Definition of nominal stress-based FAT classes of complex welded steel structures using the Peak Stress Method

The purpose of this paper is to report some industrial applications of the local approach Peak Stress Method (PSM) in fatigue strength assessment of complex welded steel joints adopted in amusement park structures, focusing on roller coasters. This method is an application of the N-SIFs approach and it is based on the singular, linear elastic peak stresses calculated from FE analyses with coarse free meshes. For fatigue strength assessment of large structures like roller coasters, companies often prefer using FE beam models and compare nominal stresses with fatigue strength values (FAT classes) available in design standards. Roller coasters present many types of complex welded joints that differ in (i) technological parameters, e.g. weld penetration, and (ii) geometrical parameters, e.g. track pipes number, shape and number of connection elements between track pipes (tie beam, cross beam, lattice structures, etc.). Due to complex geometries and limited number of FAT classes available in design standards, finding appropriate FAT classes consistent with the real geometries is frequently troublesome. To overcome this problem, in this paper some applications of the mentioned local approach are proposed; the outcome is the definition of FAT classes in terms of nominal stress starting from the design curve calibrated in the context of the PSM. The advantage is that it is possible to perform FE analysis with beam elements, having in hands FAT classes derived from a robust local approach and faithful to real geometries.