The Master S-N curve approach for fatigue assessment of welded bridge structural details

The verification of the Master S-N curve approach using an equivalent structural stress parameter, has been carried out for welded joints with different geometries, thickness and loading modes, mainly representative of pressure vessels and piping components. The application of the Master S-N curve to a new field requires the reassessment of the relevant fatigue data, due to different manufacturing conditions, detailing techniques across industries and typical loading modes. In this paper, the referred verification will be further extended by means of available fatigue strength data from large-scale tests of typical bridge structural details, mainly based on girder type specimens. The reassessment can allow the consideration of loading modes and stress conditions closer to those which develop in actual girder steel bridges. An approach based on experimental S-N data is also useful because it can take into account residual stresses, construction misalignments, local notch and weld bead geometry effects in the estimated fatigue lives. In this manner, the aim is to reassess the robustness and effectiveness of the equivalent-structural stress technique combined with the Master S-N curve approach through the finite element numerical modelling of large-scale tests of welded bridge details subjected to in-plane and out-of plane loading. The approach is applied to four fatigue test setup programs representative of welded bridge details, which were used as the basis for the establishment of the current nominal S-N curves available in American codes. A performance comparison with the nominal and the hot-spot stress method is carried out. The method proved to collapse nearly 300 datapoints of fatigue failures of welded bridge details into a narrow scatter band.