This work deals with a case study performed by the University of Trieste and Fincantieri for the improvement of a structural detail where welded joints are subject to a proportional biaxial variable-amplitude loading causing high plastic strains. The critical detail is located on a longitudinal bulkhead at the corner of the opening for the access, from the deck, to a stairs trunk. After discussing how the S-N method should be modified to allow for stress multiaxiality and elastic-plastic strain, a critical examination is carried out following alternatives for effective equivalent stress hypothesis. A comparison between the elastic stress approach and the nonlinear elastic-plastic strain approach is then developed, with the aim of calibrating a design-oriented assessment procedure. Different design solutions are proposed involving gross or local geometry modifications, with impact respectively on the arrangement of the openings along the trunk and on the layout of each opening (i.e., th
