| Orthotropic steel decks(OSDs)have been extensively applied worldwide.However,fatigue cracking is frequently observed in the welded joint of OSDs,especially in the rib-to-deck(RD)welded joint.A series of distinctive works have been carried out by considerable researchers.Among these works,the application of thickened edge U-ribs(TEUs)is expected to improve the fatigue performance of RD joints,which in turns can lead to the technical improvement in fatigue-resistance of OSDs.However,currently there is a lack of theoretical and experimental studies on this innovative structure.Further studies are urgently required to be conducted with deeper insights,in seek of a significant improvement.This study has been focused on three levels including the theory,experiment and application.Systematic investigations have been made on the fatigue performance of the OSD with TEUs.The study consists of the following three aspect:study on the fatigue resistance model of the welded joint,study on the service life of the welded joint under the vehicle loads and study on the dynamic fatigue reliability at the operation stage.The following issues have been addressed in the thesis:(1)Comparative fatigue tests have been performed on the RD joints,which is the crucial fatigue-critical detail in OSDs,to investigate the fatigue performance of OSD with TEUs.Through the local full-scale fatigue test,comparison has been made between the OSD with conventional U-ribs(CUs)and the OSD with TEUs.The result suggests that,in terms of both the nominal stress approach and hot spot stress approach,the fatigue strength of RD joints could be effectively enhanced through the application of TEUs.Meanwhile,the effectiveness of the OSD with TEUs in the actual application has been further validated by the test data of two full-scale segment model.(2)Refined finite element model has been established for the test specimen,to investigation the mechanism of enhancement in fatigue strength of the RD joint in the OSD with TEUs.The model is used to perform the detailed analysis of local stress in the welded joint,in accordance with the equivalent notch stress approach.The result shows that the TEU specimen is relatively insensitive to the change in un-fused length in presence of negative welding error.This attribute ensures the robustness of fatigue performance of RD joints,which in turns lead to the improvement in the fatigue strength.Meanwhile,the proper increase in the open angle of welded joints is expected to further enhance the fatigue performance of RD joints when it is applicable in terms of the welding technology.(3)Large-scale numerical evaluation and statistics have been carried out using the experimental-numerical combined method,to establish the stress-life(S-N)curve of the RD joint in the OSD with TEUs.At first,a further fatigue crack propagation test has been performed based on the previous fatigue test.The result shows that,the fatigue crack keeps almost the semi-elliptical shape during the propagation.On this end,the multi-site fatigue crack propagation model has been established for this type of fatigue issues.In the model,the propagation and coalescence of multiple cracks have also been considered.At the same time,the uncertainties in both material properties,initial flaws,and local geometries has been considered.The proposed model is then verified through the experimental data.Based the validated model,a large-size database has been established for the S-N relation within a short time period.Finally,the design S-N curve has been derived for the RD joints based on the statistical analysis on the established database.The comparison has been conducted between the design S-N curve derived from numerical analysis and the curve obtained by the regression on the test data.The result shows the good agreement between the two types of S-N curves,while the one obtained numerically is more conservative.Based on the above analysis,it is suggested that in the fatigue evaluation of RD joints in the OSD with TEU,the FAT 85 and FAT 110 design curves could be applied with the power index m=2.89 in terms of the nominal stress and hot spot stress,respectively.(4)Fatigue life analysis has been performed on the steel deck of the Chengdu Fenghuanshan viaduct bridge,which is a continuously supported steel bridge with the hollow section,to investigate the fatigue performance of RD joints in the innovative OSD under the actual vehicle loads.In the analysis,the stress spectra have been established using the stochastic traffic model.At first,the framework of the stochastic traffic model has been derived considering the mechanic feature of welded joints in OSDs.The default traffic model is then established using the vehicle model and traffic volume data from the code of practice.At the same time,the default model has been modified using the measured data.Besides,an innovative solution method the has been established based on the multi-scale finite element model and influence surface technology,to solve the vehicle-induced stress history in RD joints.The vehicle data have been first generated using the Monte Carlo simulations on the stochastic traffic model.Combining the proposed approach with the generated vehicle data,the stress spectra have been derived for welded joints in various lateral position.Finally,with the fatigue resistance model previously proposed,analysis and comparison have been made on the actual fatigue life of RD joints in the OSD with CU and the OSD with TEUs,respectively.The result shows that the application of the OSD with TEUs can notably improve the fatigue life of RD joints under the vehicle loads.(5)Incorporating the reliability theory,the dynamic fatigue performance of OSDs has been investigated,based on the dynamic Bayesian network(DBN).The influence of the number of joints on the reliability is quantitatively considered through the serviceability-based multi-joint fatigue reliability model.In the study,a DBN model has been established aiming at the fatigue reliability of OSDs,which enables the fatigue reliability prediction,information-based confusion and parameter updating.In order to improve the computational efficiency,the discrete model with direct inference algorithm has been employed in the DBN model.At the same time,the inference algorithm has been adapted in a parallel way to further improve the computation efficiency.Based on the proposed DBN model,the following investigation are carried on the fatigue reliability of the Chengdu Fenghuanshan viaduct bridge,including the effect of enhancement in fatigue strength,the information-based confusion using the observed traffic data,the inspection results-based parameter updating and the effect of fatigue-treatment approaches.The result shows that the enhancement in the fatigue strength of RD joints could significantly improve the fatigue reliability of OSDs,and the effect is much better than the post-cracking treatments such as the maintenance and traffic control.Meanwhile,through the information-based confusion using the observed traffic data,it shows the significant influence of the change in traffic model on the fatigue reliability.Besides,the inspection results-based parameter updating suggests that the proposed DBN model can be self-calibrated using the inspection results,which in turns can continuously improve its accuracy with the input of inspection results.It is worth noting that,the fatigue reliability of OSDs can be effectively improved under the proper maintenance.However,when the design and fabrication of the OSD are poor,considerable efforts have to be applied in order to maintain the fatigue reliability at a reasonable level.Similarly,the fatigue deterioration rate can be ameliorated through the traffic control,which can in turns postpone the decrease in the fatigue reliability.Nevertheless,the traffic control is only recommended as an alternative when the maintenance is hard to be conducted since the function of the bridge will be intendedly restricted.Based on the research of this thesis,the outputs related to the innovative OSD with TEUs have already been incorporated in the code "Cold-formed U-rib steel section for steel structure of bridge".Meanwhile,the TEU has also been successfully applied in several engineering projects,including Chengdu Fenghuangshan Viaduct Bridge,the reconstruction of Beijing Sanyuan Bridge,Taiyuan Wuyi Road Viaduct Bridge,the reconstruction of Huzhou Wuyi Bridge,Yushan Bridge in Ningbo Zhoushan Harbor Main Passway,Beijing Capital Ring Road Viaduct Bridge and Wuhan Yangsigang Yangtze River Bridge. |
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