Australian Steel Design Standards

AS 4100:2020 replaced AS 4100:1998 (the first limit state edition, which replaced the working-stress AS 1250). The 2020 edition incorporated amendments from the 2012 and 2016 revisions, added new hollow section grades, and harmonised section classification with international practice. It is the legally referenced standard for structural steelwork under the NCC 2022.

AS 4100 Capacity Factors (φ)

The capacity factors in AS 4100:2020 Table 3.4 are a fundamental design input — they differ from AISC and EN 1993. Australian designers must use the correct φ values:

Key differences from AISC 360: AS 4100 uses φ = 0.90 for most member checks (consistent with AISC φ_b = φ_c = 0.90). For bolts, AS 4100 uses φ = 0.80 vs AISC φ = 0.75 — a 7% less conservative factor reflecting Australian bolting practice. For fillet welds, AS 4100 uses φ = 0.80 (directional method in butt welds) vs AISC φ = 0.75. The 0.80 factor applies when weld strength is determined by the directional method (Cl. 9.7.3.10); otherwise φ = 0.60 for the simplified method.

AS/NZS 3678/3679.1 Steel Grades

Australian structural steel grades are specified to AS/NZS 3678 (plates) and AS/NZS 3679.1 (hot-rolled sections). Grade designation gives minimum yield stress in MPa.

Grade 300PLUS is the proprietary brand name for Grade 300 sections produced by InfraBuild (formerly OneSteel/Arrium). It is chemically identical to AS/NZS 3679.1 Grade 300 with guaranteed minimum Fy = 300 MPa, and is the default hot-rolled section grade for Australian construction. Grade 350 is used where higher strength is justified (long-span roof trusses, heavily loaded transfer girders).

L0 and L15 designations: The suffix L0 or L15 indicates Charpy V-notch impact testing temperature:

• No suffix (standard Grade 250/300/350): No Charpy requirement. Suitable for non-fracture-critical members in normal service.
• L0: Charpy 27J minimum average at 0°C. Standard for primary structural members in multi-storey buildings per AS 4100 Cl. 4.4.
• L15: Charpy 27J minimum average at -15°C. Specified for exposed structures in cold climates (Alpine regions, Tasmania) and fracture-critical members.

Grade 250 vs Grade 300: Grade 300 gives a 20% increase in yield strength over Grade 250 for a negligible cost premium. It is now the default structural grade in Australia — Grade 250 plates are typically only specified for base plates, stiffeners, and secondary members where the higher strength provides no design benefit.

Australian Structural Sections

Hot-Rolled Sections to AS/NZS 3679.1

Australian hot-rolled sections are produced exclusively in Grade 300 (300PLUS):

The Australian market predominantly uses UB and UC sections. TFB (Taper Flange Beams) are a legacy section from the BS 4-1 imperial series, still available but progressively replaced by UB sections in modern construction. WB and WC (Welded Beam/Column) sections are fabricated plate girders in standardised sizes produced by InfraBuild.

Hollow Sections to AS/NZS 1163

C450L0 (Fy = 450 MPa) hollow sections are now standard for structural applications in Australia. The higher strength compared to hot-rolled Grade 300 sections makes hollow sections efficient for compression members and truss chords.

AS 1170 Loading Standards

Load Combinations to AS 1170.0

The principal ultimate limit state combinations:

ψ_l = 0.4 for residential/office, 0.6 for storage/assembly. ψ_E = 0.3 (residential/office), 0.6 (storage/assembly).

Wind Actions — AS 1170.2:2021

Australia uses a regional wind speed map with design wind speeds for Serviceability (25-year return), Ultimate (500-year for housing, 1,000-year for Importance Level 3, 2,000-year for IL4):

V_des,θ = V_R × M_d × (M_z,cat × M_s × M_t)
p = 0.5 × ρ_air × V_des,θ^2 × C_fig × C_dyn

ρ_air = 1.2 kg/m^3
C_fig = shape factor (pressure coefficient)
C_dyn = dynamic response factor

Australia has four wind regions: A (non-cyclonic, most of the continent), B (intermediate), C (tropical cyclones, NW coast), and D (severe tropical cyclones, Pilbara coast). The design wind speed V_R ranges from V_25=28 m/s (Region A1 inland) to V_1000=80 m/s (Region D).

Earthquake Actions — AS 1170.4:2007

Australia is an intraplate setting with low-to-moderate seismicity. The hazard factor Z ranges from 0.03 (most of the continent) to 0.22 (Meckering/Newcastle regions). The design response spectrum uses:

Spectral shape: C(T) = Z × S(T) / (k_p × Z × S(T) + 1)
k_p = spectral shape factor
S(T) = site hazard spectrum ordinate

Steel moment frames: R = μ/S_p = 4.0 (Special MRF) or 2.5 (Intermediate MRF). The low seismicity means wind often governs over earthquake for most Australian locations except Newcastle, Meckering (WA), and Tennant Creek regions.

