The same table, with a smaller ε
Cross-section classification in fire uses Table 5.2 of EN 1993-1-1, but with a reduced factor ε (EN 1993-1-2, cl. 4.2.2):
The factor 0.85 approximates the ratio √(kE,θ / ky,θ): at elevated temperatures the modulus of elasticity drops faster than the yield strength, so thin plates buckle locally more easily. As a result all c/t limits drop by 15%, and some sections move into a weaker class.
c/t limits by steel grade
The class boundaries for the most common parts of a section (outstand flange in compression, web in compression, web in bending and circular hollow section in compression). For each part the upper limit of Class 1 and of Class 3 is given; beyond the latter the section is Class 4:
| Grade | ε | Flange, Class 1 (9ε) | Flange, Class 3 (14ε) | Web compr., Class 1 (33ε) | Web compr., Class 3 (42ε) | Web bend., Class 1 (72ε) | Web bend., Class 3 (124ε) | CHS, Class 1 (50ε²) | CHS, Class 3 (90ε²) |
|---|---|---|---|---|---|---|---|---|---|
| S235 | 0.850 | 7.65 | 11.90 | 28.1 | 35.7 | 61.2 | 105.4 | 36.1 | 65.0 |
| S275 | 0.786 | 7.07 | 11.00 | 25.9 | 33.0 | 56.6 | 97.4 | 30.9 | 55.6 |
| S355 | 0.692 | 6.22 | 9.68 | 22.8 | 29.0 | 49.8 | 85.8 | 23.9 | 43.0 |
| S420 | 0.636 | 5.72 | 8.90 | 21.0 | 26.7 | 45.8 | 78.8 | 20.2 | 36.4 |
| S460 | 0.608 | 5.47 | 8.51 | 20.0 | 25.5 | 43.7 | 75.3 | 18.5 | 33.2 |
- For the flange (outstand in compression), c = (b − tw − 2r) / 2, divided by tf. The Class 2/3 boundary is 10ε.
- For the web, c = h − 2tf − 2r, divided by tw. The Class 2/3 boundary is 38ε in compression and 83ε in bending.
- For a circular hollow section (CHS), the ratio is d/t and the Class 2/3 boundary is 70ε².
- The walls of square and rectangular hollow sections are classified like webs, with c = h − 3t.
For elements thicker than 40 mm the nominal yield strength drops (for example 335 N/mm² instead of 355 for S355), so ε changes too.
What each class means in fire
- Classes 1 and 2 use the plastic modulus Wpl.
- Class 3 uses the elastic modulus Wel.
- For Class 4 without a detailed calculation (Annex E), EN 1993-1-2 recommends a critical temperature of 350 °C. This usually means much thicker protection.
Example
HEB 300 in S355, in compression: ε = 0.692. The flange has c/t = 117.5 / 19 = 6.18 ≤ 9ε = 6.22, so Class 1, close to the limit. The web has c/t = 208 / 11 = 18.9 ≤ 33ε = 22.8, so Class 1 too. The full calculation is in the worked example.
Informative material summarising the provisions of EN 1993-1-2. It does not replace the text of the standard, the National Annex or verification by a qualified engineer.