Investigation and Retrofit of Distortion-Induced Fatigue Cracks in a Double-Deck Cantilever-Suspended Steel Truss Bridge

This paper discusses a comprehensive investigation and retrofit for extensive fatigue cracks in the end connections of floor beams on a double-deck, cantilever-suspended steel truss bridge. The investigation involved three-dimensional (3D) finite-element analyses using global and local models, field measurement of strains and displacements due to live load and temperature, laboratory testing of steel samples for material properties and high-stress low-cycle fatigue characteristics, as well as development of an effective retrofit based on the analytical, experimental, and field testing results. It was concluded that the cracks were a result of distortion induced fatigue in the floor beam web due to interactive deformations of the global structural system under live load and temperature variations. The retrofit entailed removing the fatigue susceptible weld terminations and reinforcing local areas of the floor beam web for out-of-plane distortion. Construction of the fatigue retrofit was completed in July 2011. The repaired structure has performed satisfactorily since then.