ε Values per Steel Grade

The classification parameter ε depends on the yield strength:

ε = √(235 / f_y)

Higher strength steels have lower ε, making the classification limits tighter. An S460 section must have lower width-to-thickness ratios to achieve the same class as an S235 section.

Flange Classification (Outstand Element)

Rolled I and H Sections — Compression Flange

Where c = (b - t_w - 2r)/2 for rolled sections, or c = (b - t_w)/2 for welded sections.

Worked Example — IPE 300 in S355

• b = 150 mm, t_f = 10.7 mm, r = 15 mm, t_w = 7.1 mm
• c = (150 - 7.1 - 2×15)/2 = 56.45 mm
• c/t_f = 56.45 / 10.7 = 5.28
• ε = 0.81
• 9ε = 7.29, 10ε = 8.1, 14ε = 11.3
• Class 1 (5.28 < 7.29)

Worked Example — HEA 300 in S355

• b = 300 mm, t_f = 14 mm, r = 27 mm, t_w = 8 mm
• c = (300 - 8 - 2×27)/2 = 119 mm
• c/t_f = 119 / 14 = 8.50
• ε = 0.81
• Class 2 (8.50 > 7.29 but < 8.1)

Web Classification (Internal Compression Element)

Web in Pure Bending

Where c_w = distance between fillet welds (rolled) or clear web depth (welded).

Web in Pure Compression

CHS Classification (Circular Hollow Sections)

Example — CHS 219.1×10 in S355

• d = 219.1 mm, t = 10 mm
• d/t = 21.91
• ε² = 0.81² = 0.656
• 50ε² = 32.8
• Class 1 (21.91 < 32.8)

SHS/RHS Classification

Flange (compression)

Web (bending)

Classification for Combined Loading

When the web is subject to combined bending and compression (beam-columns), the classification limits are modified:

• Determine α (ratio of depth in compression to total web depth)
• For Class 1: c_w/t_w ≤ 396ε / (13α - 1)
• For Class 2: c_w/t_w ≤ 456ε / (13α - 1)
• For Class 3: c_w/t_w ≤ 42ε / (0.67 + 0.33ψ)

Where ψ is the stress ratio (σ₂/σ₁) across the web depth. These formulas give tighter limits as the compression zone increases.

Practical Classification Summary

Frequently Asked Questions

What is the definition of ε in EN 1993 section classification?

ε = √(235/f_y) where f_y is the nominal yield strength of the steel in MPa. ε accounts for the influence of yield strength on local buckling resistance. Higher strength steels (lower ε) have tighter classification limits because the higher yield stress leaves less margin before local buckling occurs.

When is a section classified as Class 4?

A section is Class 4 when the width-to-thickness ratio exceeds the Class 3 limit. Class 4 sections experience local buckling before the yield stress is reached, and design must use effective section properties per EN 1993-1-5. Typical Class 4 sections include slender welded plate girders and thin-walled cold-formed sections.

How does combined bending and compression affect section classification?

Combined loading increases the compression zone in the web (α > 0.5), which tightens the web classification limits. A web that is Class 1 in pure bending may become Class 2 or Class 3 when axial compression is present. EN 1993-1-1 Table 5.2 provides modified limits based on the stress distribution factor ψ and compression depth ratio α.

Related Pages

• EN 1993 Beam Design — Flexural design per Eurocode 3
• Column Design Guide — Compression per EN 1993-1-1
• European Beam Sizes — IPE, HEA, HEB dimensions
• Lateral-Torsional Buckling — LTB per Clause 6.3.2
• All European References

Educational reference only. Classification limits per EN 1993-1-1:2005 Table 5.2. Verify actual section dimensions against mill certificates. For Class 4 sections, follow EN 1993-1-5 for effective width calculation. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification.