SHS 140×140×7.1 vs CF SHS 140×140×8
SHS 140×140×7.1 is an SHS Hot-finished section from the UK Blue Book (SCI P363); CF SHS 140×140×8 is the closest equivalent in the UK Blue Book (SCI P363). CF SHS 140×140×8 carries more bending: 194 cm³ of plastic modulus against 184 cm³, 5.2% more. SHS 140×140×7.1 is the lighter of the two at 29.2 kg/m against 31.4 kg/m, a saving of 7.0% on steel weight. Both are about 140 mm deep, so either fits the same construction zone.
| Property | SHS 140×140×7.1 SHS Hot-finished | CF SHS 140×140×8 SHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 29.2 kg/m | 31.4 kg/m | -7.0% |
| Depth | 140 mm | 140 mm | same |
| Width | 140 mm | 140 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 37.2 cm² | 40 cm² | -7.0% |
| Second moment, major (Iy) | 1,090 cm⁴ | 1,130 cm⁴ | -3.5% |
| Plastic modulus, major (Wpl,y) | 184 cm³ | 194 cm³ | -5.2% |
| Elastic modulus, major (Wel,y) | 155 cm³ | 161 cm³ | -3.7% |
| Second moment, minor (Iz) | 1,090 cm⁴ | 1,130 cm⁴ | -3.5% |
| Plastic modulus, minor (Wpl,z) | 184 cm³ | 194 cm³ | -5.2% |
| Radius of gyration, minor (iz) | 5.4 cm | 5.3 cm | +1.9% |
| Torsion constant (It) | 1,710 cm⁴ | 1,900 cm⁴ | -10.0% |
SHS 140×140×7.1 to BS EN 10210-2:2006; CF SHS 140×140×8 to BS EN 10219-2:2006. Difference is expressed relative to CF SHS 140×140×8.
How to choose between them
On stiffness, CF SHS 140×140×8 has the larger second moment of area: 1,130 cm⁴ against 1,090 cm⁴, a difference of 3.5%. 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 (6.3 against 6.2 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 140×140×8 is the stronger section and SHS 140×140×7.1 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.