W 18X50 vs S 18X54.7
W 18X50 is an W section from the American AISC Shapes Database; S 18X54.7 is the closest equivalent in the American AISC Shapes Database. S 18X54.7 carries more bending: 1704.3 cm³ of plastic modulus against 1655.1 cm³, 2.9% more. W 18X50 is the lighter of the two at 74.4 kg/m against 81.4 kg/m, a saving of 8.6% on steel weight. Both are about 457 mm deep, so either fits the same construction zone.
| Property | W 18X50 W | S 18X54.7 S | Difference |
|---|---|---|---|
| Mass | 74.4 kg/m | 81.4 kg/m | -8.6% |
| Depth | 457.2 mm | 457.2 mm | same |
| Width | 190.5 mm | 152.4 mm | +25.0% |
| Web thickness | 9 mm | 11.7 mm | -23.1% |
| Flange thickness | 14.5 mm | 17.6 mm | -17.6% |
| Area | 94.8 cm² | 103.2 cm² | -8.1% |
| Second moment, major (Iy) | 33,299 cm⁴ | 33,340 cm⁴ | -0.1% |
| Plastic modulus, major (Wpl,y) | 1,655.1 cm³ | 1,704.3 cm³ | -2.9% |
| Elastic modulus, major (Wel,y) | 1,456.8 cm³ | 1,458.4 cm³ | -0.1% |
| Second moment, minor (Iz) | 1,669 cm⁴ | 861.6 cm⁴ | +93.7% |
| Plastic modulus, minor (Wpl,z) | 272.03 cm³ | 198.28 cm³ | +37.2% |
| Radius of gyration, minor (iz) | 4.19 cm | 2.9 cm | +44.5% |
| Torsion constant (It) | 51.61 cm⁴ | 96.98 cm⁴ | -46.8% |
W 18X50 to AISC Shapes Database v16.0; S 18X54.7 to AISC Shapes Database v16.0. Difference is expressed relative to S 18X54.7.
How to choose between them
On stiffness, S 18X54.7 has the larger second moment of area: 33,340 cm⁴ against 33,299 cm⁴, a difference of 0.1%. 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 18X50 is the more efficient section: 22.2 against 20.9 cm³ per kg/m, 6.3% 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 18X50 has 272.03 cm³ of minor-axis plastic modulus against 198.28 cm³, 37.2% 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.
In short, S 18X54.7 is the stronger section and W 18X50 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.