RHS 300×100×8.8 vs CF RHS 300×200×6
RHS 300×100×8.8 is an RHS Hot-finished section from the UK Blue Book (SCI P363); CF RHS 300×200×6 is the closest equivalent in the UK Blue Book (SCI P363). They are close to interchangeable in bending: 594 cm³ against 588 cm³ of plastic modulus, a difference of 1.0%. CF RHS 300×200×6 is the lighter of the two at 45.2 kg/m against 52.2 kg/m, a saving of 15.5% on steel weight. Both are about 300 mm deep, so either fits the same construction zone.
| Property | RHS 300×100×8.8 RHS Hot-finished | CF RHS 300×200×6 RHS Cold-formed | Difference |
|---|---|---|---|
| Mass | 52.2 kg/m | 45.2 kg/m | +15.5% |
| Depth | 300 mm | 300 mm | same |
| Width | 100 mm | 200 mm | -50.0% |
| Web thickness | 0 mm | 0 mm | same |
| Flange thickness | 0 mm | 0 mm | same |
| Area | 66.5 cm² | 57.6 cm² | +15.5% |
| Second moment, major (Iy) | 6,840 cm⁴ | 7,370 cm⁴ | -7.2% |
| Plastic modulus, major (Wpl,y) | 594 cm³ | 588 cm³ | +1.0% |
| Elastic modulus, major (Wel,y) | 456 cm³ | 491 cm³ | -7.1% |
| Second moment, minor (Iz) | 1,160 cm⁴ | 3,960 cm⁴ | -70.7% |
| Plastic modulus, minor (Wpl,z) | 266 cm³ | 446 cm³ | -40.4% |
| Radius of gyration, minor (iz) | 4.18 cm | 8.29 cm | -49.6% |
| Torsion constant (It) | 3,320 cm⁴ | 8,120 cm⁴ | -59.1% |
RHS 300×100×8.8 to BS EN 10210-2:2006; CF RHS 300×200×6 to BS EN 10219-2:2006. Difference is expressed relative to CF RHS 300×200×6.
How to choose between them
On stiffness, CF RHS 300×200×6 has the larger second moment of area: 7,370 cm⁴ against 6,840 cm⁴, a difference of 7.2%. 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 of steel, CF RHS 300×200×6 is the more efficient section: 13.0 against 11.4 cm³ per kg/m, 12.5% 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: CF RHS 300×200×6 has 446 cm³ of minor-axis plastic modulus against 266 cm³, 40.4% 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 300×100×8.8 is the stronger section and CF RHS 300×200×6 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.