Within the scope of a German research project supported by Deutsche Forschungsgemeinschaft (DFG), two thick-walled hot rolled Rectangular Hollow Sections (RHS) were welded together side by side in longitudinal direction to act as top girders for crane runways. For systematic investigations of the complex stress state, component tests were carried out for two different weld designs. These experiments were performed for stationary wheel loads and roll-over tests. This realistic loading leads to a multi-axial stress condition with longitudinal, transverse and shear stresses. For the various failure modes found during the experiments fatigue design lines were derived. It was found that, within the range of practical relevant dimensions of the hollow sections and the rails, this type of crane runway girders shows an unexpected high fatigue resistance. Here the main results of this research are presented, which was part of a joint project together with University of Stuttgart and MPA Stuttgart.