Australian Seismic Design — AS 1170.4:2007 Earthquake Actions for Steel Structures

Complete reference for seismic design of steel structures in Australia per AS 1170.4:2007 (Structural Design Actions — Earthquake Actions in Australia). Earthquake hazard factor Z, site classification, ductility classes for steel moment-resisting frames (SMRF, IMRF, OMF), structural regularity requirements, the response reduction factor (μ), and AS 4100 seismic detailing provisions with worked examples for low to moderate seismic regions.

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Australian Seismic Design Context

Australia is a continent with low to moderate seismicity. Unlike Japan, New Zealand, or the US West Coast, most Australian regions have low earthquake hazard. However, several significant historical earthquakes — Newcastle 1989 (M_w 5.6, 13 deaths), Meckering 1968 (M_w 6.5), Tennant Creek 1988 (M_w 6.6) — demonstrate that seismic design is necessary for certain regions.

AS 1170.4:2007 (with the 2012 amendment) provides the seismic design framework for Australia. The standard uses a simplified hazard model calibrated to Australian conditions. For steel structures, AS 4100:2020 Clause 3 references AS 1170.4 for seismic actions, and AS 4100 Appendix D provides specific detailing requirements for earthquake resistance.

Earthquake Hazard Factor (Z)

The earthquake hazard factor Z represents the peak ground acceleration (PGA) expressed as a proportion of gravity (g) with a 1:500-year return period:

Location Z (PGA as fraction of g) Seismicity Level
Sydney 0.07-0.09 Low
Melbourne 0.08-0.10 Low
Brisbane 0.05-0.07 Very low
Adelaide 0.09-0.12 Moderate
Perth 0.07-0.09 Low
Newcastle 0.10-0.14 Moderate
Wollongong 0.08-0.11 Low-moderate
Canberra 0.07-0.10 Low
Hobart 0.05-0.07 Very low
Darwin 0.05-0.06 Very low
Meckering (WA) 0.18-0.22 Highest in Australia
Tennant Creek (NT) 0.15-0.20 Very high

For most Australian steel buildings in Sydney, Melbourne, Perth, and Brisbane, seismic actions are low and wind loads typically govern the lateral system design. However, for buildings in Newcastle, Adelaide, Meckering, and Tennant Creek regions, seismic design may govern, particularly for low-rise heavy structures.

AS 1170.4 Seismic Action Calculation

AS 1170.4 defines the seismic base shear for a building as:

V = k_p × Z × C_h(T_1) × μ^(-1) × W_t × S_p

Where:

Site Classification

AS 1170.4 defines site classes A_E based on the average shear wave velocity over the top 30 m (V_s30):

Class Description V_s30 Range (m/s) Typical Australian Sites
A Strong rock > 1,500 Granite, basalt, massive sandstone
B Rock 760-1,500 Sandstone, limestone, competent sedimentary rock
C Shallow soil 350-760 Stiff clay, dense sand (typical Sydney basin)
D Deep soil 150-350 Deep clay deposits (Melbourne, Adelaide)
E Very soft soil < 150 Reclaimed land, soft estuarine deposits

Site class significantly amplifies ground motion. A building on Site Class E will experience approximately 2-3 times the spectral acceleration of the same building on Site Class A.

Spectral Shape Factor C_h(T_1)

C_h(T_1) is calculated from the site class and the building fundamental period T_1:

For a steel building, the approximate fundamental period:

T_1 = 1.25 × k_t × h_n^(3/4)

Where:

For a 6-storey steel moment frame building (h_n = 24 m): T_1 = 1.25 × 0.085 × 24^(3/4) = 1.25 × 0.085 × 11.08 = 1.18 seconds

The spectral shape factor C_h(T_1) is then read from AS 1170.4 Figure 3.2. For Site Class C at T_1 = 1.18 s, C_h ≈ 0.5-0.6.

Structural Ductility Factor (μ)

The ductility factor μ (response reduction factor) accounts for the structure's ability to dissipate seismic energy through inelastic deformation:

Steel Lateral System μ Description
Special Moment-Resisting Frame (SMRF) 4.0 Ductile — designed for significant inelastic deformation
Intermediate Moment-Resisting Frame (IMRF) 3.0 Moderate ductility
Ordinary Moment-Resisting Frame (OMF) 2.0 Limited ductility
Concentrically Braced Frame (CBF) — ductile 3.0 Buckling-controlled energy dissipation
Concentrically Braced Frame (CBF) — ordinary 2.0 Elastic design
Eccentrically Braced Frame (EBF) 4.0 Link-beam energy dissipation
Cantilever column (inverted pendulum) 1.25 Limited redundancy
Fully elastic 1.0 No ductility assumed

Higher μ values reduce the design base shear (V design = V elastic / μ). However, they require more stringent detailing to ensure the ductility capacity is achieved. AS 4100 Appendix D specifies the detailing requirements for each ductility class.

