The number of aging bridges in Japan is on the rise, prompting a growing trend in research focused on predicting the load-carrying capacity of bridges using 3D FEM. This approach aims to enhance the efficiency of bridge maintenance and management. However, when modeling steel bridges with 3D FEM, modeling of secondary members, such as lateral bracing and sway bracing, varies depending on analysts. The effect of different modeling of secondary members on the analytical results is not clear. In addition, complex analytical models such as full bridge models should be as simple as possible to reduce computation time. This study investigated the effect of secondary member modeling on analytical results in FEM analysis, with focusing on end sway bracings. These bracings notably influence the load-carrying capacity of steel bridges. Firstly, the authors created an analytical model of end sway bracings that could simulate mechanical behaviors observed by previous experiment. Next, a parametric analysis was performed varying the following analytical conditions: element type of secondary members and gusset plates; eccentricity of the connection. Secondary members and gusset plates modeled with shell elements were scrutinized to assess the feasibility of converting them into beam elements. As analytical results, we identified the effect of model simplification on load-carrying capacity and rigidity.