The Influence of Rib Distortion on the fatigue resistance of orthotropic plated bridge decks

Orthotropic bridge decks contain many places that are prone to fatigue damage due to the crossing of wheel loads of vehicles. Most research has focused on the rib to deck plate detail that has limited fatigue strength, especially due to the joint weld between these two parts. Less attention has been given to the effect of the alternating distortion stresses in the web plate of closed ribs. This distortion occurs as the rib passes through the web plate of the cross beams and is particularly pronounced if there is a high cope hole in that web plate. Experiments have been carried out in on this distortion effect at Ghent University. During 2 series of tests, similar results for the vertical rib web stress were found. However, these stresses due to distortion combine with longitudinal bending stress of the rib web and with shear. The corresponding strains were found from rosette measurements. These stress variations must be combined for fatigue assessment. This requires to use hot spot stress. In theory, the variation of the principal stress should be compared with the hot spot stress limit. This limit value is of course different for the 2 principal directions of the stress variations. This type of multi‐directional stress variations is addressed in codes, yet not exactly applicable in the present problem. Therefore, the modified critical plane offers a solution and gives more favourable results with longer fatigue endurance. In particular the Findley criterion that uses normal stress instead of shear, proves to be efficient in this type of problem. In order to link the results to simpler calculations, an attempt has been made to constitute a graph, which should allow a quick approximation for a first design.