This paper proposes a numerical tool for residual life assessment of metal active gas (MAG) welded structures such as gantry crane forestay and boom connections. These structures are submitted to cyclic loadings, which can cause fatigue damage around microcracks in the heat-affected zones (HAZ) of the welded joints, leading to safety and reliability issues. The numerical implementation of a crack propagation methodology in welded structures is complex due to the need for experimental validation tools, determination of the microstructural and physical properties in the HAZ, and efficient fracture mechanics simulation tools. In that context, a 3D tool – Z cracks – has been used to simulate the rate and direction of crack growth of a predefined flaw along the HAZ, the latter also acting as a stress raiser. Z-cracks is a finite element crack analysis toolbox for Stress Intensity Factors (SIF) computation. It uses the G-θ method in the Linear Elastic Fracture Mechanics (LEFM) regime and a three-dimensional automatic adaptive mesh refinement algorithm for crack advancement. The numerical results are compared with fracture analysis obtained from experimental test bench results on a reduced-sized replica of the forestay and boom substructure. The numerical tool implementation procedure has allowed to identify the prospects and limitations of the use of LEFM-based crack growth simulation tools and perspectives to further tailor the experimental procedures of such welded structures. Based in a better understanding of the crack propagation, the results obtained should contribute to the improvement of engineering practices for the prediction of the residual service life of welded structures and to the definition of improved procedures maintenance procedures.