Column Interaction Diagram

A column interaction diagram is the failure envelope plotting all combinations of axial load (P) and bending moment (M) that a given reinforced-concrete column section can just safely resist. Its characteristic shape runs from the pure-axial capacity at the top, bulges outward to the 'balanced point' (where concrete crushing and steel yielding occur simultaneously, giving the maximum moment capacity), and down to the pure-bending capacity. Any (Pu, Mu) point inside the curve is safe; outside, the section fails.

Columns almost always carry combined axial load and moment (uniaxial or biaxial), so design uses these diagrams rather than simple formulae. SP 16 (Design Aids to IS 456) provides non-dimensional interaction charts for standard bar arrangements and grades; software generates them directly. They make column reinforcement design, optimisation and the biaxial-bending check (Bresler's load-contour with αn) practical.