UB 457×191×89 vs S 18X70
UB 457×191×89 is an UB section from the UK Blue Book (SCI P363); S 18X70 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 2010 cm³ against 2032 cm³ of plastic modulus, a difference of 1.1%. UB 457×191×89 is the lighter of the two at 89.3 kg/m against 104.2 kg/m, a saving of 14.3% on steel weight. Both are about 460 mm deep, so either fits the same construction zone.
| Property | UB 457×191×89 UB | S 18X70 S | Difference |
|---|---|---|---|
| Mass | 89.3 kg/m | 104.2 kg/m | -14.3% |
| Depth | 463.4 mm | 457.2 mm | +1.4% |
| Width | 191.9 mm | 158.8 mm | +20.8% |
| Web thickness | 10.5 mm | 18.1 mm | -42.0% |
| Flange thickness | 17.7 mm | 17.6 mm | +0.6% |
| Area | 114 cm² | 132.3 cm² | -13.8% |
| Second moment, major (Iy) | 41,000 cm⁴ | 38,418 cm⁴ | +6.7% |
| Plastic modulus, major (Wpl,y) | 2,010 cm³ | 2,032 cm³ | -1.1% |
| Elastic modulus, major (Wel,y) | 1,770 cm³ | 1,687.9 cm³ | +4.9% |
| Second moment, minor (Iz) | 2,090 cm⁴ | 999 cm⁴ | +109.2% |
| Plastic modulus, minor (Wpl,z) | 338 cm³ | 234.34 cm³ | +44.2% |
| Radius of gyration, minor (iz) | 4.29 cm | 2.74 cm | +56.6% |
| Torsion constant (It) | 90.7 cm⁴ | 170.7 cm⁴ | -46.9% |
UB 457×191×89 to BS EN 10365:2017; S 18X70 to AISC Shapes Database v16.0. Difference is expressed relative to S 18X70.
How to choose between them
On stiffness, UB 457×191×89 has the larger second moment of area: 41,000 cm⁴ against 38,418 cm⁴, a difference of 6.7%. 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, UB 457×191×89 is the more efficient section: 22.5 against 19.5 cm³ per kg/m, 15.4% 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: UB 457×191×89 has 338 cm³ of minor-axis plastic modulus against 234.34 cm³, 44.2% 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 S 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 14.3% weight.
In short, S 18X70 is the stronger section and UB 457×191×89 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.