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Concrete filled tube (CFT) system provides excellent structural performance thanks to the composite effect between the tube and the concrete. However, its closed sections impose complication on the fabrication of the column-to-beam connection and only a small number of connection methods have been developed. In square CFT column-to-beam connections, internal diaphragms reinforce the inside of the column for the loads and moments delivered from the beam. It is required to develop various connection types to secure structural capacity and concrete filling. In the process of making the square Hollow steel section (HSS), diaphragms are placed in channel members in advance and then the two channel members are welded longitudinally to produce a square HSS. It is superior to other connection types in terms of the degree of Complication in placing the internal diaphragms, economical efficiency and workability. In this study, strength formulas were suggested based on the yield line theory to evaluate the load capacity of the column-to-beam connections. For the connection of horizontal diaphragms, the strength of steel tube faces, internal diaphragms, and concrete cone failure were accumulated. For the connection of vertical perforated plates, the strength of steel tube faces, concrete cone failure, and perfobond were accumulated. Although there was a slight difference between the formula and test result, it can be used to estimate the strength of column-to-beam connections.