TECHNICAL 08 · MATERIAL
Oval or round for the arch: what the section actually buys you.
Oval tube is the standard arch section in multi-span houses because a flattened section resists bending in the plane of the roof better than a round tube of the same mass. That is a real property — but the number depends on the axis you measure.
Bending stiffness is proportional to the second moment of area, I. For a round tube, I is the same in every direction. For an oval, I is larger about the strong axis and smaller about the weak axis. An arch is loaded mostly in the plane of the roof, so orienting the strong axis there is a genuine gain.
| Criterion | Round | Oval |
|---|---|---|
| Bending in roof plane | Baseline | Higher for the same mass |
| Bending across the roof | Baseline | Lower — orientation matters |
| Erection tolerance | Forgiving | Must be oriented correctly |
| Fixing with clamps | Standard clamps | Needs matching clamp shape |
| Cost per kg | Similar | Similar |
| Common use | Tunnels, braces, posts | Multi-span arches |
The number to ask for
Ask for the second moment of area (I) about the strong axis, and for the section modulus (W). Then compare per kilogram, not per metre. A supplier quoting an oval “stronger than 60 mm round” is usually comparing a heavier oval against a lighter round; per kilogram the difference is real but smaller.
What we will not claim
We do not claim our oval equals a named competitor's high-strength oval. Different mills roll different shapes and the section properties differ. Ask for the I and W of the exact profile you are being quoted, from whoever quotes it.
Orientation is the failure mode
An oval arch installed with the strong axis in the wrong plane behaves like a weaker section, and nobody notices until snow. The clamp shape and the bending mark on the arch are what keep it right; if the arch has no orientation mark, the site crew will guess.
Read next
Pages that answer the question this page raises.
- 04.02Oval arch tubeOval steel tube for greenhouse arches: 30×60, 30×70, 40×80, 40×90, 50×100 mm, wall 1.5–2.5→
- 08.13Purlin spacingGreenhouse purlin spacing: film, polycarbonate and glass span tables, C and Z profiles, an→
- 08.15Steel gradesQ235 and Q355 steel for greenhouse frames: yield strength, section choice, weldability and→
- 05.01Multi-span film houseMulti-span film greenhouse frames: 6.2–12 m span, gutter-connected, galvanised oval arches→
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