Overall buckling of steel I-section members is usually in a lateral-torsional mode, in which the Vlasov assumption that the cross-section does not change shape during buckling is assumed. While in many cases this assumption is vindicated, members whose compression flange is restrained only by the stiffness of the web may buckle in a lateral-distortional mode, in which the mode of buckling is usually associated with distortion of the web in the plane of the member cross-section. Lateral-distortional buckling is prevalent in the hogging region of continous composite beams, in beams with partial end restraint, and in columns and rafets in industrial porta frame buildings where the clading combined with purlins or girst forms a diaphragem restraint to the tension flange.
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