Fatigue Life Assessment of Welded Steel Bridge Connections Under Variable Amplitude Loading

Welded steel connections are the most fatigue-critical components in highway bridge structures, and their reliable assessment under realistic variable amplitude loading remains a fundamental challenge in structural engineering practice. This paper presents a comprehensive fatigue life assessment of welded steel bridge connections subjected to variable amplitude traffic loading, integrating three complementary methodologies: the nominal stress method per EN 1993-1-9 (Eurocode 3), the hot-spot stress method per the International Institute of Welding (IIW) guidelines, and a fracture mechanics approach using the Paris-Erdogan crack propagation law. Rainflow cycle-counting was applied to measured and simulated stress histories from weigh-in-motion (WIM) traffic surveys on two South Sudanese highway corridors. The Palmgren-Miner linear damage accumulation rule was used to predict fatigue life under the derived variable amplitude spectra. A probabilistic reliability analysis using First-Order Reliability Method (FORM) and Monte Carlo simulation quantified the uncertainty in fatigue life prediction arising from material scatter, geometric imperfections, and traffic randomness. Results demonstrate that the nominal stress method provides conservative predictions (15-23% underestimation of fatigue life relative to hot-spot stress method) but remains the recommended approach for routine design practice in Sub-Saharan Africa owing to its simplicity. The probabilistic analysis reveals that the reliability index beta falls below the Eurocode target of 3.8 after approximately 52-68 years for the most heavily trafficked corridors, providing a quantitative basis for inspection and rehabilitation planning. The effect of weld quality class (as-welded vs. post-weld treated) on fatigue life