W 21X48 vs UB 533×165×75
W 21X48 is an W section from the American AISC Shapes Database; UB 533×165×75 is the closest equivalent in the UK Blue Book (SCI P363). UB 533×165×75 carries more bending: 1810 cm³ of plastic modulus against 1753.4 cm³, 3.1% more. W 21X48 is the lighter of the two at 71.4 kg/m against 74.7 kg/m, a saving of 4.4% on steel weight. Both are about 526 mm deep, so either fits the same construction zone.
| Property | W 21X48 W | UB 533×165×75 UB | Difference |
|---|---|---|---|
| Mass | 71.4 kg/m | 74.7 kg/m | -4.4% |
| Depth | 523.2 mm | 529.1 mm | -1.1% |
| Width | 206.8 mm | 165.9 mm | +24.7% |
| Web thickness | 8.9 mm | 9.7 mm | -8.2% |
| Flange thickness | 10.9 mm | 13.6 mm | -19.9% |
| Area | 91 cm² | 95.2 cm² | -4.4% |
| Second moment, major (Iy) | 39,917 cm⁴ | 41,100 cm⁴ | -2.9% |
| Plastic modulus, major (Wpl,y) | 1,753.4 cm³ | 1,810 cm³ | -3.1% |
| Elastic modulus, major (Wel,y) | 1,524 cm³ | 1,550 cm³ | -1.7% |
| Second moment, minor (Iz) | 1,611 cm⁴ | 1,040 cm⁴ | +54.9% |
| Plastic modulus, minor (Wpl,z) | 244.17 cm³ | 200 cm³ | +22.1% |
| Radius of gyration, minor (iz) | 4.22 cm | 3.3 cm | +27.9% |
| Torsion constant (It) | 33.42 cm⁴ | 47.9 cm⁴ | -30.2% |
W 21X48 to AISC Shapes Database v16.0; UB 533×165×75 to BS EN 10365:2017. Difference is expressed relative to UB 533×165×75.
How to choose between them
On stiffness, UB 533×165×75 has the larger second moment of area: 41,100 cm⁴ against 39,917 cm⁴, a difference of 2.9%. 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 (24.6 against 24.2 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.
The two diverge much more about the minor axis: W 21X48 has 244.17 cm³ of minor-axis plastic modulus against 200 cm³, 22.1% 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 UB 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 4.4% weight.
In short, UB 533×165×75 is the stronger section and W 21X48 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.