| Steel bridge has many advantages and is widely used in modern bridges,especially in long-span cable-stayed bridges and suspension bridges.However,the fatigue cracking is a prominent problem of steel bridges.Particularly,the fatigue cracking often occurs in orthotropic steel deck with complex composition and numerous welds after a short period of service life.In order to calculate the static and dynamic responses of a long-span steel bridge accurately and evaluate the fatigue damage of critical structural details,a multiscale finiteelement(FE)model is necessary to be developed.However,the initial FE model needs to be updated because it cannot represent the real state of the structure in service due to the model errors and the change in structure state of the bridge.Oriented to fatigue evaluation of longspan steel bridges,a multiscale model updating method for long-span steel bridges is developed and the influence of longitudinal and transverse multi-vehicle effect on fatigue damage is analyzed in this thesis.The main contents are as follows:(1)After the investigation and comparison of relevant theories,the multiscale model updating method for long-span steel bridges is developed by a combination of Kriging surrogate model,Latin hypercube sampling experimental design method,R2 indicator-based multiobjective optimization and evolution control strategy.The global model and local models will be updated simultaneously in this method,which lays a foundation for the model updating of an actual bridge in service.(2)The developed method is applied to a long-span cable-stayed bridge.Using the modal frequencies,displacement influence lines and stress influence lines measured in the system loading test to define the objective functions,the initial FE model of the cable-stayed bridge is updated to demonstrate the feasibility and accuracy of this method.The results show that Kriging model can accurately replace FE model to improve the calculation efficiency;and R2indicator-based optimization algorithm has more advantages than other algorithms in the problem of more objectives.The developed multiscale model updating method for long-span steel bridges can coordinate the conflict among objectives and provide good accuracy for all the calculated results.(3)Based on the updated FE model,a fatigue failure pattern with different control stress directions is selected for analysis from rib-to-deck welded joints,rib-to-diaphragm welded joints and rib butt welded joints,respectively.In order to consider the influence of global stiffness change on local fatigue stress amplitude and reflect the superposition effect of the first system,the longitudinal multi-vehicle loading is used in the fatigue stress history calculation of the critical details.The changes of fatigue stress amplitude before and after model updating and the changes under single and multiple vehicle loading are investigated.The longitudinal multi-vehicle fleet load is established from the measured traffic load data.In addition,a correction factor γ is used to define the transverse multilane effect in fatigue assessment according to relevant research and specifications.(4)Based on the Palmgren-Miner linear cumulative damage theory and nominal stress SN curves,the fatigue life of each critical detail is evaluated with the superposition of longitudinal and transverse multi-vehicle effect.The fatigue life calculated in the updated multiscale FE model is less than that in the initial model,which proves that the multiscale model updating technology can provide a more reliable basis for local detail response problems of long-span steel bridges such as fatigue damage assessment. |
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