The U-shaped rib-to-deck welded joint in orthotropic steel decks (OSDs) is prone to multiple fatigue cracks, which threaten structural integrity. Current design codes worldwide stipulate a minimum weld penetration rate of 75% or 80%, but practical challenges exist in meeting these requirements due to welding process limitations and machining errors. This study aims to investigate the influence of weld penetration rate on the fatigue performance of such joints, providing a theoretical basis for optimizing design and construction schemes.

Firstly, finite element models of U-shaped rib-to-deck welded joints with penetration rates of 65%, 75%, 85% and double-sided welds were established to analyze mechanical characteristics under typical loading conditions. Fatigue tests were then conducted on specimens with different penetration rates, and phased array ultrasonic testing technology was employed to assess penetration depth. Finally, fatigue S-N curves were regressed for partial penetration and double-side welded joints to quantify fatigue performance.

Stress concentration at the weld root decreases with the increase of penetration rate in partial penetration welds, while double-sided welds eliminate the weld root and further reduce stress concentration. Penetration depth exhibits significant dispersion even within the same weld. Partial penetration welds mainly initiate cracks from the weld root and propagate along the weld throat, whereas double-sided welds initiate cracks from the inner weld toe and propagate along the rib web. The fatigue strength of double-sided welds (143.9 MPa) is significantly superior to that of partial penetration welds (96.9 MPa) at 23106 cycles.

This study fills the gap of limited experimental data on the fatigue performance difference between partial penetration and double-side rib-to-deck welded joints. The findings clarify the influence mechanism of penetration rate on fatigue behavior, providing valuable theoretical support and technical reference for the design optimization, construction quality control and maintenance decision-making of OSD welded joints.

This research addresses the diverse characteristics of existing railway steel bridges in China, including variations in construction age, design standards, structural types, manufacturing processes, materials and service conditions. It also focuses on prominent defects and challenges related to heavy transportation conditions, particularly low live haul reserves and severe fatigue problems.

The study encompasses three key aspects: (1) Adaptability assessment: It begins with assessing the suitability of existing railway steel bridges for heavy-haul operations through comprehensive analyses, experiments and engineering applications. (2) Strengthening: To combat frequent crack defects in the vertical stiffener end structure of girder webs, fatigue performance tests and reinforcement scheme experiments were conducted. These experiments included the development of a hot-spot stress S-N curve for this structure, validating the effectiveness of methods like crack stop holes, ultrasonic hammering and flange angle steel. (3) Service life extension: Research on the cruciform welded joint structure (non-fusion transfer type) focused on fatigue performance over the long life cycle. This led to the establishment of a fatigue S-N curve, enhancing Chinese design codes.

The research achieved several significant outcomes: (1) Successful implementation of strengthening and retrofitting measures on a 64-m single-span double-track railway steel truss girder on an existing heavy-duty line. (2) Post-reinforcement, a substantial 26% to 32% reduction in live haul stress on bridge members was achieved. (3) The strengthening and retrofitting efforts met design expectations, enabling the bridge to accommodate vehicles with a 30-ton axle haul on the railway line.

This research systematically tackles challenges and defects associated with Chinese existing railway steel bridges, providing valuable insights into adaptability assessment, strengthening techniques and service life extension methods. Furthermore, the development of fatigue S-N curves and the successful implementation of bridge enhancements have practical implications for improving the resilience and operational capacity of railway steel bridges in China.