The early fatigue cracking of orthotropic steel decks has been a long-standing problem for heavy traffic regions of China’s long-span bridges. However, the actual fatigue life is much lower than the fatigue life calculated by fatigue design codes. In this study, a modified full range S-N curve, the M-S-N method, is proposed to evaluate welded joint fatigue life, considering mean stress, which is based on metal fatigue theory. As an example, a combined shell-solid finite-element model of a segmental bridge is first built for the Jiangyin Bridge to obtain the real stress, integrating welding residual stress and vehicle load stress. The fatigue life is then calculated and compared in order to consider the influence of different vehicle loads and deck-plate thicknesses. The calculation results show that the fatigue life of the deck-rib welded joint in the Jiangyin Yangtze River Bridge decreases nonlinear at a staggering rate with the increase in vehicle load. Under the condition of severe overload heavy vehicles, the Jiangyin Bridge fatigue life is only 2.17 years according to the M-S-N method, which matches practical situations. Deck-rib fatigue life obviously increases with the increase in deck thickness. The current steel bridge deck design with a 16-mm-thick plate is not safe under heavy traffic conditions.

An orthotropic steel deck (OSD) has a complicated structure, and its fatigue life is mainly determined by various welding details. Fatigue assessment of deck-to-rib welded joints under long-term train loads is an important concern for engineers. Using the stress range–number of cycles (S-N) curves that are recommended by existing specifications, it is difficult to consider welding residual stress. As a type of initial stress, welding residual stress reduces structural fatigue life by influencing the mean stress of the external cyclic load. However, the effect of mean stress is often overlooked by traditional welding fatigue theory. In this paper, a full-range S-N curve model of steel fatigue resistance integrating welding residual stress is proposed by adopting the Soderberg formula to equivalently transform the stress amplitude and mean stress. The Nanjing Dashengguan Bridge, a six-line railway steel arch bridge and the longest steel arch bridge in the world, is used as an example to demonstrate the fatigue life evaluation procedure of the proposed model in detail. First, the distribution of the welding residual stress of deck-to-rib welded details is obtained based on a refined finite-element model (FEM), and its accuracy is verified experimentally. Second, the partial welded joint model is embedded into the whole multiscale FEM of the steel deck. The structural/real stress spectrum of the welded details under a passenger/freight train is obtained by integrating the coupling effect of the train load stress and residual stress. Finally, fatigue life evaluation of the deck-to-rib welded details of the Dashengguan Bridge is carried out based on the proposed S-N curve model, integrating the residual stress and annual train volume. The results show that (1) the longitudinal residual stress (${\sigma }_Z$ ) on the top and bottom surfaces of the deck vary with its welding direction respectively (perpendicular or parallel); (2) the maximum value of the real stress under the coupling effect of train load stress and residual stress is less than the yield strength ($f_y$ ), so the influence of mean stress on the fatigue life of welded joints should still be considered; and (3) the fatigue life calculated by the proposed model is more accurate and conservative than a common standard. In summary, the proposed model provides a basis for determining whether the Dashengguan Bridge can allow freight trains to pass.

The steel truss arch is an important structural type for high-speed railway bridges with long spans. The fatigue assessment of rigid hangers under long-term train loads is an important concern. In this study, the Nanjing Dashengguan Bridge, a six-line railway steel arch bridge with three planes of truss arches and the largest span in the world, was taken as a case study. First, a fatigue assessment of one short hanger was carried out based on dynamic strain monitoring data in 2015. The influence of bending behavior, train lane, number of carriages, and train speed on the fatigue performance of the short hanger were investigated. Second, the fatigue damage distribution of full-field hangers induced by a single train was analyzed based on a calibrated finite-element model. The influence of driving direction on the fatigue performance of rigid hangers was discussed. Finally, an engineering approach for fatigue performance assessment of full-field hangers was proposed based on annual train volume. Consequently, it was shown that the bending behavior has considerable influence on the fatigue effects of short hangers. In addition, train lane and driving direction also have a significant influence on the fatigue performance of hangers. The fatigue damage of short hangers in the middle truss arch was the most remarkable, followed by the side truss arch on the downstream side (the Beijing–Shanghai High Speed Railway) and then by the side truss arch on the upstream side (the Shanghai–Wuhan–Chengdu Railway), which should be paid greater attention to in daily maintenance.

The welded joints in steel bridges have a complicated structure, and their fatigue life is mainly determined by the real stress under the coupling effect of vehicle load stress, as well as weld-induced residual stress and its relaxation. Traditional fatigue analysis methods are inadequate for effectively accounting for weld-induced residual stress and its relaxation, resulting in a significant discrepancy between the predicted fatigue life and the actual fatigue cracking time. A fatigue damage assessment model of welded joints was developed in this study, considering weld-induced residual stress and its relaxation under vehicle load stress. A multi-scale finite element model (FEM) for vehicle-induced coupled analysis was established to investigate the weld-induced initial residual stress and its relaxation effect associated with cyclic bend fatigue due to vehicles. The fatigue damage assessment, considering the welding residual stress and its relaxation, was performed based on the S–N curve model from metal fatigue theory and Miner’s linear damage theory. Based on this, the impact of variations in traffic load on fatigue life was forecasted. The results show that (1) the state of tension or compression in vehicle load stress notably impacts the residual stress relaxation effect observed in welded joints, of which the relaxation magnitude of the von Mises stress amounts to 81.2% of the average vehicle load stress value under tensile stress working conditions; (2) the predicted life of deck-to-rib welded joints is 28.26 years, based on traffic data from Jiangyin Bridge, which is closer to the monitored fatigue cracking life when compared with the Eurocode 3 and AASHTO LRFD standards; and (3) when vehicle weight and traffic volume increase by 30%, the fatigue life significantly drops to just 9.25 and 12.13 years, receptively.