UB 533×210×109 vs W 21X73
UB 533×210×109 is an UB section from the UK Blue Book (SCI P363); W 21X73 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 2830 cm³ against 2818.6 cm³ of plastic modulus, a difference of 0.4%. They weigh essentially the same, 109 kg/m and 108.6 kg/m. Both are about 539 mm deep, so either fits the same construction zone.
| Property | UB 533×210×109 UB | W 21X73 W | Difference |
|---|---|---|---|
| Mass | 109 kg/m | 108.6 kg/m | +0.4% |
| Depth | 539.5 mm | 538.5 mm | +0.2% |
| Width | 210.8 mm | 210.8 mm | same |
| Web thickness | 11.6 mm | 11.6 mm | same |
| Flange thickness | 18.8 mm | 18.8 mm | same |
| Area | 139 cm² | 138.7 cm² | +0.2% |
| Second moment, major (Iy) | 66,800 cm⁴ | 66,597 cm⁴ | +0.3% |
| Plastic modulus, major (Wpl,y) | 2,830 cm³ | 2,818.6 cm³ | +0.4% |
| Elastic modulus, major (Wel,y) | 2,480 cm³ | 2,474.4 cm³ | +0.2% |
| Second moment, minor (Iz) | 2,940 cm⁴ | 2,939 cm⁴ | same |
| Plastic modulus, minor (Wpl,z) | 436 cm³ | 435.9 cm³ | same |
| Radius of gyration, minor (iz) | 4.6 cm | 4.6 cm | same |
| Torsion constant (It) | 126 cm⁴ | 125.7 cm⁴ | +0.2% |
UB 533×210×109 to BS EN 10365:2017; W 21X73 to AISC Shapes Database v16.0. Difference is expressed relative to W 21X73.
How to choose between them
On stiffness, UB 533×210×109 has the larger second moment of area: 66,800 cm⁴ against 66,597 cm⁴, a difference of 0.3%. 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 (26.0 against 26.0 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.