Fatigue behavior at flange thickness transitions with cope holes - design model for engineering practice

Flange thickness transitions are common constructional details in welded girders, such as crane runway girders or main girders of highway and railway bridges, to adapt the bending moment capacity of the girder to variable bending moment distributions. At these details the transverse butt weld is often decisive for the fatigue verification of the member. Based on comprehensive studies at Graz University of Technology, including also two fatigue tests, stress concentration factors for the fatigue verification of the butt welds at the girder flange were developed. However, due to the complex local stress fields at the flange thickness transition, cope holes were not included in these studies.

Within a research cooperation of Graz University of Technology and Ruhr University Bochum, flange thickness transitions with cope holes are studied recently. Based on comprehensive numerical FE- calculations within the practically relevant geometric parameter range, the complex local stress fields near the cope hole are analyzed both, within the web and the flange plate. For validation of the FE-model also strain measurements were carried out at large-scale test girders. Applying the hot spot stress method, the decisive hot spots are determined, and a simplified design model is developed for an accurate prediction of these hot spot stresses.

The paper presents this design model in detail and shows its accuracy - compared to the FE-results - for a wide range of different geometrical conditions, as used in practice (e.g. variation of girder dimensions, flange thickness ratio, tapering to the outside and to the inside).

Finally, also the results of full-scale fatigue tests on two different welded girders with flange thickness transitions and cope holes are presented. This provides an accurate fatigue resistance for the proposed design model, based on the hot spot stress method.