Fatigue Analysis Of The WEB Gap Of Steel Girder Bridges Considering Vehicle-bridge Interaction

Out-of-plane distortion of steel bridge is one of the most difficult problems in the fatigue design of steel bridges.Web gaps,which are constantly subjected to out-of-plane distortion,experience large-amplitude cyclic stresses,resulting in the initiation and propagation of cracks between the webs and girder flanges.It is expected that the cracks will pose significant threats to the bridge fatigue life.The out-of-plane distortion-induced fatigue problem of web gaps has aroused great interests among researchers.They typically adopt the static analysis method which introduces an impact factor to consider the vehicle dynamic effect.However,when the road surface roughness is poor,this method may underestimate the effect of dynamic vehicle loading especially for fatigue design,which brings about an overestimated fatigue life of web gaps.In order to study the effect of dynamic vehicle loading on the fatigue life of web gap,the paper has done a larger amount of work which includes:(1)The finite element models for simply-supported steel ?-girder bridges with 4 typical length of span and 3D model for fatigue truck were established based on AASHTO Code.The stress level and distribution law in the web gap under moving loads from running vehicle were investigated.The effects of bridge span length and web gap length on the stress in the web gap were investigated based on the static analysis.Results show that,the web gap stress is significantly affected by the bridge span length while the effect of the web gap length is relatively small.It can be concluded that the bridge span length is the main factor for affecting web gap stress.(2)The deterioration process of the road surface condition under the given traffic and environmental condition was investigated.The number of truck passages and the proportion of time for bridge deck roughness in different grades for the road surface condition to deteriorate from one level to another were obtained.(3)According to the interactive force relationship and the displacement relationship at the contact points of the three-dimensional vehicle and bridge model,a three-dimensional vehicle-bridge coupled system was established and was solved by the modal superposition technique and the fourth-order Runge-Kutta method in the time domain.The stress,stress range,impact factor of stress and impact factor of stress range under different road surface condition were obtained using the developed vehicle-bridge coupled system.Results show that,the impact factor of stress range in the web gap is larger than that of stress;the specified impact factor of 0.15 for fatigue design in the AASHTO LRFD code may underestimate the vehicle dynamic loading effect in practice,especially when the road surface roughness is poor.Based on the results,the current paper proposed a recommended value of the impact factor of stress range for fatigue life evaluation of web gaps.(4)A program for modeling random traffic flow developed by MATLAB was used to count the number of stress ranges of web gap generated when the vehicle passing.The fatigue life of web gaps of steel bridge was estimated based on S-N curve and Palmgren-Miner linear cumulative damage criterion.Then the fatigue life of the web gaps of the bridges was estimated considering the dynamic effect of the fatigue truck.It was found that regular maintenance of the bridge deck could reduce the fatigue damage and increase the fatigue life of the web gap.The suggested impact factor of stress range of web gap and the time of pavement maintenance are important to evaluate and prolong the fatigue life.