Australian Section Compactness Limits — AS 4100 Table 5.2

Complete reference for section classification per AS 4100:2020 Clause 5.2 and Table 5.2. The section compactness classification determines which moment capacity formula applies: plastic section capacity (M_s), effective section modulus (Z_e), or effective section capacity for slender sections. Correct classification is mandatory before any member capacity calculation.

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Section Classification Framework — Clause 5.2

AS 4100:2020 Clause 5.2 classifies steel cross-sections into four categories based on the width-to-thickness ratio of the plate elements (flange and web) that compose the section:

1. Compact (C): The section can develop its full plastic moment capacity (M_s) and sustain plastic rotation beyond initial yield. The flange and web plate elements are stocky enough to prevent local buckling before the plastic moment is attained and through sufficient inelastic rotation to permit moment redistribution in continuous beams.

2. Non-compact (NC): The section can develop first yield at the extreme fibre (effective section modulus, Z_e) but cannot sustain plastic rotation. Local buckling of one or more plate elements occurs after first yield but before the full plastic moment is reached.

3. Slender (S): One or more plate elements buckle elastically before first yield is reached. The section capacity is governed by elastic local buckling of the slender element(s), and an effective width approach must be used per Clause 5.2.5.

The classification is performed element-by-element: the most critical element (flange or web) determines the overall section classification.

Flange Slenderness Limits — Table 5.2, Part 1

For I-Sections (UB, UC, WB, WC series)

Flange outstand is defined as b_f / (2 t_f) for I-sections, where b_f is the flange width and t_f is the flange thickness. The outstand is the portion of the flange from the web face to the flange tip.

For Grade 300 steel (f_y = 300 MPa for t_f <= 12 mm, 280 MPa for t_f > 12 mm):

Standard UB Section Flange Compactness

All standard Australian UB and UC hot-rolled sections have compact or non-compact flanges. Welded plate girders with thin wide flanges are the primary case where flange slenderness governs.

Web Slenderness Limits — Table 5.2, Part 2

For I-Sections in Bending

Web slenderness is defined as d_1 / t_w, where d_1 is the clear depth of the web between flange fillets, and t_w is the web thickness. For hot-rolled sections, d_1 = d - 2(t_f + r), where r is the root radius.

For Grade 300 steel (f_y = 320 MPa for web, t_w <= 12 mm typical):

Compact web limit: 82 x sqrt(250/320) = 72.6

Standard UB Section Web Compactness

All standard hot-rolled UB sections have compact webs in bending. Web slenderness typically controls only in welded plate girders with deep slender webs.

Hollow Sections — Table 5.2, Part 3

RHS/SHS (Rectangular/Square Hollow Sections)

For Grade 350 RHS (f_y = 350 MPa for cold-formed RHS):

Compact flange limit: 34 x sqrt(250/350) = 28.8 Compact web limit: 82 x sqrt(250/350) = 69.3

CHS (Circular Hollow Sections)

For Grade 350 CHS (f_y = 350 MPa):

Compact: d_o/t <= 50 x (250/350) = 35.7 Non-compact: 35.7 < d_o/t <= 120 x (250/350) = 85.7 Slender: d_o/t > 85.7

Capacity Formulas by Section Classification

Effective Section Modulus for Non-Compact Sections

For non-compact sections, the effective section modulus Z_e is linearly interpolated between the plastic modulus S and the elastic modulus Z:

Z_e = Z + (lambda_ey - lambda_e) / (lambda_ey - lambda_ep) x (S - Z)

where lambda_e is the actual plate element slenderness, lambda_ep is the compact limit, and lambda_ey is the non-compact (yield) limit. When lambda_e <= lambda_ep, Z_e = S (full plasticity). When lambda_e >= lambda_ey, Z_e = Z (elastic only, local buckling at first yield).

Worked Example: Section Classification

Problem: Classify a 310UB40.4 Grade 300 section and determine the applicable moment capacity formula. f_y = 300 MPa (flange, t_f = 10.2 mm <= 12 mm), f_y = 320 MPa (web, t_w = 6.1 mm <= 12 mm).

