| 1 | Plastic design sections — can develop plastic hinge with rotation capacity | Zx × Fy | Mp | | 2 | Compact sections — can develop plastic moment but limited rotation | Zx × Fy | Mp (limited) | | 3 | Non-compact — yielding governs before local buckling | Sx × Fy | My | | 4 | Slender — local buckling occurs prior to yielding | Seff × Fy | Reduced My |

The classification is element-specific: a beam may have a Class 1 web but Class 2 flange. The overall section is classified by the least favourable element classification.

Table 1 — Width-to-Thickness Limits (CSA S16:24)

Flanges in Flexural Compression

Webs in Pure Flexural Compression

Flanges in Axial Compression

Webs in Pure Axial Compression

Note: For webs in axial compression, Classes 1 and 2 have the same limit (670/sqrt(Fy)).

Numerical Limits for Common Grades

Limits for 350W Steel (Fy = 350 MPa)

Limits for 300W Steel (Fy = 300 MPa)

Limits for 400W Steel (Fy = 400 MPa)

W-Shape Classification Examples (350W)

Beam Classification (Flexure)

Most W-shapes used for beams are Class 1 or 2, allowing full plastic moment Mr = phi × Zx × Fy.

Column Classification (Axial)

Columns are more likely to be Class 3 or 4 because the web axial compression limits are more restrictive (670/sqrt(Fy) = 35.8). Light sections like W310×39 have Class 3 webs in axial compression due to their slender web.

Class 3 Web in Axial Compression

For columns where h/w > 35.8 (for Fy = 350 MPa), the web is Class 3 or 4 in axial compression and the effective area must be used:

Aeff = A × (750/sqrt(Fy)) / (h/w) for Class 3 (limited to 750/sqrt(Fy))

For W310×39 in 350W:

• h/w = 46.5
• 750/sqrt(350) = 40.09
• Aeff = A × 40.09/46.5 = 0.862 × A

This reduces the axial compression capacity by approximately 14% compared to the gross section. The CISC Handbook includes this reduction in the column capacity tables.

HSS Section Classification

HSS sections are classified similarly but with different limit expressions per CSA S16 Table 1:

For HSS in Flexure

For HSS in Axial Compression

Note: Round HSS in axial compression has a different limit expression (22000/Fy) treating Classes 1 and 2 identically.

Worked Example — Classification Check

Problem: Classify W530x82, Grade 350W for beam bending and column axial compression.

Section Data:

• bf = 191 mm, tf = 13.3 mm, d = 529 mm, w = 9.3 mm
• b/2tf = 191/(2×13.3) = 7.18
• h/w = (529 - 2×13.3 - 2×11.43)/9.3 = 480/9.3 = 51.6

For Flexure: Flange: 7.18 ≤ 7.75 (145/sqrt(350)) → Class 1 Web (flexure): 51.6 ≤ 58.8 (1100/sqrt(350)) → Class 1 Overall: Class 1 → Mr = phi × Zx × Fy

For Axial Compression: Flange: 7.18 ≤ 7.75 → Class 1 Web (axial): 51.6 > 35.8 (670/sqrt(350)) → Class 3 Overall: Class 3 → Effective area required: Aeff = 10,400 × (750/sqrt(350))/51.6 = 10,400 × 40.09/51.6 = 8,080 mm^2

The beam-column interaction check uses the appropriate classification for the combined stress state per CSA S16 Clause 13.8.

FAQ

What is the difference between Class 1 and Class 2 in CSA S16? Both achieve plastic moment Mp, but Class 1 has sufficient rotation capacity for plastic analysis and redistribution of moments (typically requiring 3-5 times the yield rotation). Class 2 reaches Mp but has limited rotation capacity (typically 1-2 times yield rotation). For most design cases, both use Zx × Fy for Mr, but Class 1 is needed for plastic hinge mechanisms in seismic design.

When is a section Class 4 in CSA S16? When either the flange ratio exceeds 200/sqrt(Fy) (for flexure) or the web ratio exceeds 1900/sqrt(Fy) (for flexure), the section is Class 4 — slender. For 350W, this means b/2tf > 10.69 or h/w > 101.6. This occurs in custom plate girders with thin webs, cold-formed sections per CSA S136, and some light-wall HSS sections.

How do you calculate effective section properties for Class 4 sections? Per CSA S16 Clause 13.5.3, the effective width be for a Class 4 compression element is: be = (200/sqrt(Fy)) / (b/t) × b for flanges in flexure. The reduced section properties (Seff, Aeff) are calculated using the effective width. This is a simplified reduction — the effective width is a fraction of the gross width, applied to the compression portions of the section.

Are all W-shapes Class 1 for beam bending? Most standard W-shapes in 350W are Class 1 for flexure (b/2tf ≤ 7.75 and h/w ≤ 58.8). Exceptions include: W360×33 (b/2tf ≈ 8.5, Class 2 flange), some wide-flange light sections, and sections in higher-strength steel (400W, 480W) where the limits are tighter. Always verify — do not assume classification.

Related Pages

• CSA S16 Beam Design — Flexural Guide
• CSA S16 Lateral-Torsional Buckling
• CSA S16 Column Design
• Canadian Beam Sizes — W-Shape Table
• CSA S16 Combined Loading
• All Canadian References

This page is for educational reference. Classification limits per CSA S16:24 Table 1. Verify limits for the applicable Fy and code edition. Class 4 sections require special consideration per Clause 13.5.3. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent PE/SE verification.