Studies on enhancing the strength and usability of steel members for high-rise buildings and long-span bridges have been conducted (Xiong and Liew, 2015; Khan et al., 2015; Mikkola et al., 2015). Furthermore, numerous experiments and numerical analyses of the damage assessment of steel columns, beams, bracings, and other structural members under repeated loading such as an earthquake load have been conducted (Nguyen et al., 2015; Park, 1993; Gao and Usami, 2000). Owing to the emergence of ultrahigh-strength steels, structural steel shapes are a very important design parameter considering the local buckling, and buckling expressions according to the structural steel shapes and load conditions have been proposed (Seif and Schafer, 2010). In addition, a trilinear kinematic hardening model was proposed to simulate the behavior of ultrahigh-strength steel columns with an H-section under cyclic loading (Wang et al., 2015; Chen et al., 2016). A steel member that has a low slenderness ratio is less affected by additional flexure owing to the axial force, and lateral buckling is caused by local buckling. Therefore, the local buckling of plate elements is the major reason for failure. According to an experimental study by Park (1993), a steel member accumulates damage during repeated loading, and local buckling leads to buckling of the entire member (Park, 1993). As a very large amount of plastic deformation repetitively occurs in the steel member, the steel member will eventually fracture. It was found that a steel member element can fail when such a large amount of plastic deformation occurs in a steel member element damaged by local buckling, even when the number of repeated loading cycles is low. In this study, a damage index is developed to quantitatively comprehend the degree of damage due to a large amount of plastic deformation under the repeated loading of steel members. By using this damage index, a method for analyzing the damage to box-type steel members is developed.