UC 356×368×129 vs H-350x350x12x19-135
UC 356×368×129 is an UC section from the UK Blue Book (SCI P363); H-350x350x12x19-135 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 2480 cm³ against 2515 cm³ of plastic modulus, a difference of 1.4%. UC 356×368×129 is the lighter of the two at 129 kg/m against 135 kg/m, a saving of 4.4% on steel weight. Both are about 353 mm deep, so either fits the same construction zone.
| Property | UC 356×368×129 UC | H-350x350x12x19-135 MB & MC | Difference |
|---|---|---|---|
| Mass | 129 kg/m | 135 kg/m | -4.4% |
| Depth | 355.6 mm | 350 mm | +1.6% |
| Width | 368.6 mm | 350 mm | +5.3% |
| Web thickness | 10.4 mm | 12 mm | -13.3% |
| Flange thickness | 17.5 mm | 19 mm | -7.9% |
| Area | 164 cm² | 172 cm² | -4.7% |
| Second moment, major (Iy) | 40,200 cm⁴ | 39,847 cm⁴ | +0.9% |
| Plastic modulus, major (Wpl,y) | 2,480 cm³ | 2,515 cm³ | -1.4% |
| Elastic modulus, major (Wel,y) | 2,260 cm³ | 2,277 cm³ | -0.7% |
| Second moment, minor (Iz) | 14,600 cm⁴ | 13,583 cm⁴ | +7.5% |
| Plastic modulus, minor (Wpl,z) | 1,200 cm³ | 1,176 cm³ | +2.0% |
| Radius of gyration, minor (iz) | 9.43 cm | 8.89 cm | +6.1% |
| Torsion constant (It) | 153 cm⁴ | 186 cm⁴ | -17.7% |
UC 356×368×129 to BS EN 10365:2017; H-350x350x12x19-135 to Continental Steel. Difference is expressed relative to H-350x350x12x19-135.
How to choose between them
On stiffness, UC 356×368×129 has the larger second moment of area: 40,200 cm⁴ against 39,847 cm⁴, a difference of 0.9%. 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, UC 356×368×129 is the more efficient section: 19.2 against 18.6 cm³ per kg/m, 3.2% 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. UC sections are specified to BS EN 10365:2017 and MB & MC to 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 4.4% weight.
In short, H-350x350x12x19-135 is the stronger section and UC 356×368×129 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.