A compression member may buckle overall or locally. It is possible that the thin flanges or webs of a column may buckle locally in compression much before the calculated buckling strength.
Depending upon the width to thickness ratio of compression elements and depending upon whether the elements are stiffened or not, the elements buckle at different stress situations. If the elements are so thin that local buckling occurs, then the strength corresponding to any buckling mode cannot be developed. This local buckling can however be avoided by limiting the width to thickness ratio as discussed in chapter 1.
But when a column buckles wholly (and not locally) then this buckling occurs in any one of the following three ways viz.
Flexural Buckling
It is also called as Euler buckling Euler buckling when elastic behavior predominates.
In this, deflection occurs due to bending or flexure (without rotation) about an axis corresponding to the largest slenderness ratio. This axis is the minor principal axis i.e. the one with the smallest radius of gyration.
Compression members of any type of cross-sectional shape can fail in this type of buckling.
Torsional Buckling
Thin walled members with open cross-sectional shapes are weak in torsion and hence undergo buckling by twisting rather than bending (as compared to flexural buckling).
Torsional buckling occurs when torsional rigidity of the member is very small as compared to flexural rigidity.
Failure occurs due to rotation about longitudinal axis (xx-axis) of the member.
This occurs only with doubly symmetric sections with very slender cross sectional elements.
Standard hot rolled sections are not prone to torsional buckling but a member made of thin plate elements must be checked for possible torsional buckling.
Torsional buckling is quite complex and thus it is always avoided. This is done by proper arrangement of the members and by providing bracing to prevent lateral movement and twisting. If sufficient end supports and intermediate lateral braces are provided, flexural buckling will always control over torsional buckling.
Flexural Torsional Buckling
This type of buckling failure is caused by combination of flexural buckling and torsional buckling.
Herein the member bends and twists simultaneously.
This type of failure occurs with unsymmetrical sections including both with one axis of symmetry like channels, T-sections, double angle sections etc. and with no axis of symmetry at all.
Usually an analysis of torsional or flexural-torsional buckling only is made as and when it seems to
be appropriate.