Structural Regularity Requirements

AS 1170.4 requires that steel buildings meet regularity criteria in plan and elevation to use the equivalent static analysis method. Irregular buildings require dynamic analysis (response spectrum or time history).

Plan Regularity

Parameter Regular Limit Irregularity
Plan aspect ratio L/B ≤ 3 Torsional effects significant
Setback asymmetry Setback ≤ 20% of dimension on each side Eccentric mass/stiffness
Diaphragm discontinuity Cut-out area ≤ 30% of gross floor area Reduced load path
Re-entrant corners Depth ≤ 25% of plan dimension in each direction Wing buildings — potential stress concentration
Torsional eccentricity e_0 ≤ 0.15 × b Irregular — requires 3D dynamic analysis

Vertical Regularity

Parameter Regular Limit Irregularity
Stiffness irregularity Storey stiffness ≥ 70% of storey above Soft storey — prohibited in SMRF
Mass irregularity Storey mass ≤ 150% of storey above Mass concentration — dynamic amplification
Setback elevation Single setback ≤ 30% of plan dimension Weak storey at setback
Vertical geometric irregularity Column line offset ≤ 25% of bay width Reduced load path
Weak storey Storey strength ≤ 80% of storey above Prohibited in all ductile systems

Steel buildings with vertical irregularity (common in buildings with a soft ground storey for parking) must be designed with a minimum μ = 1.25 (effectively elastic) unless dynamic analysis demonstrates ductility capacity.

AS 4100 Seismic Provisions

AS 4100:2020 Appendix D provides the supplementary detailing requirements for steel structures in seismic applications. The key provisions depend on the ductility class:

SMRF (μ = 3.0-4.0) Detailing Requirements

Component Requirement AS 4100 Reference
Beam section Class 1 plastic section (b/2t_f ≤ λ_eb) Clause 5.2, Table 5.2
Column section Class 1 plastic section with N* / φN_s ≤ 0.30 Clause 6.2, Appendix D
Beam-column connection Full-strength welded or bolted connection ≥ 1.2 × M_p of beam Appendix D Clause D3
Column splices Full-strength connection ≥ 1.2 × M_p of column Appendix D Clause D4
Panel zone (web) Panel zone shear capacity ≥ 0.8 × sum(M_p) / (h_b - t_fb) Appendix D Clause D5
Lateral bracing Beam bracing at plastic hinge zones ≤ L_b = 0.09 × r_y × E / F_y Appendix D Clause D2

IMRF (μ = 2.0-3.0) Detailing Requirements

Component Requirement
Beam section Class 1 or 2 compact section
Column section Class 1, 2, or 3 section with N* / φN_s ≤ 0.50
Beam-column connection Capacity ≥ moment at beam hinge formation or at least ≥ M_p of beam
Lateral bracing Bracing at plastic hinge zones — less restrictive than SMRF

OMF (μ = 1.25-2.0) Detailing Requirements

The ductility class selection significantly affects steel tonnage and connection cost. A SMRF building may require 10-15% heavier beams and columns (to meet the Class 1 compact section and N*/φN_s limits) and more expensive moment connections compared to an OMF design using the same forces. However, the design base shear for the SMRF is approximately half that of the OMF (μ = 4.0 vs μ = 2.0), which reduces the total lateral force.

Worked Example: Steel Building in Newcastle

Problem: Determine the seismic design actions for a 4-storey steel office building in Newcastle (moderate seismicity).

Building data:

Step 1 — Fundamental period:

T_1 = 1.25 × 0.085 × 14.4^(3/4) = 1.25 × 0.085 × 8.35 = 0.89 seconds

Step 2 — Spectral shape factor:

For Site Class C at T_1 = 0.89 s, using AS 1170.4 Figure 3.2: The constant acceleration plateau (C_h = 2.0-3.0) applies for T ≤ 0.5 seconds. At T_1 = 0.89 s: C_h ≈ 1.4 (from the descending branch of the spectrum)

Step 3 — Design base shear:

V = 1.0 × 0.12 × 1.4 × 1/4.0 × 17,000 × 0.7

(S_p = 0.7 for SMRF — accounts for overstrength and higher-mode effects)

