Fatigue Failures at Stiffener Connections in Highway Composite Bridges

Many large highway bridges on the route through the Alps of Austria are designed as open-section composite plate girder bridges with two main girders and a thin pre-stressed concrete slab that protrudes to both sides. A number of these show numerous fatigue cracks of the welds connecting the vertical web-stiffeners with the upper flanges. A research project was carried out by the Austrian Highway Authority ASFINAG to clarify the background of the failures and to investigate proper solutions for retrofitting existing structures and for the improvement of future designs. Numerical simulations using non-linear FEM techniques, measurements of strains and deflections on site and experimental fatigue testing on large-scale specimens were performed. These investigations made clear that the lifetime of the studied weld connection is influenced by numerous structural effects, such as the bending stiffness of the concrete slab, the stiffness of the shear connectors, the thickness of the upper flange and the position and and stiffness of the transversal bracing system of the bridge. Different types of weld connection and weld improvement methods have been tested and evaluated. This paper gives a summary of the most significant findings of this project.