This paper presents a new method for computing the fiexural rigidity, El, of prestressed concrete columns at their ultimate capacity. Flexural rigidity is needed to evaluate the critical buckling load which is used in the magnification formula. The proposed methodology for computing El is based on two fundamental relationships: (1) the moment versus curvature relationship at a given load eccentricity, and (2) buckling under the assumption of pure concentric load. The proposed El model was used with the moment magnification formula to obtain moment versus axial load interaction diagrams for a number of slender prestressed concrete columns. The diagrams were compared with those obtained from a finite element analysis and very good agreement was observed. Also good agreement was observed with available experimental test results. Comparisons with current code formulations and PCI recommendations are made, and a new procedure is proposed. Several numerical design examples, illustrating the new method, are provided.
