Fatigue-based performance study of arch rib joints in CFST truss arch bridges

Concrete-filled steel tubular (CFST) truss arch bridges are widely used in long-span bridge construction due to their high strength, good stiffness, and excellent durability. However, arch rib joints, as critical load-bearing components, are prone to fatigue cracking under long-term vehicle loading, posing a threat to the safe service of the bridge. This study investigates a 15-year-old in-service CFST truss arch bridge with fatigue cracking at its arch rib joints. Based on field inspection and static load test results, a refined finite element (FE) model was developed and validated for accuracy. Stress influence lines at the joints were extracted, and fatigue life prediction was conducted using the rainflow counting method and Miner’s linear cumulative damage theory. Traffic flow statistics were incorporated to analyze factors influencing joint fatigue performance. Based on these findings, structural optimization strategies were proposed to improve fatigue resistance. Results show that fatigue cracks primarily occur near the arch crown and at the cross-section located one-eighth of the clear span from the support. The predicted fatigue life closely matches observed crack locations and propagation sequences. Daily traffic volume and heavy vehicle ratios have a significant impact on fatigue life, with the proportion of heavy vehicles identified as the main cause of premature cracking. Increasing the wall thickness and diameter of branch tubes effectively improves joint stress distribution, extending fatigue life by more than tenfold and enhancing overall structural durability. This study establishes a fatigue life prediction and optimization design method for bridge joints based on finite element analysis, traffic simulation, and cumulative damage theory, providing theoretical guidance and engineering reference for fatigue design, long-term health monitoring, and reinforcement of CFST truss arch bridges.