Causes and Retrofits of Fatigue Cracking in the Braced Truss of Steel Box-Girder Longitudinal Diaphragms on a Cable-Stayed Bridge

Abstract

A finite element model of steel box girder with angle steel and round pipe braced trusses, which matched the construction details of the Sutong Yangtze River Bridge, was created to investigate the causes of cracks in braced trusses of longitudinal diaphragms. The stress characteristics and deformation modes of fatigue crack details on the braced trusses were studied in conjunction with nominal stress test results obtained from an actual bridge. Additionally, two retrofit interventions were proposed: replacing the braced trusses with channel steel and adding vertical braces. Based on the traffic flow data from an actual bridge, the fatigue life of four braced truss forms was predicted and compared. Finally, an optimized retrofit intervention was recommended. The results indicate that the inner side joint details between the braced trusses and upper gusset plates are particularly susceptible to fatigue cracking. Under cyclic loads from traffic, axial tensile and compressive stresses are the main contributors to fatigue. Local stress concentration, combined with initial welding defects of the braced trusses, plays a crucial role in the initiation of fatigue cracks. The use of channel steel sections can enhance the fatigue performance of joint details, while the impact of adding vertical braces on fatigue performance is minimal.