Mainly in the last 30 years so much research has been done on Fe-based calculation of seam welded thin-sheet structures fatigue life. However, available prediction methods have been developed for a limited range of geometries under ideal load conditions. Extrapolating to complex real world geometries and load conditions such those resultant from, for example, ground vehicles dislocation over rough surfaces, are least documented. One example of the application of seam welded thin-sheet structures in the ground vehicle industry is the powertrain installation bracketry. Such brackets are subject to variable amplitude loading sourced from powertrain and road surface irregularities and their fatigue strength is tightly dependent on the strength of their joints. In this paper, a FE-based force/moment method has been used for numerically predicting fatigue life of powertrain installation bracketry of a commercial truck submitted to variable amplitude loading. A modal transient analysis has been utilized for stress calculation. Predictions results have shown good agreement with confirmation bench tests results, not only in relation to the location of the onset of failure but also regarding to the calculated fatigue life itself.