Dynamic Stress Analysis And Fatigue Performance Assessment Of Bridges Based On Vehicle-Bridge Coupling Vibration

Fatigue is an issue related to the nature of the steel. According to the American Society of Civil Engineers,80-90% of failures in steel structures are associated with fatigue. There are a large volume of steel bridges in both highway and railway traffics of China. As the durations of their service increase, fatigue has been an appearing issue for these steel bridges. Consequently, effective assessment of fatigue damage and remaining service life of the existing bridges can be significantly helpful in ensuring their safety of service and further maintaining the normal operation of transportation networks. In addition, the strategy of developing high-speed and heavy-haul rails has been implemented by China’s railway administration. To date, the railway traffic in China has experienced six major speed promotions. The largest high-speed rail network in the world has also been in place. The coupling dynamic effects between trains and bridges become more significant with the increase in vehicle speed, axle weight, and overall train loading, resulting in greater fatigue load effects, and therefore, less bridge service length than expected.Fatigue stresses are the foundation of the evaluation of fatigue damage and service life. Accurate calculation of the stress response of bridge members to moving trains can significantly improve the reliability of fatigue assessment results. In order to capture the increasing dynamic interactions of trains and bridges and further to accurately identify the fatigue load effects on bridge members, it is necessary to establish a more advanced analysis model. To this end, a framework based on vehicle-bridge coupling vibrations is formulated for the dynamic stress analysis and fatigue performance assessment of bridges. Additionally, effects of vehicle speed, track irregularities, lateral vibrations, and traffic evolution on the fatigue stresses, residual fatigue life, and fatigue reliability are investigated. The main contents and contributions of this dissertation are as follows:(1) Mechanisms of bridge resonance and vibration cancellation and their correlation.Train loads are represented by a series of moving concentrated forces. The closed-form solutions of vibrations on the simply-supported beams are derived in terms of free vibration, on the basis of which, the mathematical critical-speed expressions of resonance and two kinds of cancellation are obtained. The phenomenon of the second type of cancellation is discovered for the first time. Mechanisms of resonance and vibration cancellation and their correlation are thoroughly explored. A new mathematical expression associated with generating conditions of the resonance disappearance is proposed. The analytical findings are validated by the numerical case study of simply-supported girder bridges for the high-speed railway. In the case study, the influence of train speed on the bridge dynamic effects is surveyed based on the calculated curve of dynamic factor versus speed, which serves as references for the following work.(2) Bridge stress calculation based on the dynamic analysis of coupled vehicle-bridge system.A 3D vehicle model is established using multi-body dynamics, and the bridge model is established by the finite element method. The direct stiffness method is used to build the equation of motion of the bridge. Incorporating the wheel-rail dynamic interaction model, the equation of motion of the vehicle-bridge system is built and solved to obtain the displacement time histories of bridge nodes, based on which the element dynamic stresses are calculated according to the elastic finite element theories. The proposed calculation approach based on the coupling vehicle-bridge vibration can properly take into account the dynamic interactions between train and bridge, and therefore, is more capable of accurately computing the dynamic stress response of bridges under train loads, compared to the conventional methods. Finally, the proposed approach is illustrated on an 80m-span steel truss girder bridge located in the passenger-dedicated railway line. The stress response of various bridge members is calculated using the proposed approach and stress amplification factors are investigated.(3) Experimental validation and comparison of stress calculation methods for bridges under train loads.The proposed approach based on the vehicle-bridge coupling vibrations is validated by using the field measuring stress/strain data from a concrete T-shaped beam bridge in the Shenchi-Huanghua Railway Line and the Baihe Bridge in the Beijing-Tongliao Railway Line. Comparisons are made among the proposed approach, the existing static influence line method, and the moving concentrated load method. In order to investigate the effect of vehicle speed on dynamic stresses, bridge strain responses to moving trains with different speeds are computed by the three considered methods. Bridge stress response is obtained, considering various track conditions represented by different samples of track irregularities. Then, the influence of track irregularities on the stress response is analyzed using various samples of track irregularities. In addition, the influence of lateral train loading and lateral bridge vibrations is also explored by comparing the stress time histories obtained from analyses with and without lateral vibrations considered. Accuracies and applicabilities of the proposed approach, the static method, and the moving load method are discussed, aiming to provide references for their engineering applications.(4) Fatigue damage and remaining life assessment considering the coupling vibration effects of vehicle and bridge.Integrating the coupled vehicle-bridge vibration theory into the fatigue analysis of bridges, a framework for the assessment of bridge fatigue damage and remaining service life is formulated. This framework can take into consideration the dynamic interactions between vehicle and bridge, and provide a novel methodology for the fatigue evaluation of existing bridges. Fatigue load effects are identified by using the proposed approach for calculating fatigue stresses of bridge members based on the coupling vibrations of the vehicle-bridge system. A unified formula in terms of the equivalent stress range, aimed for calculating the fatigue damage accumulation and suited for both the single-slope and bilinear S-N lines, is presented. Subsequently, the remaining service life of bridges is estimated, integrating the historical bridge traffic information. The methodology is applied to the Baihe Bridge in the Beijing-Tongliao Railway Line. Fatigue damage of the critical member is analyzed, based on which the fatigue life is predicted. Additionally, the impact of train speed, track irregularities, and lateral vibrations on the bridge fatigue life is investigated. The coupling effects of train speed and track irregularities on the fatigue performance are revealed.(5) Fatigue reliability analysis of bridges based on random vibrations of coupled vehicle-bridge system.A novel approach to fatigue reliability assessment of bridges is proposed based on probabilistic dynamic analysis of a coupled vehicle-bridge system. Train speed and track irregularities, which are proven capable of significantly impacting the fatigue damage accumulation, are selected as basic random variables. Probability models of the equivalent stress range and the number of stress cycles corresponding to the fatigue-critical detail are determined by performing the random vibration analysis of the vehicle-bridge system. Treating the critical damage accumulation index, the fatigue detail coefficient, the equivalent stress range and its cyclic count as random variables, the fatigue limit state function is established through the S-N approach. The fatigue reliability index is then solved via Monte Carlo simulation. The proposed approach is used in conjunction with the Baihe Bridge. Probabilistic fatigue assessment is conducted for the fatigue-critical stringer. The influence of vehicle speed, track irregularities, cutoff stress level, and traffic evolution on the probability models of fatigue load effects and fatigue reliability is discussed.