ABSTRACT
Suspension bridges and cable-stayed bridges are dominant structures for super long-span bridges, where steel box girders, known as stiffened girders, are commonly used. Despite their advantages of light weight and easy construction, steel box girders are prone to suffer from severe fatigue damage. Excessive traffic volume and truck overloads are one of the external factors for the girders to have premature fatigue cracks. Critical details including U-ribs and truss diaphragms are reported to have the most fatigue cracks for their geometric features, seen at their welded joints. In this paper, fatigue performance of U-rib to deck joints, belonging to a suspension bridge, and truss-type longitudinal diaphragms, belonging to a cable-stayed bridge, is analyzed. Multi-scale finite element models (FEM) of the suspension and cable-stayed bridges are developed to calculate the fatigue life of the details under different load cases, based on cumulative fatigue damage law and S-N curves. The results are validated by field inspection records of the background bridges. Then, to prolong the life-cycle of the fatigue details, two maintenance actions are proposed and applied respectively. The thickness of the deck and U-rib of the suspension bridge is discussed and optimized through comparative study. On the other hand, the steel pipes of the diaphragms are replaced by bolted steel channels and the effects are proved to be satisfactory. These replacement strategies provide practical instructions for steel box girder maintenance and similar structures.
