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 2×106 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.