ESDU 89007 gives the cubic equation for the buckling stress of a strut of arbitrary open section under the interaction of the flexural and torsional-flexural modes. When the section has an axis of symmetry, buckling will occur in either the flexural mode or in the torsional-flexural mode, the buckling stress for which is given by the solution of a quadratic equation. For sections in which the centroid and the shear centre coincide (which are those with axes of symmetry or with point symmetry such as a Z-section with equal flanges) there is no interaction and buckling will occur at the least stress of the two Euler buckling stresses and the torsional buckling stress. A fully worked example illustrates the use of the equations.