W 8X18 vs S 8X18.4
W 8X18 is an W section from the American AISC Shapes Database; S 8X18.4 is the closest equivalent in the American AISC Shapes Database. W 8X18 carries more bending: 278.58 cm³ of plastic modulus against 270.39 cm³, 3.0% more. W 8X18 is the lighter of the two at 26.8 kg/m against 27.4 kg/m, a saving of 2.2% on steel weight. Both are about 205 mm deep, so either fits the same construction zone.
| Property | W 8X18 W | S 8X18.4 S | Difference |
|---|---|---|---|
| Mass | 26.8 kg/m | 27.4 kg/m | -2.2% |
| Depth | 206.8 mm | 203.2 mm | +1.8% |
| Width | 133.4 mm | 101.6 mm | +31.3% |
| Web thickness | 5.8 mm | 6.9 mm | -15.9% |
| Flange thickness | 8.4 mm | 10.8 mm | -22.2% |
| Area | 33.9 cm² | 34.8 cm² | -2.6% |
| Second moment, major (Iy) | 2,576 cm⁴ | 2,393 cm⁴ | +7.6% |
| Plastic modulus, major (Wpl,y) | 278.58 cm³ | 270.39 cm³ | +3.0% |
| Elastic modulus, major (Wel,y) | 249.08 cm³ | 235.97 cm³ | +5.6% |
| Second moment, minor (Iz) | 331.7 cm⁴ | 153.6 cm⁴ | +116.0% |
| Plastic modulus, minor (Wpl,z) | 76.364 cm³ | 52.111 cm³ | +46.5% |
| Radius of gyration, minor (iz) | 3.12 cm | 2.1 cm | +48.6% |
| Torsion constant (It) | 7.16 cm⁴ | 13.94 cm⁴ | -48.6% |
W 8X18 to AISC Shapes Database v16.0; S 8X18.4 to AISC Shapes Database v16.0. Difference is expressed relative to S 8X18.4.
How to choose between them
On stiffness, W 8X18 has the larger second moment of area: 2,576 cm⁴ against 2,393 cm⁴, a difference of 7.6%. 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 8X18 is the more efficient section: 10.4 against 9.9 cm³ per kg/m, 5.3% 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 8X18 has 76.364 cm³ of minor-axis plastic modulus against 52.111 cm³, 46.5% 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, W 8X18 is the stronger section and W 8X18 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.