Structural Stability Theory and Practice

6Torsional Buckling and Lateral Buckling of Beams

6.1 Introduction

So far we have assumed that buckling of members occurs due to bending only, where the bending deformation takes place in the plane of one of the principal axes and there is no rotation. This occurs if the bending rigidity of a member EI is smaller than the torsion rigidity GJ. Closed thin-walled and solid sections usually fail in this type of failure. However, beams and columns can also fail by buckling due to torsion alone or by a combined torsion-flexure mode. This type of failure occurs in sections where torsion rigidity is low in comparison to the flexural rigidity. Open thin-walled sections made of narrow rectangles, such as I, channel, T, angle, etc. sections usually have low torsion rigidity and are thus likely to buckle through torsion. A pure torsion buckling can occur in a section with two axes of symmetry where the centroidal and shear center axes coincide. For other sections, such as angle and channel sections, where the centroidal and shear center axes do not coincide, combined torsion-flexure mode of buckling occurs. The torsion-flexural buckling also occurs in transversely loaded beams, when the compression flange becomes unstable and wants to buckle laterally but the tension flange is stable and straight. We will consider in this chapter both axially loaded columns and transversally loaded beams for different buckling modes.

6.2 Pure Torsion of Thin-Walled Cross-Sections

In the case of a circular ...