The number in a steel grade is a minimum yield strength, and for S355 it applies to thin material. Rolling thick plate and heavy flanges gives a coarser grain structure, so the guaranteed yield strength steps down as the thickness grows.
The thickness bands
EN 10025-2 gives the minimum yield strength of S355 by nominal thickness :
Thickness (mm) | (MPa) |
|---|---|
355 | |
345 | |
335 | |
325 | |
315 |
The drop in the second band is only about 3%, which is exactly why it is easy to forget and why it matters in a check that passes at 99%.
Common sections that cross 16 mm
For rolled sections the flange is usually the thickest element, so it decides the band:
Section | Flange (mm) | for S355 (MPa) |
|---|---|---|
HE 300 A | 14 | 355 |
HE 300 B | 19 | 345 |
IPE 600 | 19 | 345 |
533x210x122 UB | 21.3 | 345 |
Heavy columns and the deeper beams are therefore designed at 345 MPa or lower, and so are thick end plates, base plates and stiffeners.
Which table to use
EN 1993-1-1 Table 3.1 gives a simplified version with only two bands, mm and mm. The recommended approach, and the choice several National Annexes make, is to take the values from the product standard itself, which is the table above. Check your National Annex: the UK annex, for example, points to the product standard values.
Practical habits
Read from the section table before choosing , not after the check.
For a built-up or welded section, use the thickest plate that the check relies on.
Base plates and end plates are often 20 to 40 mm, so start them at 345 MPa.
The tensile strength has its own values in the same standard; do not assume it follows the same steps.
Takeaways
S355 means 355 MPa only up to 16 mm.
Many everyday sections have flanges between 16 and 40 mm and use 345 MPa.
Take the band from the element that governs the check, usually the flange.
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