The interaction formulae for the plastic load carrying capacity of rectangular hollow sections under the action of normal forces, biaxial bending and shear and torsion are derived from the equilibrium conditions for the fully plastic cross section. This calculation gives also the position of the neutral axis which is required for the classification of the cross section, i.e. a criterion for the applicable plastic analysis type. It is shown that the neglect of the radius of the corner bend in deriving these formulae gives results which are very close to the exact solution and on the safe side. The comparison with ENV 1993-1-1 shows good agreement with the two exceptions that ENV 1993-1-1 may be on the unsafe side for some cases and it gives no rules for biaxial shear.
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