What the code means
In a grade such as S355J2, the "S" means structural steel, "355" is the minimum yield strength in N/mm² (for thicknesses up to 16 mm), and "J2" is the impact toughness grade.
S275 and S355
- S275 has a minimum yield strength of 275 N/mm². It was the traditional grade for many beams
- S355 has a minimum yield strength of 355 N/mm², about 30% higher. It is now the most common grade for UK sections
The strength falls slightly for thicker material, so very heavy sections and thick plates have lower guaranteed values.
Toughness: JR, J0, J2 and K2
Toughness is the steel's resistance to brittle fracture, measured with an impact test:
- JR - 27 joules at room temperature (20°C)
- J0 - 27 joules at 0°C
- J2 - 27 joules at -20°C
- K2 - 40 joules at -20°C
The engineer chooses the toughness from the thickness, the stresses and the lowest temperature the steel will see. External and thick members often need J2 or better.
Hollow sections and other grades
Hot-finished hollow sections are usually S355J2H to BS EN 10210; cold-formed ones are to BS EN 10219. Weathering steel (for example S355J2W) forms a protective rust layer and is used unpainted in some bridges and facades.
Can grades be swapped?
Only with the engineer's written approval. Using a higher grade rarely causes problems structurally, but a lower grade can be unsafe, and a change of toughness or standard can matter for welding and fatigue.
Traceability
Every structural piece we make can be traced to a mill certificate (inspection certificate 3.1 to BS EN 10204) showing the grade, chemistry and test results, as required by our BS EN 1090 Factory Production Control.