Cross-Section Class Check

Automate EN 1993-1-1 §5.5 section classification for I/H/box/angle sections under combined bending and compression.

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Cross-Section Class Check

Cross-section classification is the first check in any EN 1993-1-1 design. It determines whether the section can develop its full plastic resistance (Class 1–2) or is limited to elastic resistance (Class 3), or requires effective properties (Class 4).

Why it matters

The class of a section determines:

**Class 1–2**: Plastic hinge analysis is permitted; moment redistribution up to 40% is allowed
**Class 3**: Only elastic analysis; no redistribution
**Class 4**: Effective width calculation required; reduced section properties

Using the wrong class gives incorrect resistance values.
How FrameAI classifies a section

For a hot-rolled I/H section under combined NEd and MEd, FrameAI applies EN 1993-1-1 Table 5.2.

**Web in compression** (ε = √(235/fy)):

ε = √(235 / 355) = 0.814 (for S355)

Internal part (web in bending + compression):

c/tw ≤ 33ε → Class 1 c/tw ≤ 38ε → Class 2 c/tw ≤ 42ε → Class 3 c/tw > 42ε → Class 4

Outstand (flange):

c/tf ≤ 9ε → Class 1 c/tf ≤ 10ε → Class 2 c/tf ≤ 14ε → Class 3 c/tf > 14ε → Class 4

The controlling class is the most restrictive (web or flange).

📸 Screenshot: The section class panel showing web and flange class checks for HEB 300. TODO: replace with actual screenshot

Pure compression (uniform compression)

When NEd acts alone, the web and flange are both fully in compression:

Internal (web): c/t ≤ 30ε → Class 1 c/t ≤ 35ε → Class 2 c/t ≤ 42ε → Class 3 c/t > 42ε → Class 4
Outstand (flange): c/t ≤ 9ε → Class 1 c/t ≤ 10ε → Class 2 c/t ≤ 14ε → Class 3 c/t > 14ε → Class 4

Class 4 — effective section

If the section is Class 4, FrameAI calculates effective widths per EN 1993-1-5 §4.3 (internal parts) and §4.4 (outstands):

ρ = (0.55 + 0.025·λ̅) but ρ ≥ 0.13 + 0.002·λ̅ beff = ρ · b

Effective section modulus Weff is used in the buckling check instead of the gross section modulus.

Manual override

If you specify a different material grade or cross-section that FrameAI hasn't modelled, click the member → edit → change class. The override is saved with an audit trail.

Common issues

**Flange slenderness in thin plates**: If your flange c/tf exceeds the Class 3 limit but the web is Class 1, the section is still Class 3 overall — the flange controls.

**Welded sections**: The γM0 factor for welded sections is applied at material selection. Rolled sections use γM0 = 1.00; welded sections may use γMw = 1.10 depending on your National Annex.

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