W 6X15 vs S 6X17.25
W 6X15 is an W section from the American AISC Shapes Database; S 6X17.25 is the closest equivalent in the American AISC Shapes Database. W 6X15 carries more bending: 176.98 cm³ of plastic modulus against 172.06 cm³, 2.9% more. W 6X15 is the lighter of the two at 22.3 kg/m against 25.7 kg/m, a saving of 13.2% on steel weight. Both are about 152 mm deep, so either fits the same construction zone.
| Property | W 6X15 W | S 6X17.25 S | Difference |
|---|---|---|---|
| Mass | 22.3 kg/m | 25.7 kg/m | -13.2% |
| Depth | 152.1 mm | 152.4 mm | -0.2% |
| Width | 152.1 mm | 90.7 mm | +67.7% |
| Web thickness | 5.8 mm | 11.8 mm | -50.8% |
| Flange thickness | 6.6 mm | 9.1 mm | -27.5% |
| Area | 28.6 cm² | 32.6 cm² | -12.3% |
| Second moment, major (Iy) | 1,211 cm⁴ | 1,091 cm⁴ | +11.0% |
| Plastic modulus, major (Wpl,y) | 176.98 cm³ | 172.06 cm³ | +2.9% |
| Elastic modulus, major (Wel,y) | 159.28 cm³ | 143.22 cm³ | +11.2% |
| Second moment, minor (Iz) | 387.9 cm⁴ | 95.32 cm⁴ | +306.9% |
| Plastic modulus, minor (Wpl,z) | 77.839 cm³ | 38.51 cm³ | +102.1% |
| Radius of gyration, minor (iz) | 3.68 cm | 1.71 cm | +115.2% |
| Torsion constant (It) | 4.2 cm⁴ | 15.44 cm⁴ | -72.8% |
W 6X15 to AISC Shapes Database v16.0; S 6X17.25 to AISC Shapes Database v16.0. Difference is expressed relative to S 6X17.25.
How to choose between them
On stiffness, W 6X15 has the larger second moment of area: 1,211 cm⁴ against 1,091 cm⁴, a difference of 11.0%. 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 6X15 is the more efficient section: 7.9 against 6.7 cm³ per kg/m, 18.5% 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 6X15 has 77.839 cm³ of minor-axis plastic modulus against 38.51 cm³, 102.1% 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.
In short, W 6X15 is the stronger section and W 6X15 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.