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One way will be to use sections designer in CSi softwares (Sap 2000, ETABS, Csi Col). Model four angles with re-bars by locating them at the centroid of of angles. You will have to define material for that reinforcement. This will be in addition to the existing reinforcement in the cross-section You can easily calculate the moment-capacity provided by four angles by manual calculation keeping in mind the crushing strength of concrete, and add it to existing capacity.