Fatigue tests under four-point bending were carried out on low alloy steel welded I-beams under constant and variable amplitude loading. Four different types of stiffeners welded to flanges and webs were analysed. Particular attention was paid to the effect of configuration at the stiffener-ends. It was shown that the stiffener-ends were the most critical points. Fatigue cracks initiated and propagated in both tension and compression flanges. The fatigue life to crack initiation was calculated on the basis of the local stress strain analysis. Both theoretically and experimentally estimated stress concentration factors were used for calculation. The predicted fatigue lives were compared with the respective values obtained in experiments.