WT 12X42 vs ST 12X40
WT 12X42 is an WT section from the American AISC Shapes Database; ST 12X40 is the closest equivalent in the American AISC Shapes Database. ST 12X40 carries more bending: 550.61 cm³ of plastic modulus against 532.58 cm³, 3.3% more. ST 12X40 is the lighter of the two at 59.5 kg/m against 62.5 kg/m, a saving of 5.0% on steel weight. Both are about 306 mm deep, so either fits the same construction zone.
| Property | WT 12X42 WT | ST 12X40 ST | Difference |
|---|---|---|---|
| Mass | 62.5 kg/m | 59.5 kg/m | +5.0% |
| Depth | 307.3 mm | 304.8 mm | +0.8% |
| Width | 229.1 mm | 177.8 mm | +28.9% |
| Web thickness | 11.9 mm | 12.7 mm | -6.3% |
| Flange thickness | 19.6 mm | 22.1 mm | -11.3% |
| Area | 80 cm² | 75.5 cm² | +6.0% |
| Second moment, major (Iy) | 6,909 cm⁴ | 6,743 cm⁴ | +2.5% |
| Plastic modulus, major (Wpl,y) | 532.58 cm³ | 550.61 cm³ | -3.3% |
| Elastic modulus, major (Wel,y) | 299.88 cm³ | 304.8 cm³ | -1.6% |
| Second moment, minor (Iz) | 1,965 cm⁴ | 874.1 cm⁴ | +124.8% |
| Plastic modulus, minor (Wpl,z) | 267.11 cm³ | 170.43 cm³ | +56.7% |
| Radius of gyration, minor (iz) | 4.95 cm | 3.4 cm | +45.6% |
| Torsion constant (It) | 76.59 cm⁴ | 101.6 cm⁴ | -24.6% |
WT 12X42 to AISC Shapes Database v16.0; ST 12X40 to AISC Shapes Database v16.0. Difference is expressed relative to ST 12X40.
How to choose between them
On stiffness, WT 12X42 has the larger second moment of area: 6,909 cm⁴ against 6,743 cm⁴, a difference of 2.5%. 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, ST 12X40 is the more efficient section: 9.3 against 8.5 cm³ per kg/m, 7.9% 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: WT 12X42 has 267.11 cm³ of minor-axis plastic modulus against 170.43 cm³, 56.7% 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, ST 12X40 is the stronger section and ST 12X40 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.