Multi-lane Fatigue Load Model And Fatigue Life Assessment Of Bridges Under Multi-lane Random Traffic Loadings

Fatigue is a critical damage mechanism of bridge,and vehicle loads are the dominant external fatigue load on operational bridges.In Specifications and a large number of studies,the fatigue load models are characterizing as fatigue damage of bridge under actual random traffic loadings.However,the problem of significant variability in multi-lane load caused by vehicle traveling in separate lanes is not considered.In addition,the S-N curve method is mainly used to assess the accumulated damage of bridge under the long-term cyclic action of practical vehicle loads,which cannot answer the fatigue damage evolution characteristics of bridge structure details over time and how to develop management and maintenance countermeasures so as to explore for main research.Therefore,the multi-lane load disparity characteristics under random traffic are considered to develop multi-lane fatigue load model,and different bridge fatigue life assessment methods are used to evaluate the fatigue performance of bridge components under random traffic.The main research contents and results are as follows:(1)Fatigue assessment methods based on S-N curves and fracture mechanics are summarized and derived.Firstly,the fatigue assessment methods based on S-N curves of various codes are systematically sorted out and the fatigue evaluate results of different codes are compare with the actual WIM data at the same fatigue detail.Afterword,different mentalities based on fracture mechanics theory are divided into cyclic-cycle approach and time-scale approach and develop numerical procedures respectively.Finally,the prediction consequences of the procedure are compared with the literature examples to demonstrate the validity and accuracy of the procedure.The results show that there is significant variability in the assessment results for the same fatigue detail due to the different national specifications specify different value of fatigue strength for various details categories.However,comparison with the fatigue life for practical traffic flow yields consequences based on the specification that are on the conservations side.In addition,the prediction of crack growth variation under diverse materials based on two methods of fracture mechanics is compared with literature data showing that predicted results by the self-programing are in good agreement with the literature.(2)A multi-lane fatigue load model for bridge considering lane-load disparity is established.On accounting of multiple sets of weigh-in-motion data,multi-lane load differentiation under WIM data loading is analyzed,and a multi-lane fatigue load model consisting of multi-lane coefficients and fatigue vehicle model is established.Finally,the fatigue life assessment of the recommended fatigue load model compared with the Chinese code based fatigue load model Ⅱand actual traffic load data are upload in the same bridge project case respectively.The results show that there is significant lane load variability in fatigue damage of different girder-component under multiple lane WIM data loading.Fatigue life of bridge members under the action of the recommend fatigue load model are compared with the actual traffic data and the specification-based fatigue load model.The results show that the recommended fatigue load model better shows the load variability of the lanes and is consistent with the calculation consequences of the actual traffic data.(3)The fatigue assessment systems of bridge under random traffic loading are established on account of the theory of fracture mechanics for the cyclic-cycle and time-scale methods,respectively.Based on whether the parameters clearly include the fatigue life assessment framework and the fatigue reliability assessment framework,comparing with the assessment results of the S-N curve method.The prediction results in terms of crack length variation showed that the gap between the prediction of fatigue crack size based on the cyclic-cycle method and the time-scale method are significant,and the crack expansion rate and reliability level of the time-scale method were lower than those of the cyclic-cycle method,but the trend of crack variation along time was the same for both,i.e.,the crack length variation curve increased nonlinearly,while there was significant variability in the distribution of crack size for different girder members.In addition,the fatigue life of each girder component under stochastic traffic loadings is calculated based on the S-N curve method and the fracture mechanics method,and the fatigue life of the girder member under the outer-slow-lane exceeded the critical crack size of 10 mm after 100-year of use,indicating that its fatigue life less than 100 years and the structure is damaged at that time and needed to be repaired and strengthened in time.