UB 533×210×101 vs W 21X68
UB 533×210×101 is an UB section from the UK Blue Book (SCI P363); W 21X68 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 2610 cm³ against 2621.9 cm³ of plastic modulus, a difference of 0.5%. They weigh essentially the same, 101 kg/m and 101.2 kg/m. Both are about 536 mm deep, so either fits the same construction zone.
| Property | UB 533×210×101 UB | W 21X68 W | Difference |
|---|---|---|---|
| Mass | 101 kg/m | 101.2 kg/m | -0.2% |
| Depth | 536.7 mm | 535.9 mm | +0.1% |
| Width | 210 mm | 210.1 mm | same |
| Web thickness | 10.8 mm | 10.9 mm | -0.9% |
| Flange thickness | 17.4 mm | 17.4 mm | same |
| Area | 129 cm² | 129 cm² | same |
| Second moment, major (Iy) | 61,500 cm⁴ | 61,602 cm⁴ | -0.2% |
| Plastic modulus, major (Wpl,y) | 2,610 cm³ | 2,621.9 cm³ | -0.5% |
| Elastic modulus, major (Wel,y) | 2,290 cm³ | 2,294.2 cm³ | -0.2% |
| Second moment, minor (Iz) | 2,690 cm⁴ | 2,693 cm⁴ | -0.1% |
| Plastic modulus, minor (Wpl,z) | 399 cm³ | 399.84 cm³ | -0.2% |
| Radius of gyration, minor (iz) | 4.57 cm | 4.57 cm | same |
| Torsion constant (It) | 101 cm⁴ | 102 cm⁴ | -1.0% |
UB 533×210×101 to BS EN 10365:2017; W 21X68 to AISC Shapes Database v16.0. Difference is expressed relative to W 21X68.
How to choose between them
On stiffness, W 21X68 has the larger second moment of area: 61,602 cm⁴ against 61,500 cm⁴, a difference of 0.2%. 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 (25.8 against 25.9 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.