Unstiffened Elements as Limiting Case of Distortional Buckling of Partially Stiffened Elements

Lip stiffened flanges in cold-formed steel compression members may undergo local or distortional buckling depending on the slenderness of the flange and rigidity of the lip. In distortional buckling, the flange-lip assembly moves perpendicular to the flange. The decrease in stiffness (or depth) of the lip increases the vulnerability of such elements to distortional buckling. In the limiting case of zero lip depth, the plate behaves as an unstiffened element. Hence the behavior of the unstiffened element may be intuitively understood as the limiting case of distortional buckling of lip stiffened element as the lip depth tends to zero. In this study, experimental and numerical results are used to show that both elastic buckling and ultimate strength behavior of a uniformly compressed unstiffened element may also be represented as distortional buckling of the lip stiffened element of zero lip depth. In addition to the effective width method, the modified direct strength method equation developed for distortional buckling is also shown to accurately estimate the ultimate strength of uniformly compressed unstiffened elements. (C) 2020 American Society of Civil Engineers.