W 12X30 vs S 12X31.8
W 12X30 is an W section from the American AISC Shapes Database; S 12X31.8 is the closest equivalent in the American AISC Shapes Database. W 12X30 carries more bending: 706.28 cm³ of plastic modulus against 684.98 cm³, 3.1% more. W 12X30 is the lighter of the two at 44.6 kg/m against 47.3 kg/m, a saving of 5.7% on steel weight. W 12X30 is deeper, 312.4 mm against 304.8 mm, which matters where the floor zone is tight.
| Property | W 12X30 W | S 12X31.8 S | Difference |
|---|---|---|---|
| Mass | 44.6 kg/m | 47.3 kg/m | -5.7% |
| Depth | 312.4 mm | 304.8 mm | +2.5% |
| Width | 165.6 mm | 127 mm | +30.4% |
| Web thickness | 6.6 mm | 8.9 mm | -25.8% |
| Flange thickness | 11.2 mm | 13.8 mm | -18.8% |
| Area | 56.7 cm² | 60.1 cm² | -5.7% |
| Second moment, major (Iy) | 9,906 cm⁴ | 9,032 cm⁴ | +9.7% |
| Plastic modulus, major (Wpl,y) | 706.28 cm³ | 684.98 cm³ | +3.1% |
| Elastic modulus, major (Wel,y) | 632.54 cm³ | 593.21 cm³ | +6.6% |
| Second moment, minor (Iz) | 844.9 cm⁴ | 388.3 cm⁴ | +117.6% |
| Plastic modulus, minor (Wpl,z) | 156.66 cm³ | 105.53 cm³ | +48.5% |
| Radius of gyration, minor (iz) | 3.86 cm | 2.54 cm | +52.0% |
| Torsion constant (It) | 19.02 cm⁴ | 36.55 cm⁴ | -48.0% |
W 12X30 to AISC Shapes Database v16.0; S 12X31.8 to AISC Shapes Database v16.0. Difference is expressed relative to S 12X31.8.
How to choose between them
On stiffness, W 12X30 has the larger second moment of area: 9,906 cm⁴ against 9,032 cm⁴, a difference of 9.7%. 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, W 12X30 is the more efficient section: 15.8 against 14.5 cm³ per kg/m, 9.4% 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: W 12X30 has 156.66 cm³ of minor-axis plastic modulus against 105.53 cm³, 48.5% 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 12X30 is the stronger section and W 12X30 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.