Non-linear biaxial bending of steel Z-beams

An unconstrained beam under biaxial bending deflects and twists as soon as loading commences and reaches very large deformations as a limiting moment is approached. The large deformations cause premature yielding before the limiting moment is reached. Pre-buckling effects reduce these large deformations somewhat, but not sufficiently that the limiting moment can be safely used in first yield design. Instead, interaction equations may be used. The linear interaction equation is unnecessarily conservative, and may be replaced by a parabolic interaction equation.