RHS 300×100×12.5 vs RHS 300×100×12
RHS 300×100×12.5 is an RHS Hot-finished section from the UK Blue Book (SCI P363); RHS 300×100×12 is the closest equivalent in the European (Continental) range. RHS 300×100×12.5 carries more bending: 806 cm³ of plastic modulus against 779 cm³, 3.5% more. RHS 300×100×12 is the lighter of the two at 70.3 kg/m against 72.3 kg/m, a saving of 2.8% on steel weight. Both are about 300 mm deep, so either fits the same construction zone.
| Property | RHS 300×100×12.5 RHS Hot-finished | RHS 300×100×12 RHS | Difference |
|---|---|---|---|
| Mass | 72.3 kg/m | 70.3 kg/m | +2.8% |
| Depth | 300 mm | 300 mm | same |
| Width | 100 mm | 100 mm | same |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 92.1 cm² | 88.7 cm² | +3.8% |
| Second moment, major (Iy) | 9,100 cm⁴ | 8,818 cm⁴ | +3.2% |
| Plastic modulus, major (Wpl,y) | 806 cm³ | 779 cm³ | +3.5% |
| Elastic modulus, major (Wel,y) | 607 cm³ | 588 cm³ | +3.2% |
| Second moment, minor (Iz) | 1,490 cm⁴ | 1,447 cm⁴ | +3.0% |
| Plastic modulus, minor (Wpl,z) | 354 cm³ | 343 cm³ | +3.2% |
| Radius of gyration, minor (iz) | 4.02 cm | 4.04 cm | -0.5% |
| Torsion constant (It) | 4,350 cm⁴ | 4,223 cm⁴ | +3.0% |
RHS 300×100×12.5 to BS EN 10210-2:2006; RHS 300×100×12 to EN 10219 · Continental Steel. Difference is expressed relative to RHS 300×100×12.
How to choose between them
On stiffness, RHS 300×100×12.5 has the larger second moment of area: 9,100 cm⁴ against 8,818 cm⁴, a difference of 3.2%. 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, the two are equally efficient (11.1 against 11.1 cm³ per kg/m), so on a pure steel-tonnage basis there is nothing to choose between them. The decision comes down to availability, connection detailing and what the rest of the frame uses.
These come from different catalogues, so treat the swap as a design change rather than a like-for-like substitution. RHS Hot-finished sections are specified to BS EN 10210-2:2006 and RHS to EN 10219 · Continental Steel; the steel grades, tolerances and available lengths differ, connection details and bolt gauges are set out differently, and the two are rarely both stocked in the same market. Check availability before changing a section on a drawing to save 2.8% weight.
In short, RHS 300×100×12.5 is the stronger section and RHS 300×100×12 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.