V = 0.12 × 1.4 × 0.25 × 17,000 × 0.7 = 500 kN

Step 4 — Compare with wind base shear:

For the same building, the wind base shear from AS 1170.2 (Newcastle is Region A2, V_R ≈ 37 m/s):

Assume windward pressure C_p,n = 0.9, leeward C_p,n = -0.5, M_z,cat ≈ 0.85 at roof level:

p = 0.5 × 1.2 × (37 × 0.95 × 0.85 × 1.0 × 1.0)² = 0.53 kPa at roof

Total wind shear = 0.5 × 18 × (0.9 + 0.5) × 0.53 × 1.0 per level ≈ 670 kN (estimated)

Wind base shear (670 kN) > Seismic base shear (500 kN). Wind governs for this building in Newcastle.

Step 5 — Conclusion:

Even in moderate-seismicity Newcastle, wind governs for a 4-storey SMRF steel building. However, the seismic load combinations (G + ψ_l × Q + E_u per AS 1170.0 Combination 6a) must still be checked, and the SMRF detailing requirements (Class 1 sections, full-strength connections) must be satisfied regardless of whether wind or seismic produces higher forces.

For taller steel buildings (10+ storeys), the fundamental period increases, reducing the spectral acceleration C_h(T_1), and seismic forces become proportionally smaller relative to wind. In low-seismicity cities like Brisbane, seismic design for steel buildings up to 15 storeys is almost never governing.

Steel Connection Design for Seismic

AS 4100 Appendix D and AS 1170.4 Clause 6 require that connections in ductile steel frames be designed for the capacity of the connected members, not just the elastic seismic forces:

Capacity Design Principle:

Connection design force = Ω_0 × V_elastic or 1.2 × R_p of the connected member (whichever is larger)

Where Ω_0 is the overstrength factor (typically 1.25-1.5 for steel SMRF).

Moment Connection Detailing (SMRF)

For SMRF beam-column moment connections per AS 4100 Appendix D:

  1. Welded connections: Full-penetration butt welds at beam flanges with continuity plates in the column. Weld metal overmatch (E55XX electrode for 300PLUS).
  2. Bolted connections: End-plate connections stiffened to prevent plate yielding before bolt failure. Bolts in tension designed for 1.25 × beam flange force at M_p.
  3. Panel zone: Column web stiffeners (continuity plates) at beam flanges. Panel zone thickness ≥ (d_c - 2×t_fc)/50 for unstiffened webs.

Brace Connection Detailing (CBF)

For concentrically braced frames in seismic design:

Frequently Asked Questions

How does Australian seismic design compare to ASCE 7 or NZS 1170.5?

Australian seismic hazard (Z = 0.03-0.22) is significantly lower than New Zealand (Z up to 0.60 for Wellington) or the US West Coast (S_s up to 2.5g for Los Angeles). The AS 1170.4 format resembles NZS 1170.5 (both use ductility factor μ and spectral shape factor C_h), whereas ASCE 7 uses R (response modification coefficient) and S_a (spectral acceleration). The Australian hazard factors are lower because intraplate earthquakes are less frequent in Australia than in plate-boundary regions like New Zealand and Japan. For steel buildings up to 8 storeys in most Australian cities, wind loads govern the lateral system design.

Is seismic design required for all steel buildings in Australia?

No. AS 1170.4 Clause 1.3 exempts buildings in Importance Level 1 (low consequence) and 2 (normal) if Z × C_h(T_1) × μ^(-1) < 0.05. For a typical 4-storey SMRF in Sydney (Z = 0.08, C_h ≈ 1.5, μ = 4.0): 0.08 × 1.5 / 4.0 = 0.03 < 0.05 — seismic design is not required. However, AS 4100 Appendix D requires that if seismic design is invoked, all ductility class detailing provisions apply, even if the seismic force is below the threshold.

What is the most common lateral system for steel buildings in Australia?

Steel moment-resisting frames (SMRF or IMRF) are the most common lateral systems for low to mid-rise steel buildings in Australia, designed primarily for wind loads. Concentrically braced frames and eccentric braced frames are used for taller buildings (10-20 storeys) where drift control becomes critical. Buckling-restrained braces (BRB) are rare in Australia but gaining acceptance. Typical seismic ductility is μ = 2.0-3.0 (IMRF) for most buildings, with μ = 4.0 (SMRF) only for critical facilities in moderate-seismicity regions.

Related Pages

Educational reference only. Seismic design methodology per AS 1170.4:2007 and AS 4100:2020 Appendix D. Verify site-specific hazard factor Z, site class, and importance level for your project. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification.