The welded joints between orthotropic steel bridge decks and longitudinal ribs are often heavily damaged by fatigue and difficult to repair. Fatigue resistance deficiency of the traditional one-sided welded joints is the main reason for frequent cracking in these components, and the double-sided welded joints are expected to significantly improve fatigue resistances of these joints. The initial welding defects are the key factors that affect the fatigue resistances of the double-sided welding joints. This study investigated the double-sided welded joints based on linear elastic fracture mechanics theory. The fatigue crack propagation characteristics were studied on single and multiple initial defects at the welding roots. Test result for multiple crack propagation in traditional one-sided welded joints verifies that the established multiple crack propagation simulation method is feasible. The propagation mode of the single crack of a welding root at the inner or outer side of the longitudinal rib and propagation mode of the multiple cracks of the roots are all of the mixed-mode crack propagations of Modes Ⅰ, Ⅱ, and Ⅲ, led by Mode Ⅰ. The multiple crack propagation characteristic is not significant, and these cracks transform into a single crack at the initial stage of propagation owing to the rapid decrease in the amplitude of the equivalent stress intensity factor of the adjacent crack. Subsequently, the propagation mode of multiple cracks is the same as that of a single crack in the welding root at the outer side of the longitudinal rib. The variations in the shape ratios of three kinds of cracks are different at the initial stages of propagation, but with the increase in propagation depth, the variations in the amplitudes of the equivalent stress intensity factor at the descending stages are basically identical. The shapes of the cracks are flat when they reach a certain depth, and the tendency to be flat becomes more pronounced with the increase in propagation depth. The single defect of the welding root at the outer side of the longitudinal rib is the main defect that affects fatigue resistance of the double-sided welded joints between a deck and longitudinal rib, and effective measures should be implemented in the manufacturing process to avoid such defects. © 2019, Editorial Department of China Journal of Highway and Transport. All right reserved.
