The concrete filled steel tube (CFT) that shows an excellent performance can be more economically used when the steel tube has a large width-to-thickness ratio. However, national codes such as KBC or AISC limits the use of a plate with a large width-to-thickness ratio, resulting in exclusion of possibility of more effective use of CFT. The purpose of this study is to remove the limitations by applying the concept of effective area which can consider local buckling and post-buckling strength. As a starting point, the effective area concept used in pure hollow steel sections in the current codes are applied to composite CFT columns and the results indicates that the limitations can be successfully removed.