TECHNICAL 14 · DETAILING
Posts and bracing: what stops the house racking sideways.
Posts carry the gutter down to the ground. Bracing stops the whole house from leaning. A frame with adequate arches and no bracing is a frame that folds in the first crosswind.
| Brace | Location | Resists |
|---|---|---|
| Longitudinal brace | Along each gutter line, 1–2 bays | Racking along the house |
| End frame bracing | First and last bay | Racking at the ends, wind on end wall |
| Cross brace (diagonal) | Between posts, in plan | Sideways sway |
| Portal frame | Where doors interrupt bracing | Keeps the opening stiff |
| Purlin continuity | Roof plane | Diaphragm action with the cover |
The cover itself acts as a diaphragm when it is tight; a loose cover contributes nothing.
Post spacing
Posts sit under the gutter, usually at every arch (4 m) or every second arch (8 m). Every second arch halves the post count and doubles the gutter span — which is a legitimate saving only if the gutter profile is sized for it. A 250 mm gutter at 8 m span with a full snow load will deflect visibly.
The door problem
A door interrupts the end bracing exactly where the wind load is highest. The usual fix is a portal frame around the opening, which is a fabricated member and therefore a cost line that should appear in the quotation, not arrive later as a surprise.
Ask this question before ordering
“How many bays are braced, and where?” If the answer is “the ends”, ask whether the middle needs it for your house length. Long houses need intermediate bracing; short ones often do not.
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- 08.04Wind loadGreenhouse wind load: pressure and uplift, exposure, bracing, anchors and the open-side ca→
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