W 12X252 vs UC 356×406×340
W 12X252 is an W section from the American AISC Shapes Database; UC 356×406×340 is the closest equivalent in the UK Blue Book (SCI P363). They are close to interchangeable in bending: 7013.7 cm³ against 7000 cm³ of plastic modulus, a difference of 0.2%. UC 356×406×340 is the lighter of the two at 339.9 kg/m against 375 kg/m, a saving of 10.3% on steel weight. UC 356×406×340 is deeper, 406.4 mm against 391.2 mm, which matters where the floor zone is tight.
| Property | W 12X252 W | UC 356×406×340 UC | Difference |
|---|---|---|---|
| Mass | 375 kg/m | 339.9 kg/m | +10.3% |
| Depth | 391.2 mm | 406.4 mm | -3.7% |
| Width | 330.2 mm | 403 mm | -18.1% |
| Web thickness | 35.6 mm | 26.6 mm | +33.8% |
| Flange thickness | 57.2 mm | 42.9 mm | +33.3% |
| Area | 478.1 cm² | 433 cm² | +10.4% |
| Second moment, major (Iy) | 113,215 cm⁴ | 123,000 cm⁴ | -8.0% |
| Plastic modulus, major (Wpl,y) | 7,013.7 cm³ | 7,000 cm³ | +0.2% |
| Elastic modulus, major (Wel,y) | 5,784.6 cm³ | 6,030 cm³ | -4.1% |
| Second moment, minor (Iz) | 34,464 cm⁴ | 46,900 cm⁴ | -26.5% |
| Plastic modulus, minor (Wpl,z) | 3,211.9 cm³ | 3,540 cm³ | -9.3% |
| Radius of gyration, minor (iz) | 8.48 cm | 10.4 cm | -18.5% |
| Torsion constant (It) | 4,495 cm⁴ | 2,340 cm⁴ | +92.1% |
W 12X252 to AISC Shapes Database v16.0; UC 356×406×340 to BS EN 10365:2017. Difference is expressed relative to UC 356×406×340.
How to choose between them
On stiffness, UC 356×406×340 has the larger second moment of area: 123,000 cm⁴ against 113,215 cm⁴, a difference of 8.0%. 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 356×406×340 is the more efficient section: 20.6 against 18.7 cm³ per kg/m, 9.2% 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.
The two diverge much more about the minor axis: UC 356×406×340 has 3540 cm³ of minor-axis plastic modulus against 3211.9 cm³, 9.3% more. If the member sees any minor-axis bending, or if it is unrestrained over a long length and lateral-torsional buckling governs, that gap decides the comparison even though the major-axis numbers looked equivalent.
These come from different catalogues, so treat the swap as a design change rather than a like-for-like substitution. W sections are specified to AISC Shapes Database v16.0 and UC to BS EN 10365:2017; 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 10.3% weight.
In short, W 12X252 is the stronger section and UC 356×406×340 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.