AS 4100 Section Classification

AS 4100 Cl. 5.2 classifies cross-sections identically to EN 1993-1-1:

For Grade 300 hot-rolled sections, most Australian UB and UC sections are Compact (Class 2) for strong-axis bending. Grade 350 hollow sections are typically Class 1 (Plastic) for CHS and SHS, Class 2 (Compact) for thin-walled RHS.

Worked Example — 310 UC 96.8 Column

Problem: Check a 310 UC 96.8 in Grade 300PLUS as an axially loaded column. Effective length L_e = 4.5 m (both axes, simple construction). Permanent load = 450 kN, Imposed load = 320 kN.

Section Properties — 310 UC 96.8

Grade 300PLUS: F_yf = 300 MPa (t_f = 15.4 mm ≤ 20 mm), F_yw = 320 MPa

d = 308 mm, b_f = 305 mm, t_f = 15.4 mm, t_w = 9.9 mm
A_g = 12,300 mm^2
r_x = 135 mm, r_y = 77.3 mm
k_f = 1.0 (fully effective, no local buckling)

Section Classification (AS 4100 Table 5.2)

Flange: λ_e = (b_f - t_w)/(2 × t_f) × √(F_y/250) = (305-9.9)/(2×15.4) × √(300/250)
     = 9.58 × 1.095 = 10.49
Plastic limit λ_ep = 9 → 10.49 > 9 → NOT Class 1
Compact limit λ_ey = 16 → 10.49 < 16 → Class 2 (Compact)

Web: λ_e = (d - 2×t_f)/t_w × √(F_y/250) = (308-30.8)/9.9 × 1.095 = 30.66
Plastic limit λ_ep = 45 → 30.66 < 45 → Class 1
Flange governs → section is Class 2 (Compact).

Axial Capacity

Form factor k_f = 1.0
N_s = k_f × A_g × F_y = 1.0 × 12,300 × 300 = 3,690 kN

Modified slenderness:
λ_nx = (L_e/r_x) × √(k_f) × √(F_y/250) = (4500/135) × 1.0 × 1.095 = 36.5
λ_ny = (L_e/r_y) × √(k_f) × √(F_y/250) = (4500/77.3) × 1.0 × 1.095 = 63.8

α_a (compression member factor, Table 6.3.3(1)):
For λ_n = 63.8, α_a ≈ 0.784 (interpolation)

α_b is complex (Table 6.3.3(2)) — for hot-rolled UC with k_f = 1.0:
α_b ≈ 0.82 (intermediate value)

α_c = min(α_a, α_b) = 0.784

N_c = α_c × N_s = 0.784 × 3,690 = 2,893 kN

Design capacity: φN_c = 0.90 × 2,893 = 2,604 kN

Load Check

N* = 1.2 × 450 + 1.5 × 320 = 540 + 480 = 1,020 kN

φN_c = 2,604 kN > N* = 1,020 kN OK. D/C = 0.39.

The 310 UC 96.8 is adequate for this load. D/C = 0.39 indicates it is conservative — a 250 UC 72.9 (φN_c ≈ 1,400 kN) would also pass at D/C ≈ 0.73 but with less reserve for moment interaction if frames are continuous.

AS 4100 vs International Codes — Quick Reference

AS 4100 φ factors are the same as AISC 360 for member checks (0.90) but 0.80 for bolts vs AISC 0.75. AS 4100 Table 6.3.3 buckling curves are based on the same ECCS curves as EN 1993 but with α_b modified for Australian sections and fabrication practice.

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Related Pages

• Steel Grades — A36, A572, A992, 350 Grade — Cross-standard grade comparison
• Steel Beam Sizes — International Comparison — W, UB, IPE, ISMB sections
• AS 4100 Base Plate Design — Step-by-Step Guide — Full Australian base plate workflow
• AS 4100 Base Plate Worked Example — Complete worked example
• AS 4100 Bolt Group Design — Eccentric Loading — Bolt group analysis
• Wind Load Calculator — ASCE 7 & AS 1170.2 — Multi-code wind calculator
• Load Combinations — AS 1170.0 — Australian load case calculator
• Beam Capacity Calculator — Free multi-code beam capacity
• UK Steel Design Guide — BS EN 1993 & UK National Annex
• India IS 800 Steel Design Guide — IS 800 limit state design
• Disclaimer

Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for independent review by a qualified structural engineer registered with Engineers Australia (or the relevant state/territory registration authority). All real-world design must comply with the current editions of AS 4100, AS 1170 series, and the National Construction Code, verified against project-specific requirements. You are responsible for verifying inputs, validating results, and obtaining professional sign-off from a registered Chartered Professional Engineer (CPEng) or Registered Professional Engineer of Queensland (RPEQ) where required.