Given section properties (from OneSteel/InfraBuild tables):

• Flange width b_f = 165 mm, flange thickness t_f = 10.2 mm
• Web depth between flanges d_1 (approx) = 309.6 - 2(10.2 + 11.4) = 266.4 mm
• Web thickness t_w = 6.1 mm
• S = 662 x 10^3 mm^3, Z = 579 x 10^3 mm^3 (actual values per manufacturer)

Solution:

Step 1: Flange classification

Lambda_e_flange = b_f / (2 t_f) = 165 / (2 x 10.2) = 8.09

Compact limit: lambda_ep_flange = 9 x sqrt(250/300) = 9 x 0.913 = 8.22

Non-compact limit: lambda_ey_flange = 15 x sqrt(250/300) = 13.7

Since lambda_e_flange = 8.09 < lambda_ep_flange = 8.22: flange is COMPACT.

Step 2: Web classification

Lambda_e_web = d_1 / t_w = 266.4 / 6.1 = 43.7

Compact limit: lambda_ep_web = 82 x sqrt(250/320) = 82 x 0.884 = 72.5

Since lambda_e_web = 43.7 < lambda_ep_web = 72.5: web is COMPACT in bending.

Step 3: Overall section classification

Both flange and web are compact. The section is classified as COMPACT.

Step 4: Moment capacity

For a compact section: M_s = f_y x S = 300 x 662 x 10^3 x 10^(-6) = 198.6 kNm (nominal).

Design moment capacity: phi M_s = 0.90 x 198.6 = 178.7 kNm.

Verification against manufacturer's table: OneSteel tables give phi M_sx = 178 kNm for 310UB40.4 Grade 300. Matches.

Result: 310UB40.4 Grade 300 is fully compact. phi M_s = 178.7 kNm.

Frequently Asked Questions

What makes a section compact per AS 4100 Table 5.2?

A section is classified as compact when the plate element slenderness (lambda_e) for BOTH the flange and web is less than or equal to the compact limit (lambda_ep). For hot-rolled I-section flanges, lambda_ep = 9 x sqrt(250/f_y). For webs in bending, lambda_ep = 82 x sqrt(250/f_y). If either element exceeds its compact limit, the section is non-compact or slender. Compact sections can develop their full plastic moment capacity and undergo plastic rotation.

How does yield strength affect section classification in AS 4100?

The compactness limits in AS 4100 Table 5.2 are scaled by sqrt(250/f_y), which means that higher-strength steel requires tighter (more restrictive) slenderness limits. A section that is compact in Grade 300 (f_y = 300 MPa) may be non-compact in Grade 350 (f_y = 350 MPa). The factor 250 MPa is the reference yield strength against which the Australian slenderness limits were calibrated. For Grade 350: sqrt(250/350) = 0.845, reducing the compact limit by 15.5%.

Are all standard UB and UC sections compact per AS 4100?

Yes, virtually all standard hot-rolled UB (Universal Beam), UC (Universal Column), WB (Welded Beam), and WC (Welded Column) sections in the Australian range are compact in bending for Grade 300 steel. The hot-rolling process naturally produces stocky flange and web proportions. Non-compact sections in the Australian standard range are limited to a few very light UC sections (e.g., 150UC23.4) and some PFC (Parallel Flange Channel) sections. Welded plate girders with thin wide flanges are the primary case where section slenderness becomes a design issue.

What is the effective section modulus Z_e for non-compact sections?

The effective section modulus Z_e is a linearly interpolated value between the plastic section modulus S and the elastic section modulus Z, based on the actual plate slenderness relative to the compact and non-compact limits. When a flange or web is non-compact, the section can reach first yield at the extreme fibre but cannot develop full plastification before local buckling occurs. Z_e reflects this reduced capacity while still being larger than the elastic modulus Z.

Does AS 4100 classify sections differently for compression versus bending?

Yes. The web slenderness limits differ between pure compression (Clause 6.2) and pure bending (Clause 5.2). For webs under compression (columns), the compact limit is 45 x sqrt(250/f_y) compared to 82 x sqrt(250/f_y) for bending. This means a web that is compact in bending may be non-compact in compression. The section classification for a beam-column under combined loading must consider the worst classification from the relevant clause for each load component.

Educational reference only. All design values must be verified against the current edition of AS 4100:2020 and the project specification. This information does not constitute professional engineering advice. Always consult a qualified structural engineer for design decisions.