UC 356×406×634 vs H-400x400x45x70
UC 356×406×634 is an UC section from the UK Blue Book (SCI P363); H-400x400x45x70 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 14200 cm³ against 14460 cm³ of plastic modulus, a difference of 1.8%. H-400x400x45x70 is the lighter of the two at 605 kg/m against 633.9 kg/m, a saving of 4.8% on steel weight. H-400x400x45x70 is deeper, 498 mm against 474.6 mm, which matters where the floor zone is tight.
| Property | UC 356×406×634 UC | H-400x400x45x70 MB & MC | Difference |
|---|---|---|---|
| Mass | 633.9 kg/m | 605 kg/m | +4.8% |
| Depth | 474.6 mm | 498 mm | -4.7% |
| Width | 424 mm | 432 mm | -1.9% |
| Web thickness | 47.6 mm | 45 mm | +5.8% |
| Flange thickness | 77 mm | 70 mm | +10.0% |
| Area | 808 cm² | 770 cm² | +4.9% |
| Second moment, major (Iy) | 275,000 cm⁴ | 297,900 cm⁴ | -7.7% |
| Plastic modulus, major (Wpl,y) | 14,200 cm³ | 14,460 cm³ | -1.8% |
| Elastic modulus, major (Wel,y) | 11,600 cm³ | 11,960 cm³ | -3.0% |
| Second moment, minor (Iz) | 98,100 cm⁴ | 94,360 cm⁴ | +4.0% |
| Plastic modulus, minor (Wpl,z) | 7,110 cm³ | 6,724 cm³ | +5.7% |
| Radius of gyration, minor (iz) | 11 cm | 11.1 cm | -0.9% |
| Torsion constant (It) | 13,700 cm⁴ | 11,060 cm⁴ | +23.9% |
UC 356×406×634 to BS EN 10365:2017; H-400x400x45x70 to Continental Steel. Difference is expressed relative to H-400x400x45x70.
How to choose between them
On stiffness, H-400x400x45x70 has the larger second moment of area: 297,900 cm⁴ against 275,000 cm⁴, a difference of 7.7%. 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, H-400x400x45x70 is the more efficient section: 23.9 against 22.4 cm³ per kg/m, 6.3% 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 4.8% weight.
In short, H-400x400x45x70 is the stronger section and H-400x400x45x70 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.