Before you can determine the design resistance of a structural steel cross-section, you must first classify it. The classification governs how much of the cross-section’s plastic capacity you can mobilise before local buckling prevents it. Get this wrong and your design will either be unconservative (unsafe) or excessively conservative (uneconomical).
The Four Classes at a Glance
Classes 1 and 2 allow use of the plastic section modulus Wpl. Class 3 is limited to elastic section modulus Wel. Class 4 requires effective section properties from EN 1993-1-5. The difference between Wpl and Wel is typically 10–20% for I-sections, so classification directly affects economy.
The Classification Process
Classification is done element-by-element. Flanges and web are classified independently; the section takes the worst (highest) class. Under EN 1993-1-1 Table 5.2, each element is classified by its width-to-thickness ratio c/t against limiting values scaled by the strain factor ε:
S235: ε = 1.00. S275: ε = 0.924. S355: ε = 0.814. S420: ε = 0.748. S460: ε = 0.715. Higher-strength steel is more susceptible to local buckling and must have proportionally thicker elements to achieve the same class.
Limiting c/t Ratios from Table 5.2
Outstand Flanges
| Class | Rolled (c/t ≤) | Welded (c/t ≤) |
|---|---|---|
| Class 1 | 9ε | 9ε |
| Class 2 | 10ε | 10ε |
| Class 3 | 14ε | 14ε |
Worked example — IPE 300, S355 (fy = 355 MPa, ε = 0.814):
c = (b − tw)/2 − r = (150 − 7.1)/2 − 15 = 56.45 mm
Flange c/t = 56.45 / 10.7 = 5.28. Class 1 limit = 9 × 0.814 = 7.33. Result: Class 1 flange.
Internal Web in Bending
| Class | Pure bending (c/t ≤) | Pure compression (c/t ≤) |
|---|---|---|
| Class 1 | 72ε | 33ε |
| Class 2 | 83ε | 38ε |
| Class 3 | 124ε | 42ε |
IPE 300 web: cw = 300 − 2(10.7) − 2(15) = 248.6 mm. Web c/t = 248.6 / 7.1 = 35.0. Class 1 limit = 72 × 0.814 = 58.6. Result: Class 1 web. IPE 300 in S355 is overall Class 1.
Typical outcome: Most standard IPE and HEA/HEB sections in S235–S355 are Class 1 under pure bending. The critical check is usually the web under combined bending + axial force, or flanges of welded plate girders.
Combined Bending and Axial Force
When axial force shifts the neutral axis, more of the web enters compression and the stress ratio α (compression zone fraction) increases. The Class 1 web limit becomes:
As α → 1.0 (pure compression), this limit drops sharply — a section that is Class 1 as a beam may become Class 3 as a column under high axial load. Always recheck classification at the governing load combination.
Class 4 — Effective Cross-Section (EN 1993-1-5)
For slender elements, the effective width is reduced by factor ρ:
λ̅p = (c/t) / (28.4ε√kσ)
The effective area Aeff and effective section moduli Weff,min replace gross section properties throughout the design. Class 4 is common in thin-walled built-up girders, cold-formed sections, and high-strength steel applications.
Practical Reference Table
| Section | Grade | Typical Class (pure bending) | Notes |
|---|---|---|---|
| IPE 80–600 | S235–S355 | Class 1 | All standard sizes; check web under axial |
| HEA 100–1000 | S235–S355 | Class 1 | Wide flanges keep c/t very low |
| HEB all sizes | S235–S460 | Class 1 | Thick flanges, very compact |
| UPN / UPE | S235–S355 | Class 1–2 | Web may become Class 3 in compression |
| SHS / RHS (hot-formed) | S235–S355 | Class 1–3 | Depends on b/t; check EN 1993-1-1 Table 5.2 Sheet 3 |
| Welded plate girders | any | Class 3–4 | Thin webs are typically slender; use effective width |
| Cold-formed sections | any | Class 3–4 | Design to EN 1993-1-3, not EN 1993-1-1 |
Design Route by Class
- Class 1: Plastic global analysis permitted. Plastic hinges may form. Use Wpl for MRd.
- Class 2: Plastic section resistance (Wpl) but elastic global analysis only. No redistribution after first hinge.
- Class 3: Elastic section resistance Wel,min. Elastic global analysis. Typically 10–15% less moment resistance than Class 2.
- Class 4: Effective section properties (Aeff, Weff) from EN 1993-1-5. Significant design complexity and reduction in resistance.
Design tip: For welded plate girders, aim for a Class 3 web (not Class 4) by specifying a slightly thicker web plate. Using Wel instead of effective section is much simpler and the weight penalty is usually small.
Common Mistakes
- Not reclassifying under combined bending and axial force — the class can change significantly.
- Applying the rolled section c/t limits to welded sections — no root radius means a larger c dimension.
- Assuming hot-formed SHS/RHS are always compact — check b/t against Table 5.2 Sheet 3.
- Using Wpl for Class 3 sections — this is unconservative by 10–20%.
- Ignoring Class 4 requirements for thin-web plate girders in preliminary design.
References: Eurocode 3 (EN 1993-1-1), IS 800:2007. For reference only — verify against current editions before use in design.