---
title: Cross-Section Class Check
description: Automate EN 1993-1-1 §5.5 section classification for I/H/box/angle sections under combined bending and compression.
category: Eurocode
---

## 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 `N_Ed` and `M_Ed`, FrameAI applies EN 1993-1-1 Table 5.2.

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

```
ε = √(235 / 355) = 0.814  (for S355)
```

Internal part (web in bending + compression):

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

Outstand (flange):

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

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

```
<div class="screenshot-placeholder">
  📸 Screenshot: The section class panel showing web and flange class checks for HEB 300.
  TODO: replace with actual screenshot
</div>
```

## Pure compression (uniform compression)

When `N_Ed` 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
b_eff = ρ · b
```

Effective section modulus `W_eff` 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/t_f` exceeds the Class 3 limit but the web is Class 1, the section is still Class 3 overall — the flange controls.

**Welded sections**: The γ_M0 factor for welded sections is applied at material selection. Rolled sections use γ_M0 = 1.00; welded sections may use γ_Mw = 1.10 depending on your National Annex.