Australian Section Compactness — AS 4100 Table 5.2 Guide
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Reference for section compactness limits per AS 4100:2020 Table 5.2. Slenderness classification for flanges and webs determines whether the full plastic moment can be achieved.
Slenderness Classification — AS 4100 Table 5.2
| Element Type | Compact (λ ≤ λcp) | Non-Compact (λ ≤ λcn) | Slender |
|---|---|---|---|
| Flanges | |||
| Hot-rolled I-section flange, outstand | 9 | 14 | >14 |
| Welded I-section flange, outstand | 8 | 12 | >12 |
| RHS/SHS flange (compression) | 30 | 40 | >40 |
| Angle leg, outstand | 10 | 16 | >16 |
| Webs | |||
| I-section web (pure bending) | 80 | 115 | >115 |
| I-section web (axial compression) | 30 | 35 | >35 |
| RHS/SHS web (bending) | 30 | 40 | >40 |
λ = b/t or d1/tw as applicable. b = half-flange for I-sections.
Moment Capacity Based on Compactness
| Classification | Section Capacity φMs | Reference |
|---|---|---|
| Compact | φMs = φ × fy × Sx | Clause 5.2.3 |
| Non-compact | φMs = φ × fy × Ze (interpolated) | Clause 5.2.4 |
| Slender | φMs = φ × fy × Ze (effective section) | Clause 5.2.5 |
Where Sx = plastic modulus, Zx = elastic modulus, Ze = effective modulus.
Common Australian Sections Classification
| Section | flambdab/tf | Web d1/tw | Flange Class | Web Class | Section Class |
|---|---|---|---|---|---|
| 250UB37.3 | 4.5 | 38 | Compact | Compact | Compact |
| 310UB46.2 | 5.5 | 45 | Compact | Compact | Compact |
| 410UB59.7 | 6.3 | 51 | Compact | Compact | Compact |
| 610UB125 | 5.6 | 53 | Compact | Compact | Compact |
| 700UB173 | 5.4 | 55 | Compact | Compact | Compact |
| 200UC52 | 5.0 | 16 | Compact | Compact | Compact |
| 310UC137 | 7.0 | 18 | Compact | Compact | Compact |
Most Australian hot-rolled sections are compact, enabling full plastic design.
Worked Example
Problem: Classify a 410UB59.7 section (bf/tf=6.3, d1/tw=51). Grade 300 steel has fy=300 MPa.
Solution: Flange: λ=6.3. Per Table 5.2, λcp=9 for hot-rolled flange. 6.3 < 9 → Compact Web: λ=51. Per Table 5.2, λcp=80 for web in bending. 51 < 80 → Compact Section classification = Compact → full φMs = φ × fy × Sx applies.
Design Resources
- [[Australian Steel Grades|/reference/australian-steel-grades/]] | [[Australian Steel Properties|/reference/australian-steel-properties/]] | [[Australian Beam Sizes|/reference/au-beam-sizes/]] | [[Australian Bolt Capacity|/reference/australian-bolt-capacity/]] | [[AS 4100 Beam Design|/reference/as4100-beam-design-example/]] | [[All Australian References|/reference/]]
FAQ
What is the compact flange limit for hot-rolled I-sections? λ ≤ 9 (b/tf) per AS 4100 Table 5.2. Most Australian UB and UC sections are compact.
How does compactness affect design? Compact sections can achieve full plastic moment (φMs = φ × fy × Sx). Non-compact and slender sections have reduced capacity based on effective section.
What is the web compactness limit for I-sections in bending? λ ≤ 80 (d1/tw) for compact. Most Australian sections have d1/tw < 80.
Educational Use Only — This reference is for educational and preliminary design purposes only. All structural designs must be independently verified by a licensed Professional Engineer (PE) or Structural Engineer (SE) in accordance with AS 4100:2020 and all applicable Australian Standards. Results are not for construction.