Fatigue strength evaluation by measurable stresses for root cracks in U-rib to deck welded joints of orthotropic steel decks

Root cracks initiating from U-rib to deck welded joints of orthotropic steel deck bridges have become a problem for maintenance. Some fatigue strength evaluation methods for root cracks based on local stress criteria have been proposed, such as the effective notch stress method, but these methods cannot evaluate fatigue strength directly from the measurable response values of the structure. In this study, the evaluation method of crack initiation by measurable stresses was investigated by fatigue tests and FE analysis for both deck and bead propagation-type cracks, which are root cracks in U-rib to deck welded joints of orthotropic steel decks. As a result of the study, the relationship between the stress of weld root and the fatigue life was obtained by using the fillet shaped notch stress as the evaluation stress, and the S-N curve for the evaluation stress was proposed. And a method to estimate the evaluation stress of the root from the measurable stresses at four points was proposed. It was confirmed that the error of the proposed estimation method was generally within 10%. As a validation of the estimation method, it was confirmed that the fatigue test results could be properly evaluated.