Fatigue of steel bridges : New structural details and the implementation of high strength steel

Sammanfattning: The use of high strength steel has the potential to reduce the amount of steel used in bridge structures and thereby facilitate a more sustainable construction. The amount of steel and what steel grade that can be used in bridge structures and other cyclic loaded structures are often limited by a material degradation process called fatigue. The fatigue resistance of steel bridges is to a large extent depending on the design of structural details and connections. The design engineer is limited by the selection of a few pre-existing structural details with rather poor fatigue strength to choose from when designing steel bridges. Therefore, he/she is often forced to increase the overall dimensions of the structure to cope with the design requirements of fatigue.This doctoral thesis aims at increasing the fatigue resistance of fatigue prone structural details and connections by implementing new structural solutions to the already existing details given in the design standards. A typical fatigue prone detail is the vertical stiffener at an intermediate cross-beam, which will be in focus. By improving the fatigue resistance, less steel material will be required for the con-struction of new steel bridges and composite bridges of steel and concrete. It is shown in this thesis and the appended papers that the use of high strength steel for bridge structures can considerably reduce the amount of steel used, the cost of the steel material, and the harmful emissions. However, this is only true if the fatigue strength of critical details can be improved. This is shown through a survey of existing bridge structures and an optimisation of a steel and concrete composite bridge.Furthermore, a few new structural details are proposed and a few already existing solutions are highlighted that have the potential to improve the fatigue strength. The most promising structural detail could potentially increase the fatigue strength by 25 %. This is shown through numerical analyses using linear elastic fracture me-chanical models. However, the new structural details should be verified through fatigue experiments as a final step to determine their fatigue strength.