UB 533×312×273 vs W 21X182
UB 533×312×273 is an UB section from the UK Blue Book (SCI P363); W 21X182 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 7870 cm³ against 7800.2 cm³ of plastic modulus, a difference of 0.9%. They weigh essentially the same, 273.3 kg/m and 270.8 kg/m. Both are about 577 mm deep, so either fits the same construction zone.
| Property | UB 533×312×273 UB | W 21X182 W | Difference |
|---|---|---|---|
| Mass | 273.3 kg/m | 270.8 kg/m | +0.9% |
| Depth | 577.1 mm | 576.6 mm | +0.1% |
| Width | 320.2 mm | 317.5 mm | +0.9% |
| Web thickness | 21.1 mm | 21.1 mm | same |
| Flange thickness | 37.6 mm | 37.6 mm | same |
| Area | 348 cm² | 345.8 cm² | +0.6% |
| Second moment, major (Iy) | 199,000 cm⁴ | 196,877 cm⁴ | +1.1% |
| Plastic modulus, major (Wpl,y) | 7,870 cm³ | 7,800.2 cm³ | +0.9% |
| Elastic modulus, major (Wel,y) | 6,890 cm³ | 6,833.4 cm³ | +0.8% |
| Second moment, minor (Iz) | 20,600 cm⁴ | 20,104 cm⁴ | +2.5% |
| Plastic modulus, minor (Wpl,z) | 1,990 cm³ | 1,950.1 cm³ | +2.0% |
| Radius of gyration, minor (iz) | 7.69 cm | 7.62 cm | +0.9% |
| Torsion constant (It) | 1,290 cm⁴ | 1,278 cm⁴ | +0.9% |
UB 533×312×273 to BS EN 10365:2017; W 21X182 to AISC Shapes Database v16.0. Difference is expressed relative to W 21X182.
How to choose between them
On stiffness, UB 533×312×273 has the larger second moment of area: 199,000 cm⁴ against 196,877 cm⁴, a difference of 1.1%. 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 (28.8 against 28.8 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.