The Peak Stress Method (PSM) is a local, finite element-based approach for the fatigue assessment of welded joints, relying on linear-elastic peak stresses calculated at weld toes and roots and obtained from coarse-meshed 2D or 3D solid models. To streamline its application in the engineering practice, a dedicated tool, the so-called “PSM App”, has been developed within the Ansys® Mechanical environment to support finite element analysts in the fatigue design of complex welded assemblies. In this context, shell elements can be used to reduce model size and enable the efficient analysis of large-scale structures. This work presents a numerical procedure that integrates shell and solid modelling by employing a main model of the full structure meshed with shell elements and local submodels of critical regions meshed with ten-node tetrahedral solid elements, which can be assessed according to the PSM. A case study taken from the literature is presented, concerning the fatigue strength assessment of Circular Hollow Section (CHS) braces welded to Square Hollow Section (SHS) chords made of structural steel and subjected to axial loading.