SHS 120×120×6.3 vs SHS 125×125×6
SHS 120×120×6.3 is an SHS Hot-finished section from the UK Blue Book (SCI P363); SHS 125×125×6 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 120 cm³ against 122 cm³ of plastic modulus, a difference of 1.6%. They weigh essentially the same, 22.2 kg/m and 22.1 kg/m. SHS 125×125×6 is deeper, 125 mm against 120 mm, which matters where the floor zone is tight.
| Property | SHS 120×120×6.3 SHS Hot-finished | SHS 125×125×6 SHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 22.2 kg/m | 22.1 kg/m | +0.5% |
| Depth | 120 mm | 125 mm | -4.0% |
| Width | 120 mm | 125 mm | -4.0% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 28.2 cm² | 27.6 cm² | +2.2% |
| Second moment, major (Iy) | 603 cm⁴ | 641 cm⁴ | -5.9% |
| Plastic modulus, major (Wpl,y) | 120 cm³ | 122 cm³ | -1.6% |
| Elastic modulus, major (Wel,y) | 100 cm³ | 103 cm³ | -2.9% |
| Second moment, minor (Iz) | 603 cm⁴ | 641 cm⁴ | -5.9% |
| Plastic modulus, minor (Wpl,z) | 120 cm³ | 122 cm³ | -1.6% |
| Radius of gyration, minor (iz) | 4.62 cm | 4.82 cm | -4.1% |
| Torsion constant (It) | 950 cm⁴ | 1,038 cm⁴ | -8.5% |
SHS 120×120×6.3 to BS EN 10210-2:2006; SHS 125×125×6 to EN 10219 · Continental Steel. Difference is expressed relative to SHS 125×125×6.
How to choose between them
On stiffness, SHS 125×125×6 has the larger second moment of area: 641 cm⁴ against 603 cm⁴, a difference of 5.9%. 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 125×125×6 is the more efficient section: 5.5 against 5.4 cm³ per kg/m, 2.1% 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.
These come from different catalogues, so treat the swap as a design change rather than a like-for-like substitution. SHS Hot-finished sections are specified to BS EN 10210-2:2006 and SHS 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 a marginal amount of weight.
In short, these two are genuine equivalents. Pick whichever your supplier stocks and the rest of the frame already uses.