## 1 Introduction  
With the development of economy and society, traffic networks have evolved toward high operating speeds and heavier transportation weight. Bridges serving as key nodes of the transportation infrastructure network are accordingly subjected to vehicles with higher moving speeds and larger axle loads in comparison with that in the past [1; 2]. As a result, the dynamic interactions between vehicles and bridge structures have become more and more significant. These growing interactions may cause critical threats to the fatigue life of steel bridges by producing excessive stress ranges and applied cycle counts. In order to determine the fatigue load effects, the computation of dynamic stresses due to vehicle loadings is a fundamental step in fatigue evaluation of steel bridges. Both the predicted fatigue damage and fatigue life can be sensitive to the obtained stress results. In the fatigue analysis of steel railway bridges, the static method has been adopted to calculate the fatigue stresses. Railway vehicles can be treated as a set of vertical concentrated forces applied on the numerical bridge model. Subsequently, static structural analysis is performed by considering the impact effects of moving vehicles with a dynamic amplification factor [3; 4]. It was reported in [PERSON] et al. [5] that a safe dynamic amplification factor might not guarantee a safe evaluation of the bridge fatigue damage. Another method for fatigue stress computation is the moving load analysis. In this method, dynamic analysis was conducted to obtain the fatigue stresses by modeling the vehicles as vertical moving forces [6; 7; 8; 9]. Both the abovementioned methods have proven to be efficient in multiple cases such as preliminary fatigue analysis of the bridges to identify fatigue-prone members. Since the vehicle-bridge dynamic interactions are not accurately captured by the static and moving load methods, more systematic methods may be required for the stress calculation in fatigue assessment. An attempt to considerthe dynamic interactions between the train and bridge in fatigue life estimation of a steel-truss riveted railway bridge has been made by using a simplified partial bridge model and the three degrees-of-freedom (DOFs) vehicle model [10]. For fatigue assessment of highway bridges, coupled vehicle-bridge system models were developed to compute the fatigue stresses in several studies [11; 12; 13; 14]. Two important aspects of the coupled vehicle-bridge system, i.e., vehicle speed and road roughness, can significantly affect the bridge fatigue reliability [12]. A three DOFs vehicle model in the vertical plane coupled with the bridge model was built by [PERSON] et al. [5] to investigate the fatigue behavior of a composite high-speed railway bridge. Recently, a coupled train-bridge system model composed of a 3D vehicle model, a 3D bridge finite element (FE) model established with the direct stiffness method, and the wheel-rail interaction model were developed by [PERSON] et al. [15; 16; 17] to compute bridge dynamic stresses and to further perform deterministic and probabilistic fatigue evaluation. In those studies, significant differences in both the calculated fatigue stress and damage resulting from the static method, the moving load method, and coupled train-bridge dynamic analysis were found for structural members of the bridge deck system (e.g., stringer and floor beam). Furthermore, in spite of the consideration of train-bridge dynamic interactions, fatigue assessment was carried out at the structural member level (i.e., stringer) based on the calculated nominal stresses and the S-N approach. Due to complex geometric configurations and stress conditions, the structural details might not belong to any of the fatigue categories prescribed in the associated codes and therefore cannot be handled with the nominal stress based S-N approach [18]. These structural details, however, may be more critical compared with the major structural members from the fatigue point of view.