This paper presents a method for modeling the initiation and propagation of the damage in steel structures under combined fatigue loading. An existing model in the literature is used as the main key to the development of the new formulation herein proposed, where a significant computational improvement for the cracking incubation phase under high cycle fatigue (HCF) is achieved. The results show a satisfactory comparison with experimental data available in the literature and the carried parametric simulations demonstrate the effectiveness of the model once strength reduction could be estimated after 0, 20, 40, and 60 years of HCF. The reduction of the structural capacity was evaluated by the consideration of the final damage due to a high-intensity dynamic loading at the end of the HCF phase.