TECHNICAL 04 · LOADS
Wind load: the case that pulls anchors out of the ground.
Wind on a greenhouse roof is often net uplift, not downward pressure. That inverts the design logic: the arch may be fine while the anchor, the base plate and the end brace are the parts that fail.
Wind pressure depends on the local basic wind speed, the terrain and the height. On a low-rise structure the shape coefficients produce suction over much of the roof, so the roof is being pulled up while the frame's own weight pushes down. The difference is what the anchor has to hold.
| Member | Load type | Typical failure |
|---|---|---|
| Cover | suction + flapping | tears at fixing line |
| Purlin | uplift bending | sags or pulls off |
| Arch | asymmetric pressure | twists, legs spread |
| End frame | suction at the end | racks sideways |
| Anchor | uplift + lateral | pulls out |
| Base plate | lateral | bends or cracks concrete |
Exposure matters more than section
The same frame on a sheltered site and on an open coastal site behaves differently. Exposure category changes the pressure coefficient, and it changes what the bracing has to do. Two projects with identical drawings and different exposure are not the same structure — which is why we ask where the site is, not just how big it is.
The open-side case
A tunnel with both sides rolled up is a different structure from a closed one: the cover is no longer a sealed surface, the frame is exposed, and the effective area is the whole side opening. Berry growers roll sides up every day in harvest, so the open case is the normal case, not the exception.
What to ask your engineer
Ask for two figures: the design wind speed and the design uplift on the roof. If only a downward pressure is provided, the anchor design has no basis.
What we change
- Bracing: end frames and longitudinal braces, not just purlins.
- Anchors: type and length, quoted per leg.
- Base plates and their fixing: bolt size and spacing.
- Arch spacing: reduced where exposure is severe.
- Fixing detail: channel instead of clips where uplift is high.
Read next
Pages that answer the question this page raises.
- 08.03Snow loadGreenhouse snow load: ground snow load, shape and exposure coefficients, EN 1991-1-3 and E→
- 04.06.03Anchors & base platesAnchors & base plates for greenhouse frames — ground screws, concrete-in anchors, base pla→
- 07.10PhilippinesGreenhouse frames for the Philippines: Manila and Batangas ports, typhoon wind case, ancho→
- 05.02Single-span tunnelSingle-span tunnel (polytunnel) frames: 6.2 m and 8.5 m arches, 2.0–2.5 m arch spacing, ga→
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