10/10/2026
Why do steel rafters twist? It’s not weakness, it’s stability 🌀
A steel rafter can be strong enough and still fail by twisting sideways. Engineers call this lateral-torsional buckling.
How it happens:
🔻 When a rafter bends, one fl**ge is squeezed and the other is stretched
🔻 The squeezed fl**ge behaves like a slender column, and slender columns buckle
🔻 The fl**ge bows sideways, the web tilts, and the whole beam rolls over
The catch 🌬️
Under the roof’s own weight, the top fl**ge is squeezed, and the purlins hold it in place.
When wind lifts the roof, the bending flips. Now the bottom fl**ge is squeezed, and nothing is holding it.
The fix 🔧
A small angle called a fl**ge brace, running from the purlin to the bottom fl**ge. It breaks the rafter into short, stiff segments.
Spacing matters 📐
Buckling load ∝ 1 / (unbraced length)²
Halve the gap between braces, and the rafter can carry far more load before it buckles.
That’s why every ISB rafter is checked for both gravity and wind uplift, so each brace sits exactly where the steel needs it.
💬 Engineers: gravity or uplift, which one governs your rafter design more often?
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