It is well known that local deformation in a steel beam-to-column connection occurs by beam flange force. In this paper, a numerical study was conducted to get important information on elasto-plastic behaviour of connections stiffened by external diaphragm using a finite element analysis program. The connections are made of circular columns and wide flange beams, connected with external diaphragms under both axial load in column and lateral force. In order to simplify the expression of P-Delta relationship of the subassemlage with bilinear model, the properties of the connections were deduced by the analytical results of 32 subassemlages, which are the value of the initial stiffness, the secondary stiffness, the yield load and the ultimate load. Finally, it was shown that the estimated relationship of P-Delta is able to trace the analysis result very well.
