UB 533×312×219 vs W 21X147
UB 533×312×219 is an UB section from the UK Blue Book (SCI P363); W 21X147 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 6120 cm³ against 6112.4 cm³ of plastic modulus, a difference of 0.1%. They weigh essentially the same, 218.8 kg/m and 218.8 kg/m. Both are about 561 mm deep, so either fits the same construction zone.
| Property | UB 533×312×219 UB | W 21X147 W | Difference |
|---|---|---|---|
| Mass | 218.8 kg/m | 218.8 kg/m | same |
| Depth | 560.3 mm | 561.3 mm | -0.2% |
| Width | 317.4 mm | 317.5 mm | same |
| Web thickness | 18.3 mm | 18.3 mm | same |
| Flange thickness | 29.2 mm | 29.2 mm | same |
| Area | 279 cm² | 278.7 cm² | +0.1% |
| Second moment, major (Iy) | 151,000 cm⁴ | 151,092 cm⁴ | -0.1% |
| Plastic modulus, major (Wpl,y) | 6,120 cm³ | 6,112.4 cm³ | +0.1% |
| Elastic modulus, major (Wel,y) | 5,400 cm³ | 5,391.3 cm³ | +0.2% |
| Second moment, minor (Iz) | 15,600 cm⁴ | 15,650 cm⁴ | -0.3% |
| Plastic modulus, minor (Wpl,z) | 1,510 cm³ | 1,517.4 cm³ | -0.5% |
| Radius of gyration, minor (iz) | 7.48 cm | 7.49 cm | -0.1% |
| Torsion constant (It) | 642 cm⁴ | 641 cm⁴ | +0.2% |
UB 533×312×219 to BS EN 10365:2017; W 21X147 to AISC Shapes Database v16.0. Difference is expressed relative to W 21X147.
How to choose between them
On stiffness, W 21X147 has the larger second moment of area: 151,092 cm⁴ against 151,000 cm⁴, a difference of 0.1%. 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 (28.0 against 27.9 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. UB sections are specified to BS EN 10365:2017 and W to AISC Shapes Database v16.0; 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.