Study On Fatigue Life Of Steel Deck Based On Random Vehicle Flow

Since the 21 st century,with the rapid development of social economy,traffic volume and vehicle axle load increasing,the bridge structure form is more changeable,and its span capacity is constantly improving.The orthotropic steel deck with small dead weight,large bearing capacity,easy construction and other characteristics are gradually applied to long-span bridges.However,the cumulative fatigue damage caused by repeated vehicle loads and the initial weld defects aggravate the fatigue cracking of the U-rib-roof weld of orthotropic steel bridge deck.Based on the measured data of WIM and the existing theories of S-N curve and Palmgren-Miner linear cumulative damage theory,linear elastic fracture mechanics and Paris crack propagation model,a cross-scale model of fatigue crack propagation is established in this paper.The probability distribution characteristics of random vehicle parameters are analyzed,and the main parameters of random load are defined.The sampling method and its influence on the stress amplitude and the amplitude of stress cycle are also discussed.Based on the above theory,the fatigue life of orthotropic steel bridge deck welds is analyzed considering the initial depth of weld defects,the temperature of road surface and the correlation of axle load.The main research contents are as follows:(1)Based on the S-N curve of the traditional fatigue damage theory and Palmgren-Miner linear cumulative damage theory,the fatigue state of the structure under constant/variable amplitude stress cycling can be solved.The stress intensity factor(SIF)is solved by finite element method(FEM)and the macro/micro fatigue crack model is established.(2)The main parameters affecting the random vehicle load are analyzed statistically,and the probability distribution characteristics of the main parameters are studied.Based on the measured data of WIM,the above parameters were randomly sampled by MATLAB,and the random load simulation program was compiled.For the random sampling of axle loads which need to consider the correlation between axle loads,the cumulative distribution function of axle loads is obtained by non-parametric method-kernel density estimation method,and the t-Copula function which is closer to the distribution law of axle loads of typical vehicles is selected as the connection function to establish the n-dimensional joint distribution function of axle loads.(3)Based on a long-span steel box girder suspension bridge,the fatigue stress amplitude and cycle number,fatigue action parameters and fatigue damage and life of U-rib-roof weld of orthotropic steel bridge deck are analyzed by using the related theories and results of random traffic load statistical analysis and simulation and orthotropic steel bridge deck detail fatigue analysis method.(4)The difference of fatigue life and cumulative fatigue damage of weld seam between main lane and overtaking lane is compared with traditional fatigue theory,linear fracture mechanics theory and cross-scale analysis method based on BS5400 and Eurocode 3.The fatigue life calculated by linear fracture mechanics is the shortest and the traditional fatigue theory is the longest.The fatigue life is greatly affected by the initial defect depth of the weld,and the axle load and the temperature of the pavement surface should be considered in the fatigue analysis,otherwise the fatigue life will be larger.The main conclusions of this paper are as follows:(1)Considering the correlation between axle loads has an important influence on fatigue life analysis,a more realistic stochastic traffic flow is established based on WIM data.(2)In the same lane,the cumulative fatigue damage calculated by BS5400 code is greater than that calculated by Eurocode 3 code,which shows that BS5400 code is more conservative.(3)The fatigue life is the shortest when the axle load is considered,the second is when the temperature is considered,and the longest when both are ignored.Therefore,the axle load correlation and temperature effect should be considered in fatigue analysis,otherwise the result will be unsafe.(4)The cumulative fatigue damage calculated by LEFM method is greater than that calculated by S-N curve method,and the fatigue damage calculated by cross-scale analysis method is between the results of the two methods.