Seismic retrofit by the addition of H-section haunches has been developed by the authors to improve plastic deformation capacity of beam-to-column connection in existing building structures. The design criterion that the moment demand on original beam-to-column connection keeps below its plastic moment capacity, has been proposed to achieve satisfactory performance. For this purpose, the simple estimation of moment transferred by haunch is necessary. In this paper, simplified strut modelling (SSM) proposed by the authors was extended to account for the haunch connectedto RHS column through external diaphragm. The validity of extended SSM was verified by comparison to finite element analysis results.