Buckling is the sideways deflection of a column or strut under a compressive load. The member does not fail because the material is crushed, but because at a certain load it suddenly deflects to the side. From that point the deformation grows rapidly and collapse follows.
Buckling is therefore a stability problem, not a strength problem. A slender steel tube can fail at a load far below the compressive strength of steel, simply because it is too slender.
What buckling depends on
- Stiffness of the material — the modulus of elasticity E
- Shape of the cross section — the second moment of area I; a tube or hollow section is far more buckling-resistant than a solid bar of the same material
- Effective length — Lk, which follows from how the ends are restrained
- Slenderness — the ratio of effective length to radius of gyration; the more slender, the sooner it buckles
The theoretical critical load follows Euler’s formula:
F(crit) = π² × E × I / Lk²
The squared term in the denominator shows how heavily length counts: double the length and the member buckles at a quarter of the load.
Where it matters
- Columns and posts in steel, timber and concrete
- Compression diagonals in trusses and frames
- Props and struts in excavations and formwork
- Scaffold tubes and adjustable props, where buckling is the governing failure mode in practice
Practical tips
- Shortening helps most — an intermediate restraint halves the effective length and so quadruples the theoretical buckling load
- Check the weakest direction — a member buckles about the axis with the smallest second moment of area
- Load it centrally — an eccentric load or an initial bow lowers the practical buckling capacity considerably
- Look at the connections — a joint that behaves as a pin gives a longer effective length than a fixed end
- Never use damaged props or scaffold tubes — a dent reduces the buckling load immediately
Related terms
- Column
- Modulus of elasticity
- Compressive strength
- Adjustable prop
- Lateral torsional buckling
- Structure
- Eurocode
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