WT 4X20 vs ST 4X11.5
WT 4X20 is an WT section from the American AISC Shapes Database; ST 4X11.5 is the closest equivalent in the American AISC Shapes Database. They are close to interchangeable in bending: 53.258 cm³ against 52.275 cm³ of plastic modulus, a difference of 1.9%. ST 4X11.5 is the lighter of the two at 17.1 kg/m against 29.8 kg/m, a saving of 74.3% on steel weight. WT 4X20 is deeper, 104.9 mm against 101.6 mm, which matters where the floor zone is tight.
| Property | WT 4X20 WT | ST 4X11.5 ST | Difference |
|---|---|---|---|
| Mass | 29.8 kg/m | 17.1 kg/m | +74.3% |
| Depth | 104.9 mm | 101.6 mm | +3.2% |
| Width | 205 mm | 105.9 mm | +93.6% |
| Web thickness | 9.1 mm | 11.2 mm | -18.7% |
| Flange thickness | 14.2 mm | 10.8 mm | +31.5% |
| Area | 37.9 cm² | 21.8 cm² | +73.9% |
| Second moment, major (Iy) | 238.5 cm⁴ | 208.1 cm⁴ | +14.6% |
| Plastic modulus, major (Wpl,y) | 53.258 cm³ | 52.275 cm³ | +1.9% |
| Elastic modulus, major (Wel,y) | 27.694 cm³ | 28.841 cm³ | -4.0% |
| Second moment, minor (Iz) | 1,020 cm⁴ | 88.66 cm⁴ | +1050.5% |
| Plastic modulus, minor (Wpl,z) | 151.42 cm³ | 30.152 cm³ | +402.2% |
| Radius of gyration, minor (iz) | 5.18 cm | 2.02 cm | +156.4% |
| Torsion constant (It) | 23.23 cm⁴ | 11.28 cm⁴ | +105.9% |
WT 4X20 to AISC Shapes Database v16.0; ST 4X11.5 to AISC Shapes Database v16.0. Difference is expressed relative to ST 4X11.5.
How to choose between them
On stiffness, WT 4X20 has the larger second moment of area: 238.5 cm⁴ against 208.1 cm⁴, a difference of 14.6%. 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, ST 4X11.5 is the more efficient section: 3.1 against 1.8 cm³ per kg/m, 41.5% 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: WT 4X20 has 151.42 cm³ of minor-axis plastic modulus against 30.152 cm³, 402.2% 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, WT 4X20 is the stronger section and ST 4X11.5 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.