SHS 120×120×7.1 vs CF SHS 120×120×8
SHS 120×120×7.1 is an SHS Hot-finished section from the UK Blue Book (SCI P363); CF SHS 120×120×8 is the closest equivalent in the UK Blue Book (SCI P363). CF SHS 120×120×8 carries more bending: 138 cm³ of plastic modulus against 133 cm³, 3.6% more. SHS 120×120×7.1 is the lighter of the two at 24.7 kg/m against 26.4 kg/m, a saving of 6.4% on steel weight. Both are about 120 mm deep, so either fits the same construction zone.
| Property | SHS 120×120×7.1 SHS Hot-finished | CF SHS 120×120×8 SHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 24.7 kg/m | 26.4 kg/m | -6.4% |
| Depth | 120 mm | 120 mm | same |
| Width | 120 mm | 120 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 31.5 cm² | 33.6 cm² | -6.3% |
| Second moment, major (Iy) | 663 cm⁴ | 677 cm⁴ | -2.1% |
| Plastic modulus, major (Wpl,y) | 133 cm³ | 138 cm³ | -3.6% |
| Elastic modulus, major (Wel,y) | 110 cm³ | 113 cm³ | -2.7% |
| Second moment, minor (Iz) | 663 cm⁴ | 677 cm⁴ | -2.1% |
| Plastic modulus, minor (Wpl,z) | 133 cm³ | 138 cm³ | -3.6% |
| Radius of gyration, minor (iz) | 4.59 cm | 4.49 cm | +2.2% |
| Torsion constant (It) | 1,050 cm⁴ | 1,160 cm⁴ | -9.5% |
SHS 120×120×7.1 to BS EN 10210-2:2006; CF SHS 120×120×8 to BS EN 10219-2:2006. Difference is expressed relative to CF SHS 120×120×8.
How to choose between them
On stiffness, CF SHS 120×120×8 has the larger second moment of area: 677 cm⁴ against 663 cm⁴, a difference of 2.1%. 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 of steel, SHS 120×120×7.1 is the more efficient section: 5.4 against 5.2 cm³ per kg/m, 3.0% better. Over a floor of repeated beams that difference is real tonnage, though it is often outweighed by whichever section the fabricator can actually source.
In short, CF SHS 120×120×8 is the stronger section and SHS 120×120×7.1 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.