Structural Engineering & Statics
where the load goes
Concept

Critical moment — where it appears

The bending moment at which a beam buckles sideways and twists, set by the minor-axis, torsional and warping stiffnesses together. It combines three stiffnesses that come from three different section properties, and for an open section the warping term is a large part of it.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

2000400060008000100001200002004006008001000distance between lateral restraintsthey cross at 3803the plastic capacity of the sectionelastic critical momentSt Venant torsion alone — what is left at long lengthswarping dominates here

The beam that fails sideways

A deep narrow beam bending in its strong plane can, at a moment well below its capacity, swing out of that plane and twist. The failure has nothing to do with how much it can carry and everything to do with what is holding it.

stability · lateral torsional 020040060080010000100200300400brace stiffness (kN/m)critical moment (kNm)compression flangeshear centretension flange447 kN/m is enougha rigid brace buys 3.12, not two

The brace on the wrong flange

A brace on a column has one property that matters, and it is stiffness. A brace on a beam has two, and the second decides whether the first is worth anything: put the identical restraint on the tension flange and it does not reach the answer at any stiffness whatever.

stability · beam bracing

Named alongside it

The objects these essays reach for when they reach for this one.

Compression flangeEffective lengthLateral torsional bucklingWarpingBracingBuckling modeEnergy methodIdeal brace stiffnessImperfectionLateral restraintMoment diagramRestraint force

All concepts