Structural integrity of existing steel and composite bridges strongly depends on the performance of details adopted for connections. When assessing the vulnerability of viaducts featuring high-strength bolted joints, a key issue is often represented by fatigue performance of connections, i.e., due to the detrimental influence of several factors including progressive pre-tightening reduction during service life. Within such framework, this study presents a parametric analysis of the effect of pre-tightening loss on the fatigue response of high-strength bolted joints. For this purpose, a refined numerical application of the Strain Energy Density (SED) method is developed with reference to different geometrical configurations. Discovered findings indicate that pre-tightening reduction can severely impair the fatigue life of these details, i.e., reducing the expected number of cycles at failure by up to one order of magnitude in worst cases.