Question

Flexural torsional buckling cannot occur in ________

(a) unsymmetrical members

(b) cross section with one axis of symmetry

(c) cross section with no axis of symmetry

(d) doubly symmetric members

This question was posed to me in an online quiz.

The above asked question is from Behaviour and Ultimate Strength of Plates and Possible Failure Modes topic in section Plastic and Local Buckling Behaviour of Steel of Design of Steel Structures

Answer 1

Correct answer is (d) doubly symmetric members

To explain: Flexural torsional buckling is a combination of flexural and torsion buckling. The member bends and twists simultaneously. It can occur only with open sections that have unsymmetrical cross section – both with one axis of symmetry(eg: channels, double angled shapes) and those with no axis of symmetry (eg: unequal leg single angles). Since the shear centre and centroid coincide with each other, doubly symmetric or point symmetric open sections are not subjected to flexural torsional buckling. Close sections are also immune to flexural torsional buckling.