SHS 350×350×8.8 vs SHS 350×350×9
SHS 350×350×8.8 is an SHS Hot-finished section from the UK Blue Book (SCI P363); SHS 350×350×9 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 1520 cm³ against 1522 cm³ of plastic modulus, a difference of 0.1%. They weigh essentially the same, 93.6 kg/m and 94.7 kg/m. Both are about 350 mm deep, so either fits the same construction zone.
| Property | SHS 350×350×8.8 SHS Hot-finished | SHS 350×350×9 SHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 93.6 kg/m | 94.7 kg/m | -1.2% |
| Depth | 350 mm | 350 mm | same |
| Width | 350 mm | 350 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 119 cm² | 120 cm² | -0.8% |
| Second moment, major (Iy) | 23,100 cm⁴ | 22,970 cm⁴ | +0.6% |
| Plastic modulus, major (Wpl,y) | 1,520 cm³ | 1,522 cm³ | -0.1% |
| Elastic modulus, major (Wel,y) | 1,320 cm³ | 1,312 cm³ | +0.6% |
| Second moment, minor (Iz) | 23,100 cm⁴ | 22,970 cm⁴ | +0.6% |
| Plastic modulus, minor (Wpl,z) | 1,520 cm³ | 1,522 cm³ | -0.1% |
| Radius of gyration, minor (iz) | 13.9 cm | 13.8 cm | +0.7% |
| Torsion constant (It) | 35,400 cm⁴ | 36,370 cm⁴ | -2.7% |
SHS 350×350×8.8 to BS EN 10210-2:2006; SHS 350×350×9 to EN 10219 · Continental Steel. Difference is expressed relative to SHS 350×350×9.
How to choose between them
On stiffness, SHS 350×350×8.8 has the larger second moment of area: 23,100 cm⁴ against 22,970 cm⁴, a difference of 0.6%. 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 (16.2 against 16.1 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.
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.