S355 steel is a non-alloy structural steel grade made to BS EN 10025-2, the British and European standard that sets out mechanical properties for hot-rolled structural steels. The S stands for structural and the 355 is the minimum yield strength in N/mm2 (also written as megapascals, MPa) for material up to 16 mm thick, which makes S355 one of the two grades, alongside S275, that you will see on the vast majority of UK structural drawings. Our S275 vs S355 guide covers the two side by side; this page goes deeper into what S355 specifically is, what it is good for, and where people get it wrong.
What the grade actually specifies
Yield strength is the stress at which steel stops behaving elastically and starts to deform permanently. Below the yield point, a loaded member springs back to its original shape; above it, the deformation stays. S355’s minimum yield of 355 N/mm2 sits above S275’s 275 N/mm2 and well above S235’s 235 N/mm2, so for a given cross-section, S355 can carry a higher load before it starts to yield.
That yield figure is not fixed across every section. BS EN 10025-2 reduces it as material gets thicker, because thicker rolled sections cool more slowly and end up with a coarser grain structure that yields at a slightly lower stress. The table below sets out the bands.
| Nominal thickness | Minimum yield strength | Tensile strength |
|---|---|---|
| Up to 16 mm | 355 N/mm2 | 470 to 630 N/mm2 |
| Over 16 mm up to 40 mm | 345 N/mm2 | 470 to 630 N/mm2 |
| Over 40 mm up to 63 mm | 335 N/mm2 | 470 to 630 N/mm2 |
Density is 7,850 kg/m3, the same as every other structural carbon steel grade, so a heavier section in S355 weighs the same per cubic metre as the equivalent size in S275. The full range of universal beam sizes is available in either grade; the grade and the section are two separate decisions on the drawing, not one.
Why S355 exists: strength without extra bulk
The reason a structural engineer specifies S355 rather than S275 comes down to one trade-off: for the same load, a higher yield strength lets the design use a smaller or lighter section. On a long span, a heavily loaded column, or a frame where every extra kilogram of steel means more foundation, more crane time and more handling on site, that saving compounds. It is also why S355 shows up so often in portal frames, bridges and crane beams, structures where the load path concentrates stress into a small number of members and a stronger grade genuinely reduces the tonnage.
None of this replaces the engineer’s calculation. The grade is one input into the design, alongside the section size, the span, the loads it carries and the way the connections are detailed. Specifying S355 does not automatically make a structure lighter; it gives the calculation a higher allowable stress to work with, and the engineer decides whether that headroom is worth using on a particular member.
The misunderstanding that catches people out: strength is not stiffness
The single most common mix-up with S355 is assuming a stronger grade also means a stiffer one. It does not. Stiffness, meaning how much a member deflects under load, is governed by Young’s modulus, and Young’s modulus is about 210,000 N/mm2 for every structural steel grade, S235, S275 and S355 alike. Yield strength varies between grades; stiffness does not.
That matters in practice because plenty of structural designs are governed by deflection rather than by strength, floor beams that must not bounce, or long members that must not sag visibly, being the classic examples. On a deflection-critical design, swapping S275 for S355 buys nothing: the beam will deflect by exactly the same amount because the material’s resistance to bending has not changed, only its resistance to permanent yielding has. In those cases the engineer either accepts the deflection, increases the section depth, or looks at the layout rather than reaching for a higher grade. Knowing which limit governs a given member, strength or deflection, is exactly why beam sizing needs a structural engineer’s calculation rather than a rule of thumb, and why load-bearing work needs Building Control sign-off before it proceeds.
Where S355 sits alongside the other grades
S275 remains the everyday default for structural steelwork with moderate loads: extensions, small commercial buildings, brackets and general framing, where the extra cost per tonne of S355 is not repaid by a meaningful reduction in section size. S355 earns its place on longer spans, heavier loads and weight-sensitive frames, and it is common to see both grades used on the same job, S355 where the calculation benefits from it and S275 for the rest of the frame.
Within S355 itself, the sub-grade letters, JR, J0 and J2, describe impact toughness rather than strength, and our S355JR vs S355J0 vs S355J2 guide sets out how the engineer chooses between them based on service temperature and loading. For applications where wear resistance rather than structural yield is the priority, such as chutes, buckets and wear plates, Hardox is a different family entirely from S355 and the two should not be confused.
Whatever grade the drawings call for, execution has to follow BS EN 1090 and the design has to sit within Eurocode 3 (BS EN 1993). We work to BS EN 1090 with CE approval, and can provide material traceability on request, covering everything from RSJ supply and fit through to full steel frame buildings. For the underlying property values in the table above, the Steel Construction Institute’s reference guide to steel material properties is a useful independent source.
Frequently asked questions
What is S355 steel used for?
S355 steel is used wherever a structural engineer needs a stronger, lighter section: long-span beams, heavily loaded columns, portal frames, bridges, crane beams and weight-sensitive frames. Everyday steelwork with moderate loads usually still uses S275, so S355 tends to appear where it earns its keep rather than as a blanket default.
What is the yield strength of S355 steel?
S355 has a minimum yield strength of 355 N/mm2 (megapascals) for material up to 16 mm thick, made to BS EN 10025-2. The figure drops as section thickness increases: 345 N/mm2 from 16 to 40 mm, and 335 N/mm2 from 40 to 63 mm. Tensile strength runs from 470 to 630 N/mm2 depending on thickness.
What is the difference between S355JR, S355J0 and S355J2?
The letters describe impact toughness, tested by measuring energy absorbed in a notched Charpy test at a set temperature. JR is 27 joules at plus 20 degrees C, J0 is 27 joules at 0 degrees C, and J2 is 27 joules at minus 20 degrees C, with K2 going further at 40 joules at minus 20 degrees C. A colder service environment or a more demanding design generally calls for a higher sub-grade.
Is S355 steel stronger or stiffer than S275?
S355 is stronger than S275 (355 N/mm2 yield versus 275 N/mm2) but no stiffer. Stiffness comes from Young’s modulus, which is about 210,000 N/mm2 for every structural steel grade, S275 and S355 included. Where a design is governed by deflection rather than strength, S355 gives no advantage and S275 will do the same job.
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