This paper evaluates external diaphragm axial capacity in moment frame structures with square concrete-filled steel tubular (CFST) columns considering bidirectional loading. Three design methods were considered: (1) the CIDECT method; (2) the equivalent beam method; and (3) the tie method. Finite element analyses were conducted to investigate the behaviour of an external diaphragm plate connected to a square CFST column under varied bidirectional diaphragm axial forces. It is shown that the perpendicular diaphragm axial forces did not reduce the diaphragm axial capacity significantly, which is consistent with the assumptions made by the CIDECT method and the tie method. The CIDECT method, in some cases, was not conservative. Among the considered methods, the tie method was the most justifiable method, although in some cases the capacity predictions were too conservative. The tie method was later modified by considering the contribution of the steel tube in addition to the diaphragm plate in calculating the diaphragm axial capacity. The modified tie method was shown to accurately predict a lower bound estimate of the capacity of an external diaphragm connection.