UB 457×191×161 vs W 18X106
UB 457×191×161 is an UB section from the UK Blue Book (SCI P363); W 18X106 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 3780 cm³ against 3769 cm³ of plastic modulus, a difference of 0.3%. W 18X106 is the lighter of the two at 157.7 kg/m against 161.4 kg/m, a saving of 2.3% on steel weight. UB 457×191×161 is deeper, 492 mm against 475 mm, which matters where the floor zone is tight.
| Property | UB 457×191×161 UB | W 18X106 W | Difference |
|---|---|---|---|
| Mass | 161.4 kg/m | 157.7 kg/m | +2.3% |
| Depth | 492 mm | 475 mm | +3.6% |
| Width | 199.4 mm | 284.5 mm | -29.9% |
| Web thickness | 18 mm | 15 mm | +20.0% |
| Flange thickness | 32 mm | 23.9 mm | +33.9% |
| Area | 206 cm² | 200.6 cm² | +2.7% |
| Second moment, major (Iy) | 79,800 cm⁴ | 79,500 cm⁴ | +0.4% |
| Plastic modulus, major (Wpl,y) | 3,780 cm³ | 3,769 cm³ | +0.3% |
| Elastic modulus, major (Wel,y) | 3,240 cm³ | 3,343 cm³ | -3.1% |
| Second moment, minor (Iz) | 4,250 cm⁴ | 9,157 cm⁴ | -53.6% |
| Plastic modulus, minor (Wpl,z) | 672 cm³ | 991.42 cm³ | -32.2% |
| Radius of gyration, minor (iz) | 4.55 cm | 6.76 cm | -32.7% |
| Torsion constant (It) | 515 cm⁴ | 311.3 cm⁴ | +65.4% |
UB 457×191×161 to BS EN 10365:2017; W 18X106 to AISC Shapes Database v16.0. Difference is expressed relative to W 18X106.
How to choose between them
On stiffness, UB 457×191×161 has the larger second moment of area: 79,800 cm⁴ against 79,500 cm⁴, a difference of 0.4%. 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 of steel, W 18X106 is the more efficient section: 23.9 against 23.4 cm³ per kg/m, 2.0% better. Over a floor of repeated beams that difference is real tonnage, though it is often outweighed by whichever section the fabricator can actually source.
The two diverge much more about the minor axis: W 18X106 has 991.42 cm³ of minor-axis plastic modulus against 672 cm³, 32.2% 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. 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 2.3% weight.
In short, UB 457×191×161 is the stronger section and W 18X106 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.