SHS 80×80×3.2 vs CF SHS 80×80×3.5
SHS 80×80×3.2 is an SHS Hot-finished section from the UK Blue Book (SCI P363); CF SHS 80×80×3.5 is the closest equivalent in the UK Blue Book (SCI P363). CF SHS 80×80×3.5 carries more bending: 29.5 cm³ of plastic modulus against 27.9 cm³, 5.4% more. SHS 80×80×3.2 is the lighter of the two at 7.63 kg/m against 8.16 kg/m, a saving of 6.5% on steel weight. Both are about 80 mm deep, so either fits the same construction zone.
| Property | SHS 80×80×3.2 SHS Hot-finished | CF SHS 80×80×3.5 SHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 7.6 kg/m | 8.2 kg/m | -6.5% |
| Depth | 80 mm | 80 mm | same |
| Width | 80 mm | 80 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 9.7 cm² | 10.4 cm² | -6.5% |
| Second moment, major (Iy) | 95 cm⁴ | 99.8 cm⁴ | -4.8% |
| Plastic modulus, major (Wpl,y) | 27.9 cm³ | 29.5 cm³ | -5.4% |
| Elastic modulus, major (Wel,y) | 23.7 cm³ | 25 cm³ | -5.2% |
| Second moment, minor (Iz) | 95 cm⁴ | 99.8 cm⁴ | -4.8% |
| Plastic modulus, minor (Wpl,z) | 27.9 cm³ | 29.5 cm³ | -5.4% |
| Radius of gyration, minor (iz) | 3.13 cm | 3.1 cm | +1.0% |
| Torsion constant (It) | 148 cm⁴ | 161 cm⁴ | -8.1% |
SHS 80×80×3.2 to BS EN 10210-2:2006; CF SHS 80×80×3.5 to BS EN 10219-2:2006. Difference is expressed relative to CF SHS 80×80×3.5.
How to choose between them
On stiffness, CF SHS 80×80×3.5 has the larger second moment of area: 99.8 cm⁴ against 95 cm⁴, a difference of 4.8%. 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 (3.7 against 3.6 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.
In short, CF SHS 80×80×3.5 is the stronger section and SHS 80×80×3.2 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.