Lateral torsional buckling is the deflection of a loaded member in its so-called weak direction, perpendicular to the plane of bending, combined with a twist of the cross section. The beam does not simply sag downwards as intended: it also moves sideways and rotates about its own length. The Dutch term for it is kip.
The classic example
Lay a plank on edge across a ditch. On edge it is far stiffer in the load direction than flat, so it hardly sags. But as soon as you step on it, the top edge moves sideways and the plank flips over. That flip is lateral torsional buckling: the member does not fail because the material is too weak, but because the shape is unstable.
What causes it
It occurs when two properties come together:
- Low stiffness perpendicular to the load — a tall, narrow section is weak in that direction
- Low torsional stiffness — an open section (an I-profile, a deep rectangular beam) twists easily
That is why tall steel I-beams, slender glulam beams and deep timber joists are susceptible, while a closed box or tube section is much less so.
How it is prevented
- Lateral restraints — supports at regular spacing that hold the compression flange in place
- A floor or roof plate — decking, hollow-core slabs or sheathing connected to the top flange keep the beam in position
- Bracing and wind girders — prevent the sideways movement
- Fork (torsional) supports — the bearing allows deflection but not rotation of the cross section
- Stiffeners — raise the torsional stiffness locally, especially at bearings and points of load introduction
Points to watch on site
- The erection stage is often critical — a beam that has been lifted in but is not yet tied into the floor or roof plate is at its most vulnerable; temporary props and bracing are not optional
- Do not weaken the section — never cut openings in a flange, and never remove a restraint “because it is in the way”
- It is a code check — verifying lateral torsional buckling is a standard part of structural design to the Eurocodes; for loadbearing beams, such as when removing a loadbearing wall, leave it to a structural engineer
Related terms
- Beam
- Rolled steel beam
- Braced and unbraced
- Roof truss
- Reinforcement
- Concrete structures
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