S275 and S355 are the two most common structural steel grades on UK drawings, and both are made to the European standard BS EN 10025. The letter S stands for structural, and the number is the minimum yield strength in N/mm2 (also written as megapascals, MPa). So S275 has a minimum yield strength of 275 N/mm2 and S355 has 355 N/mm2, which makes S355 the stronger of the two. The grade for any given job is chosen by the structural engineer, not picked off a shelf, but understanding what the labels mean helps when you are reading a specification or a quote.
What the grade number actually means
Yield strength is the point at which steel stops springing back and starts to deform permanently under load. A higher yield strength means the steel can take more stress before that happens. Because S355 has a minimum yield of 355 N/mm2 against 275 N/mm2 for S275, it is roughly 30 per cent stronger in this respect.
That difference is useful in a practical way. For the same load, an engineer using S355 can often specify a smaller or lighter section than they would need in S275. A lighter frame can mean less steel to fabricate, handle and lift, and sometimes shallower beams that free up headroom. The trade-off is cost: S355 generally costs more per tonne than S275, so the saving in weight is weighed against the higher price of the material.
It is worth saying that the grade is only one variable. The section size, the span, the loads above it and the connections all come from the engineer’s calculation. The grade does not replace that calculation; it is one of its inputs.
What is S275 steel used for
S275 is the general-purpose workhorse of UK structural steelwork. It is widely stocked, economical and more than adequate for a large share of everyday jobs. Typical uses include beams and columns in house extensions and small commercial buildings, lintels over openings, brackets, supports, posts and general fabricated frames where loads are moderate and the section size is not tightly constrained.
Because it is readily available and cheaper per tonne, S275 is often the default unless the design specifically benefits from a stronger grade. For much of the structural steelwork we fabricate, S275 is a sensible and cost-effective choice.
What is S355 steel
S355 is the higher-strength grade. With its greater yield strength it is favoured where strength-to-weight matters: long spans, heavily loaded beams and columns, portal frames, bridges, and larger commercial or industrial structures where keeping sections light and slender pays off. It is also chosen where the engineer wants to limit deflection or fit the steel into a tight zone.
The benefit is that you can do more with less steel; the cost is a higher price per tonne and, sometimes, a slightly longer lead time depending on stock. On many projects an engineer will mix grades, using S355 only where it earns its keep and S275 for the rest of the frame.
JR, J0 and J2: the sub-grades
After the grade you will often see letters such as JR, J0 or J2 (for example S355J2). These describe impact toughness, which is the steel’s resistance to brittle fracture at low temperature. They are tested by measuring the energy absorbed in a notched impact test at a set temperature:
- JR is tested at room temperature (around 20 degrees C).
- J0 is tested at 0 degrees C.
- J2 is tested at minus 20 degrees C.
A higher sub-grade means proven toughness at a colder temperature. Steel that will be exposed to low service temperatures, or that carries demanding dynamic loads, generally calls for a higher sub-grade. Which one is specified, again, comes from the structural engineer and the design code, not from a rule of thumb.
Grade is not the same as section or finish
It is easy to confuse three separate things on a drawing: the grade (S275 or S355, the steel’s strength), the section (such as a 203 x 133 x 25 UB, its shape and size), and the way the steel was made and finished. Grade and section are independent choices; a beam of any common universal beam size can be rolled in either grade. And the grade is separate from the difference between hot rolled and cold rolled steel, which is about the forming process rather than the yield strength. The full picture on any job is the engineer’s specification: grade, sub-grade, section and connections together.
Frequently asked questions
What is the difference between S275 and S355 steel?
Both are structural carbon steel grades made to BS EN 10025. The number is the minimum yield strength in N/mm2 (megapascals): S275 has a minimum yield of 275 N/mm2 and S355 has 355 N/mm2. S355 is stronger, so an engineer can carry the same load with a smaller or lighter section, but it costs more per tonne. Which grade suits your job is the structural engineer’s decision.
What is S275 steel used for?
S275 is a general-purpose structural grade used across everyday steelwork: beams and columns in extensions and small commercial buildings, brackets, supports, lintels and frames where loads are moderate and section size is not critical. It is widely stocked and economical, which makes it a common default unless the design calls for the extra strength of S355.
What is S355 steel?
S355 is a higher-strength structural steel with a minimum yield of 355 N/mm2. Because it is stronger than S275, the same load can be carried with a smaller or lighter section, which is useful for long spans, heavily loaded members and weight-sensitive frames. It costs more per tonne, so the engineer balances the material saving against the higher price.
What do the JR, J0 and J2 sub-grades mean?
The letters after the grade describe impact toughness, which is how well the steel resists fracture at low temperature. JR is tested at room temperature (around 20 degrees C), J0 at 0 degrees C and J2 at minus 20 degrees C. A colder service condition or a more demanding design generally calls for a higher sub-grade such as S355J2. The specification comes from the structural engineer.
Not sure which grade your drawings call for? Send us the engineer’s specification and we will supply and fabricate the right steel. Get in touch for a free quote.