Proposal for a stress modification factor for the fatigue design of flange thickness transitions in welded girders

Thickness steps or transitions are often used in flanges of bridge girders in order to adapt the bending resistance of the cross-sections to variable bending moment distributions. The sudden change of the stiffness distribution within the cross-section at these details causes additional local forces and stresses, which cannot be properly taken into account by classical beam-theoretical calculations, and which lead to a deterioration of the fatigue performance of the welded details at and around the transition. A modification of nominal stresses – e.g. by the use of an appropriate stress concentration factor kf – is therefore required in order to obtain safe joint designs with respect to fatigue. ress-raising effects at flange thickness transitions are of a similar nature as the ones caused by plate or angular misalignments, for which kf-factors are provided in the literature. However, the specific configuration of flanges in – often quite deep – bridge girders causes additional (beneficial or detrimental) effects that have not been sufficiently studied before, and which are currently not correctly considered in common design practice. aper discusses the characteristic local load-carrying behaviour and stress distribution in flange thickness transitions of welded I-section bridge girders, which were studied by numerical, experimental and analytical means. The paper furthermore introduces a new, accurate and mechanically coherent analytical formulation of kf-factors for the modification of nominal longitudinal stresses in bridge girder flanges; these new kf-factors could be used for practical fatigue design verifications. The possibility of including further amendments to the proposed formulation in order to take into account for the effects of additional unintentional misalignment or dimensional tolerances at the butt weld were also discussed. Finally, an outlook is given on the need to also consider additional stress concentration factors for shear in the fillet welds in the areas surrounding a thickness transition.