The Secant Method is an application of slenderness to the moment capacity of a given interaction diagram. This method was first described in ACI Journal, Arthur R. Anderson and Saad E. MOUSTAFA, “Ultimate Strength of Prestressed Concrete Piles and Columns”. The method is derived from classical elastic buckling theory for a prismatic member.
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A basis for the selection of the applicable value for EI is determined from the curvature at a given section. Using a strain-curvature relationship the accompanying EI can be found using Mu. Using this, EI can be used in the classical theory of elastic buckling to solve for Mu. Once Mu is found the points are plotted on the interaction diagram as the slenderness line.