CHS 88.9×2.9 vs CHS 88.9×3
CHS 88.9×2.9 is an CHS Hot-finished section from the UK Blue Book (SCI P363); CHS 88.9×3 is the closest equivalent in the European (Continental) range. CHS 88.9×3 carries more bending: 22.1 cm³ of plastic modulus against 21.5 cm³, 2.7% more. CHS 88.9×2.9 is the lighter of the two at 6.15 kg/m against 6.36 kg/m, a saving of 3.3% on steel weight. Both are about 89 mm deep, so either fits the same construction zone.
| Property | CHS 88.9×2.9 CHS Hot-finished | CHS 88.9×3 CHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 6.2 kg/m | 6.4 kg/m | -3.3% |
| Depth | 88.9 mm | 88.9 mm | same |
| Width | 88.9 mm | 88.9 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 7.8 cm² | 8.1 cm² | -3.2% |
| Second moment, major (Iy) | 72.5 cm⁴ | 74.8 cm⁴ | -3.1% |
| Plastic modulus, major (Wpl,y) | 21.5 cm³ | 22.1 cm³ | -2.7% |
| Elastic modulus, major (Wel,y) | 16.3 cm³ | 16.8 cm³ | -3.0% |
| Second moment, minor (Iz) | 72.5 cm⁴ | 74.8 cm⁴ | -3.1% |
| Plastic modulus, minor (Wpl,z) | 21.5 cm³ | 22.1 cm³ | -2.7% |
| Radius of gyration, minor (iz) | 3.04 cm | 3.04 cm | same |
| Torsion constant (It) | 145 cm⁴ | 150 cm⁴ | -3.3% |
CHS 88.9×2.9 to BS EN 10210-2:2006; CHS 88.9×3 to EN 10219 · Continental Steel. Difference is expressed relative to CHS 88.9×3.
How to choose between them
On stiffness, CHS 88.9×3 has the larger second moment of area: 74.8 cm⁴ against 72.5 cm⁴, a difference of 3.1%. This matters more often than the strength comparison, because on normal floor spans deflection rather than bending capacity sets the beam size - so if the two are close on modulus but apart on inertia, the stiffer one is usually the better answer even when it is not the stronger one.
Measured as bending capacity per kilogram, the two are equally efficient (3.5 against 3.5 cm³ per kg/m), so on a pure steel-tonnage basis there is nothing to choose between them. The decision comes down to availability, connection detailing and what the rest of the frame uses.
These come from different catalogues, so treat the swap as a design change rather than a like-for-like substitution. CHS Hot-finished sections are specified to BS EN 10210-2:2006 and CHS Cold-formed to EN 10219 · Continental Steel; the steel grades, tolerances and available lengths differ, connection details and bolt gauges are set out differently, and the two are rarely both stocked in the same market. Check availability before changing a section on a drawing to save 3.3% weight.
In short, CHS 88.9×3 is the stronger section and CHS 88.9×2.9 is the lighter one; where those are the same section, it is the clear choice, and where they are not, decide on whether your design is governed by capacity or by tonnage.