RHS 300×250×7.1 vs CF RHS 300×200×8
RHS 300×250×7.1 is an RHS Hot-finished section from the UK Blue Book (SCI P363); CF RHS 300×200×8 is the closest equivalent in the UK Blue Book (SCI P363). RHS 300×250×7.1 carries more bending: 802 cm³ of plastic modulus against 757 cm³, 5.9% more. They weigh essentially the same, 59.3 kg/m and 59.1 kg/m. Both are about 300 mm deep, so either fits the same construction zone.
| Property | RHS 300×250×7.1 RHS Hot-finished | CF RHS 300×200×8 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 59.3 kg/m | 59.1 kg/m | +0.3% |
| Depth | 300 mm | 300 mm | same |
| Width | 250 mm | 200 mm | +25.0% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 75.5 cm² | 75.2 cm² | +0.4% |
| Second moment, major (Iy) | 10,300 cm⁴ | 9,390 cm⁴ | +9.7% |
| Plastic modulus, major (Wpl,y) | 802 cm³ | 757 cm³ | +5.9% |
| Elastic modulus, major (Wel,y) | 683 cm³ | 626 cm³ | +9.1% |
| Second moment, minor (Iz) | 7,750 cm⁴ | 5,040 cm⁴ | +53.8% |
| Plastic modulus, minor (Wpl,z) | 708 cm³ | 574 cm³ | +23.3% |
| Radius of gyration, minor (iz) | 10.1 cm | 8.19 cm | +23.3% |
| Torsion constant (It) | 13,600 cm⁴ | 10,600 cm⁴ | +28.3% |
RHS 300×250×7.1 to BS EN 10210-2:2006; CF RHS 300×200×8 to BS EN 10219-2:2006. Difference is expressed relative to CF RHS 300×200×8.
How to choose between them
On stiffness, RHS 300×250×7.1 has the larger second moment of area: 10,300 cm⁴ against 9,390 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, RHS 300×250×7.1 is the more efficient section: 13.5 against 12.8 cm³ per kg/m, 5.6% 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: RHS 300×250×7.1 has 708 cm³ of minor-axis plastic modulus against 574 cm³, 23.3% 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, RHS 300×250×7.1 is the stronger section and CF RHS 300×200×8 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.