Australian Steel Framing — AS 4100 Structural Systems

Complete reference for steel framing systems in Australian building design per AS 4100:2020 and AS 1170.4:2007. Covers concentrically braced frames (CBF), moment-resisting frames (MRF), eccentrically braced frames (EBF), ductility classification under AS 1170.4, structural regularity requirements, inter-storey drift limits, and practical design guidance for Australian steel construction.

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Overview of Australian Steel Framing Systems

Steel framing systems provide the primary lateral and gravity load-resisting structure for Australian buildings. The selection of framing system depends on building height, seismicity, wind loads, architectural requirements, and construction economics. AS 4100:2020 provides the structural design framework, while AS 1170.4:2007 defines seismic actions.

The three main lateral load-resisting systems used in Australian steel design are:

Many Australian buildings use dual systems combining MRF and CBF to achieve both stiffness and ductility. For low-rise industrial buildings in non-seismic regions (Adelaide Z > 0.09 is the general seismic threshold), CBF alone is usually sufficient.

Concentrically Braced Frames (CBF) — AS 4100 Clause 5.6

In a CBF, the brace members intersect the beam-column joints such that the brace centroidal axes are concentric at each joint. This eliminates moment transfer at the brace connections, with all lateral load resisted by axial tension-compression in the braces.

CBF Configuration Types

CBF Design Requirements per AS 4100

For braces in compression, the member slenderness must satisfy:

• le/ry ≤ 120 (compression braces, Clause 5.6.2)
• le/ry ≤ 180 (tension-only braces, Clause 9.1.3)

The design capacity of a concentrically braced compression member per AS 4100 Clause 6.3:

phi-Nc = phi x alpha-c x Ns ≤ Ns

Where:

• phi = 0.90 (capacity factor for compression, Table 3.4)
• alpha-c = member slenderness reduction factor (Clause 6.3.3)
• Ns = kf x An x fy (nominal section capacity)

Brace Slenderness Limits

For seismic design per AS 1170.4, brace slenderness limits are more restrictive to ensure stable cyclic behaviour. Compression brace buckling must not precipitate a loss of lateral strength exceeding 30% of the maximum base shear.

Moment-Resisting Frames (MRF) — AS 4100 Clause 5.5

In an MRF, lateral loads are resisted by flexure in beams and columns connected by rigid (moment-resisting) joints. Moment frames provide the highest ductility capacity but are less stiff than braced frames, requiring larger sections for drift control.

MRF Beam-to-Column Connection Requirements

AS 4100 Clause 9.3.8 requires that moment connections in MRFs use pretensioned bolts (Category 8.8/TB or 10.9/TB) for bolted connections, or full-penetration groove welds for welded connections. Connection rotation capacity must be sufficient to accommodate the design plastic hinge rotation.

Inter-Storey Drift Limits

AS 4100 Clause 16.4 and NCC 2022 specify drift limits for steel frames:

Where H = storey height. For a typical 4.0 m storey in a Sydney office building: H/500 = 8.0 mm serviceability drift limit under a 1-in-25 year wind event.

Eccentrically Braced Frames (EBF) — AS 4100 + AS 1170.4

EBFs combine the stiffness of braced frames with the ductility of moment frames by introducing replaceable ductile links — short beam segments between brace connections where plastic deformation is concentrated.

Link Types and Properties

Where:

• Mpl = f_y x Zx (plastic moment capacity of link section)
• Vpl = 0.6 x f_y x d x tw (plastic shear capacity)

Australian EBF design follows the AISC (US) seismic provisions approach adapted to AS 4100 material properties and AS 1170.4 ductility classification. Shear links are preferred because they provide the highest rotation capacity and more predictable inelastic behaviour.

Link Overstrength Design

AS 1170.4 requires that the frame members outside the link be designed for the overstrength capacity of the link:

Omega = link overstrength factor = 1.25 x Vpl / Vdes

Where Vdes is the design shear in the link at the ultimate limit state. The connections, beams outside the link, and columns are designed for 1.25 x the nominal link capacity to ensure the link is the only yielding element in the system.

Ductility Classification per AS 1170.4

AS 1170.4:2007 defines three ductility classes for steel framing systems:

The structural ductility factor mu is the ratio of ultimate displacement to yield displacement of the lateral system and is used to reduce the elastic seismic base shear:

Vdes = Velastic / mu

Where Velastic is the base shear calculated using an elastic response spectrum. A structure classified as DC3 (mu = 4.0) attracts only 25% of the elastic seismic force compared to a DC1 structure (mu = 1.25 attracting 80% of elastic force).

Important: Higher ductility classification requires more stringent detailing and increased connection costs. The economic optimum for Australian regions with moderate seismicity (Z = 0.08-0.12) is typically DC2 (mu = 3.0), balancing construction cost against seismic force reduction.

Structural Regularity Requirements

AS 1170.4 requires that steel frames meet structural regularity criteria to qualify for the ductility classifications above. Irregular structures require reduced ductility capacity.

