UB 686×254×170 vs H-700x300x13x20
UB 686×254×170 is an UB section from the UK Blue Book (SCI P363); H-700x300x13x20 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 5630 cm³ against 5629 cm³ of plastic modulus, a difference of 0.0%. H-700x300x13x20 is the lighter of the two at 166 kg/m against 170.2 kg/m, a saving of 2.5% on steel weight. Both are about 692 mm deep, so either fits the same construction zone.
| Property | UB 686×254×170 UB | H-700x300x13x20 MB & MC | Difference |
|---|---|---|---|
| Mass | 170.2 kg/m | 166 kg/m | +2.5% |
| Depth | 692.9 mm | 692 mm | +0.1% |
| Width | 255.8 mm | 300 mm | -14.7% |
| Web thickness | 14.5 mm | 13 mm | +11.5% |
| Flange thickness | 23.7 mm | 20 mm | +18.5% |
| Area | 217 cm² | 211 cm² | +2.8% |
| Second moment, major (Iy) | 170,000 cm⁴ | 172,400 cm⁴ | -1.4% |
| Plastic modulus, major (Wpl,y) | 5,630 cm³ | 5,629 cm³ | same |
| Elastic modulus, major (Wel,y) | 4,920 cm³ | 4,984 cm³ | -1.3% |
| Second moment, minor (Iz) | 6,630 cm⁴ | 9,030 cm⁴ | -26.6% |
| Plastic modulus, minor (Wpl,z) | 811 cm³ | 936 cm³ | -13.4% |
| Radius of gyration, minor (iz) | 5.53 cm | 6.53 cm | -15.3% |
| Torsion constant (It) | 308 cm⁴ | 260 cm⁴ | +18.5% |
UB 686×254×170 to BS EN 10365:2017; H-700x300x13x20 to Continental Steel. Difference is expressed relative to H-700x300x13x20.
How to choose between them
On stiffness, H-700x300x13x20 has the larger second moment of area: 172,400 cm⁴ against 170,000 cm⁴, a difference of 1.4%. 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, H-700x300x13x20 is the more efficient section: 33.9 against 33.1 cm³ per kg/m, 2.5% 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: H-700x300x13x20 has 936 cm³ of minor-axis plastic modulus against 811 cm³, 13.4% 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.
These come from different catalogues, so treat the swap as a design change rather than a like-for-like substitution. UB sections are specified to BS EN 10365:2017 and MB & MC to 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 2.5% weight.
In short, UB 686×254×170 is the stronger section and H-700x300x13x20 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.