SHS 70×70×3.6 vs CF SHS 70×70×3.5
SHS 70×70×3.6 is an SHS Hot-finished section from the UK Blue Book (SCI P363); CF SHS 70×70×3.5 is the closest equivalent in the UK Blue Book (SCI P363). SHS 70×70×3.6 carries more bending: 23.3 cm³ of plastic modulus against 22.2 cm³, 5.0% more. CF SHS 70×70×3.5 is the lighter of the two at 7.06 kg/m against 7.4 kg/m, a saving of 4.8% on steel weight. Both are about 70 mm deep, so either fits the same construction zone.
| Property | SHS 70×70×3.6 SHS Hot-finished | CF SHS 70×70×3.5 SHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 7.4 kg/m | 7.1 kg/m | +4.8% |
| Depth | 70 mm | 70 mm | same |
| Width | 70 mm | 70 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 9.4 cm² | 9 cm² | +4.8% |
| Second moment, major (Iy) | 68.6 cm⁴ | 65.1 cm⁴ | +5.4% |
| Plastic modulus, major (Wpl,y) | 23.3 cm³ | 22.2 cm³ | +5.0% |
| Elastic modulus, major (Wel,y) | 19.6 cm³ | 18.6 cm³ | +5.4% |
| Second moment, minor (Iz) | 68.6 cm⁴ | 65.1 cm⁴ | +5.4% |
| Plastic modulus, minor (Wpl,z) | 23.3 cm³ | 22.2 cm³ | +5.0% |
| Radius of gyration, minor (iz) | 2.7 cm | 2.69 cm | +0.4% |
| Torsion constant (It) | 108 cm⁴ | 106 cm⁴ | +1.9% |
SHS 70×70×3.6 to BS EN 10210-2:2006; CF SHS 70×70×3.5 to BS EN 10219-2:2006. Difference is expressed relative to CF SHS 70×70×3.5.
How to choose between them
On stiffness, SHS 70×70×3.6 has the larger second moment of area: 68.6 cm⁴ against 65.1 cm⁴, a difference of 5.4%. 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 (3.1 against 3.1 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, SHS 70×70×3.6 is the stronger section and CF SHS 70×70×3.5 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.