Worked Example: Frame Selection for a 10-Storey Office Building

Problem: A 10-storey office building in Sydney (Z = 0.08) has plan dimensions 60 m x 30 m with 4.0 m storey heights. Wind loads govern over seismic. Select a suitable framing system and determine preliminary member sizes.

Solution:

1. Lateral system selection: For a low-seismic Sydney location with wind-governed loading, a CBF with X-bracing is the most economical system. Provide two braced bays per direction at the building core to minimise torsional effects.

2. Brace slenderness check: Brace length at 45-degree angle in a 4.0 m bay = 4.0 x sqrt(2) = 5.66 m. For a 150 x 150 x 10 SHS (ry = 58.5 mm, Grade C450L0): le/ry = 5,660 / 58.5 = 97 < 100 (DC2 seismic limit) or < 120 (AS 4100 compression limit). Acceptable.

3. Drift check: Under service wind load (5 kPa basic wind pressure, 1-in-25 year): total drift = 10 x 4.0 m = 40 m. For a CBF with 45-degree braces, elastic drift per storey approximately = Fh / (2EA cos^3(theta)), where F is the storey shear. Preliminary brace sizes of 150 SHS would typically achieve H/500 drift at each storey (< 8 mm per floor).

4. Ductility classification: Since seismic does not govern, DC2 (mu = 3.0) is selected as the default seismic classification. The braces require ductile detailing per AS 1170.4, and the brace-to-gusset connections must develop the brace section capacity without premature fracture.

5. Frame cost comparison: A CBF solution uses approximately 35% less structural steel than an MRF solution for the same building, making it the clear choice for wind-governed low-seismic Australian regions.

Design Resources

• Australian Steel Design Guide — AS 4100 overview
• Australian Seismic Design — AS 1170.4 earthquake actions
• AS 4100 Load Combinations — Limit state design
• AS 4100 Connection Design — Bolted and welded connections
• Australian Steel Grades — Grade 300 and 350 material properties
• Australian Steel Properties — Section property tables
• Australian Bolt Capacity — Bolt shear and tension values
• Australian Wind Load — AS/NZS 1170.2 wind actions
• All Australian References

Frequently Asked Questions

What steel framing systems are most common in Australian building construction? Concentrically braced frames (CBF) are the most common system for low to mid-rise Australian buildings (1-15 storeys) due to their high stiffness and low cost. Moment-resisting frames (MRF) are used for taller buildings where bracing interferes with architectural requirements. Eccentrically braced frames (EBF) are used in higher-seismic regions (Newcastle, Adelaide, Meckering) where both stiffness and ductility are required. For most Australian regions with Z < 0.10, wind loads govern and CBF is the most economical choice.

How does AS 1170.4 classify ductility for steel frames? AS 1170.4 defines three ductility classes: DC1 (limited ductility, mu = 1.25-1.5) — ordinary braced frames and ordinary moment frames with minimal detailing; DC2 (moderate ductility, mu = 3.0) — concentrically braced frames with ductile brace connections and intermediate moment frames; DC3 (full ductility, mu = 4.0) — special moment-resisting frames and eccentrically braced frames with shear links. Higher mu reduces design base shear but requires more stringent connection detailing and member slenderness limits.

What are the inter-storey drift limits for Australian steel frames? AS 4100 Clause 16.4 and the NCC 2022 specify H/500 per storey for serviceability under wind, H/300 overall for building envelope protection, and H/150 for ultimate wind limit state. Seismic drift limits per AS 1170.4 are H/300 for serviceability (1-in-25 year earthquake) and H/50 for ultimate (1-in-500 year earthquake). For a 4.0 m storey: serviceability drift = 8.0 mm, ultimate seismic drift = 80 mm.

What is the role of eccentrically braced frames in Australian seismic design? EBFs use replaceable ductile links — short beam segments between brace connections where plastic deformation concentrates. Shear links (length e ≤ 1.6 Mpl/Vpl) provide the highest rotation capacity (up to 0.15 radians) and are preferred for Australian seismic design. The structure outside the link is designed for link overstrength (1.25 x link nominal capacity). EBFs are primarily used in regions with Z > 0.10 (Newcastle, Adelaide, Meckering) where the combination of stiffness and ductility is needed.

When does seismic design govern over wind design for Australian steel frames? For most major Australian cities (Sydney Z = 0.08, Melbourne Z = 0.09, Brisbane Z = 0.06, Perth Z = 0.08), wind loads govern the lateral system design for typical buildings. Seismic design may govern when: (1) the site is in a higher seismic region (Adelaide Z = 0.10-0.12, Newcastle Z = 0.10-0.14); (2) the building is heavy (high mass per floor, such as concrete-filled decks or library loads); (3) the building is in Seismic Design Category C or D per AS 1170.4; (4) the structure is low-rise with heavy cladding where wind base shear is low. The governing load case should always be checked for both wind and seismic at the ultimate limit state.

Educational reference only. All design values must be verified against the current edition of AS 4100:2020, AS 1170.4:2007, and NCC 2022. This information does not constitute professional engineering advice. Always consult a qualified structural engineer for design decisions.