In this study, the mechanical property on 200mm long gussets and cover plates welded to tensile flange for the purpose of increasing shear-strength of steel is presented. Fatigue crack propagation life was evaluated using numerical analsis method based on the Monte Carlo simulation technique. Safety assessment of existing steel bridge was carried out to widely employ the steel used in this study. The constant amplitude fatigue limit caculated by the numerical analysis was smaller than that obtained from experiment. In the case of members with 200mm long gussets, the constant amplitude fatigue limit was about 343$kg/cm^2$. In the case of specimens with 200mm long cover plates, the constant amplitude fatigue limit was about 506$kg/cm^2$ and it was 47.5 percent higher than the former case. The safety assessment was carried out on steel girder bridge with members similar to this test in type. According to the result, the steel girder bridge is safety although four decades have passed.