UC 356×368×202 vs H-400x400x21x21
UC 356×368×202 is an UC section from the UK Blue Book (SCI P363); H-400x400x21x21 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 3970 cm³ against 3992 cm³ of plastic modulus, a difference of 0.6%. H-400x400x21x21 is the lighter of the two at 197 kg/m against 201.9 kg/m, a saving of 2.5% on steel weight. H-400x400x21x21 is deeper, 400 mm against 374.6 mm, which matters where the floor zone is tight.
| Property | UC 356×368×202 UC | H-400x400x21x21 MB & MC | Difference |
|---|---|---|---|
| Mass | 201.9 kg/m | 197 kg/m | +2.5% |
| Depth | 374.6 mm | 400 mm | -6.3% |
| Width | 374.7 mm | 408 mm | -8.2% |
| Web thickness | 16.5 mm | 21 mm | -21.4% |
| Flange thickness | 27 mm | 21 mm | +28.6% |
| Area | 257 cm² | 251 cm² | +2.4% |
| Second moment, major (Iy) | 66,300 cm⁴ | 70,890 cm⁴ | -6.5% |
| Plastic modulus, major (Wpl,y) | 3,970 cm³ | 3,992 cm³ | -0.6% |
| Elastic modulus, major (Wel,y) | 3,540 cm³ | 3,545 cm³ | -0.1% |
| Second moment, minor (Iz) | 23,700 cm⁴ | 23,810 cm⁴ | -0.5% |
| Plastic modulus, minor (Wpl,z) | 1,920 cm³ | 1,794 cm³ | +7.0% |
| Radius of gyration, minor (iz) | 9.6 cm | 9.75 cm | -1.5% |
| Torsion constant (It) | 558 cm⁴ | 415 cm⁴ | +34.5% |
UC 356×368×202 to BS EN 10365:2017; H-400x400x21x21 to Continental Steel. Difference is expressed relative to H-400x400x21x21.
How to choose between them
On stiffness, H-400x400x21x21 has the larger second moment of area: 70,890 cm⁴ against 66,300 cm⁴, a difference of 6.5%. 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-400x400x21x21 is the more efficient section: 20.3 against 19.7 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.
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 2.5% weight.
In short, H-400x400x21x21 is the stronger section and H-400x400x21x21 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.