefficient in Chinese and American codes are compared.According to the degradation model of the road roughness,the modified value of fatigue impact coefficient of steel-concrete composite girder bridge is proposed considering the degradation of road roughness.3.Based on the fatigue analysis stress spectrum and the S-N curve,the cumulative fatigue damage calculated by Chinese,British and American codes are compared.Consequently,the fatigue life of the steel-concrete composite girder bridge is obtained based on the Miner linear fatigue cumulative damage criterion with considering the degradation of road roughness.The effects of the vehicle speed,road roughness and vehicle overload on the fatigue damage of the steel girder are also studied.4.According to the actual traffic volume data collected by the dynamic weighing system,the distributional characteristics of traffic volume parameters are statistically analyzed,and the random traffic flow model of steel-concrete composite girder bridge under the actual operation state is then generated by the self-developed algorithm.Finally,the fatigue life of steel girder of steel-concrete composite girder bridge is predicted considering the road roughness degradation and random traffic distribution characteristics.5.The crack density is introduced to reflect the stiffness reduction of the concrete slabs.Subsequently,the field investigation of the bridge is carried out and the crack density statistics of three different steel-concrete composite girder bridges are obtained.The fatigue life of such bridges considering the crack of concrete slab is then predicted.The result indicates that the fatigure life of steel-concrete composite girder bridges reduced by 37.31% and 56.61% when the degree of concrete-slab craking get to the “general” and“serious” levels.In addition,the corrosion degradation process of the steel girder is also analyzed and the fatigue strength reduction factor is introduced to characterize the degradation of the steel girder under environmental corrosion.Finally,based on the corrosion model of the steel girder,the fatigue life of the bridge under different corrosion environment is further studied.The result shows that fatigue life of the bridge reduced by38.58%,42.02% and 65.86% which respected to the low corrosion,moderate corrosion and high corrosion,respectively.