The objective of this study is to propose the skeleton curve model of the square CFT beam-columns, that is simple, easy to use and represent the actual behavior appropriately. In this study, we gave attention to tne change of the bearing axial load of steel pipe and filled concrete, and examined by conducting the elasto-plastic analysis to define the tendency of the bearing axial load. The skeleton curve model of steel beam-columns and concrete beam-columns were assumed respectively, which considered the bearing axial load. The skeleton curve model of square CFT columns is proposed by superposing the model of steel beam-columns and concrete beam-columns. Proposed model estimated the test results (Diameter-to-thickness ratio (D/t : 33-44), strength of concrete (Fc : 30, 60MPa), axial load ratio (n : 0.2-0.7), yield stress of the steel (sσy : 400MPa)) generally.
