The Perry–Robertson formula is a mathematical formula which is able to produce a good approximation of buckling loads in long slender columns or struts, and is the basis for the buckling formulation adopted in EN 1993. The formula in question can be expressed in the following form: with where: is the average longitudinal stress in the beam's cross section is the material's elastic limit is the average tension measured in the cross section which correspond to the beam's Euler load the amplitude of the initial geometrical imperfection distance from the cross section's centroid to the section's most stressed fiber the section's radius of gyration Robertson then proposed that , where represents the beam's slenderness. References Elasticity (physics)