Lateral-torsional buckling (LTB), explained simply
A steel beam's compression flange is really just a slender compression member in disguise — left unrestrained, it doesn't only bow sideways, it twists the whole beam section with it. That combined sideways-and-twisting failure is LTB.
When a steel beam bends under load, one flange goes into compression and the other into tension. That compression flange, on its own, behaves a lot like a slender compression member — and just like any slender compression member, if it isn't held in place sideways, it can buckle. The difference is that the compression flange is still physically attached to the rest of the beam section — to the web, and through the web to the tension flange below (or above) it — so it can't simply bow sideways on its own without dragging the whole cross-section into a twist along with it. That combined sideways deflection plus twist, happening together, is lateral-torsional buckling: the beam's own version of column buckling, shaped by the fact that a beam section isn't free to move independently of itself.
What actually controls how vulnerable a beam is to this is the unbraced length of the compression flange — the distance over which nothing (no floor slab bearing directly on it, no bracing member, no adjoining purlin) is holding that flange in place sideways. A short unbraced length keeps the flange essentially pinned in position at frequent intervals, leaving little length available to buckle over; a long unbraced length gives it much more room to move, and LTB capacity drops accordingly.
Why bracing matters as much as section size
This is the practical reason bracing a beam's compression flange — even at a handful of intermediate points, not necessarily continuously — can meaningfully increase how much load that exact same beam can carry, without changing the section itself at all. It's a genuinely different lever from making the section bigger or using a stronger grade of steel: it directly shortens the unbraced length that LTB depends on, rather than increasing the section's raw strength or stiffness. This is also why a beam's top flange braced continuously by a bearing floor slab is often far less at risk of LTB than the exact same beam used as, say, an unbraced roof purlin support.
Steel Beam Design checks lateral-torsional buckling directly as part of its flexural capacity check, and Section Properties is where the section's own geometric properties feeding that check — including its torsional properties — actually come from for a given standard or custom shape (see the related tools below).