W 12X35 vs S 12X40.8
W 12X35 is an W section from the American AISC Shapes Database; S 12X40.8 is the closest equivalent in the American AISC Shapes Database. S 12X40.8 carries more bending: 863.6 cm³ of plastic modulus against 839.02 cm³, 2.8% more. W 12X35 is the lighter of the two at 52.1 kg/m against 60.7 kg/m, a saving of 14.2% on steel weight. W 12X35 is deeper, 317.5 mm against 304.8 mm, which matters where the floor zone is tight.
| Property | W 12X35 W | S 12X40.8 S | Difference |
|---|---|---|---|
| Mass | 52.1 kg/m | 60.7 kg/m | -14.2% |
| Depth | 317.5 mm | 304.8 mm | +4.2% |
| Width | 166.6 mm | 133.4 mm | +24.9% |
| Web thickness | 7.6 mm | 11.7 mm | -35.0% |
| Flange thickness | 13.2 mm | 16.7 mm | -21.0% |
| Area | 66.5 cm² | 76.8 cm² | -13.4% |
| Second moment, major (Iy) | 11,863 cm⁴ | 11,238 cm⁴ | +5.6% |
| Plastic modulus, major (Wpl,y) | 839.02 cm³ | 863.6 cm³ | -2.8% |
| Elastic modulus, major (Wel,y) | 747.25 cm³ | 739.06 cm³ | +1.1% |
| Second moment, minor (Iz) | 1,020 cm⁴ | 561.9 cm⁴ | +81.5% |
| Plastic modulus, minor (Wpl,z) | 188.45 cm³ | 145.19 cm³ | +29.8% |
| Radius of gyration, minor (iz) | 3.91 cm | 2.69 cm | +45.4% |
| Torsion constant (It) | 30.84 cm⁴ | 70.34 cm⁴ | -56.2% |
W 12X35 to AISC Shapes Database v16.0; S 12X40.8 to AISC Shapes Database v16.0. Difference is expressed relative to S 12X40.8.
How to choose between them
On stiffness, W 12X35 has the larger second moment of area: 11,863 cm⁴ against 11,238 cm⁴, a difference of 5.6%. 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 12X35 is the more efficient section: 16.1 against 14.2 cm³ per kg/m, 13.2% 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 12X35 has 188.45 cm³ of minor-axis plastic modulus against 145.19 cm³, 29.8% 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 12X40.8 is the stronger section and W 12X35 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.