AS 1170.4 Seismic Design Category — Hazard Factor, Site Class & Ductility

Complete reference guide for determining the seismic design category per AS 1170.4:2007 (incorporating Amendment No. 2, 2018). Covers the seismic hazard factor Z for all Australian capital cities and regional centres, probability factor kp for annual exceedance probabilities other than 1/500, site sub-soil classification (Classes Ae through Ee), structural ductility factor mu and structural performance factor Sp, design spectrum construction, and the interaction with AS 4100 steel design provisions. Metric units throughout (kN, mm, MPa, m/s^2).

Related pages: AS 4100 Steel Column Design | AS 4100 Braced Frame Design | AS 4100 Brace Connection Design | AS 4100 Moment Frame Design | Seismic Load Calculator — ASCE 7 & AS 1170.4

Australian Seismicity — Why It's Different

Australia is an intraplate tectonic setting — unlike New Zealand, Japan, or California, earthquakes are not concentrated at plate boundaries. Australian earthquakes occur as intraplate ruptures on ancient fault lines within the Australian Plate. The consequence for structural design:

Characteristic Australia (AS 1170.4) New Zealand (NZS 1170.5) USA (ASCE 7)
Tectonic setting Intraplate Plate boundary (Alpine Fault) Plate boundary (San Andreas)
Peak hazard factor Z 0.22 (Meckering, WA) 0.60 (Wellington) 3.0 (coastal California)
Capital city Z values 0.05-0.11 0.13-0.40 0.15-1.5
Return period (design) 500 years (Importance 2) 500 years (IL2) 2,475 years (Risk Category II)
Site amplification 5 classes Ae to Ee 5 classes A to E 6 classes A to F
Ductility reduction mu up to 6 (special) mu up to 6 R up to 8 (special)
Dominant period range 0.3-1.0 s (short-period) 0.4-2.5 s (moderate-long) 0.2-2.0 s

The key takeaway: Australian seismic hazard is low to moderate by global standards. Wind loads govern the lateral force-resisting system for approximately 90% of Australian buildings. Seismic design typically only governs for structures in the higher-hazard zones (Z > 0.11) or structures over 50 m in height.

Hazard Factor Z — AS 1170.4 Section 3

The hazard factor Z represents the peak ground acceleration (PGA) on rock (Site Class Be) for a 500-year return period, expressed as a fraction of gravity (g). Values are determined from the seismic hazard map in AS 1170.4 Figure 3.2(A) and Table 3.2.

Hazard Factor Z for Australian Cities and Regions:

Location Z Hazard Level Design Implication
Perth, WA 0.09 Low Wind governs for all buildings
Darwin, NT 0.09 Low Wind (cyclonic) governs
Brisbane, QLD 0.05 Very Low Seismic rarely considered
Sydney, NSW 0.08 Low Wind governs for all buildings
Melbourne, VIC 0.08 Low Wind governs for all buildings
Hobart, TAS 0.04 Very Low Seismic negligible
Adelaide, SA 0.11 Moderate Seismic may govern for tall structures
Canberra, ACT 0.10 Moderate Seismic may govern for tall structures
Newcastle, NSW 0.11 Moderate Seismic may govern for irregular structures
Meckering, WA (epicentre) 0.22 High Seismic governs for most structures
Tennant Creek, NT 0.15 Moderate Seismic check required for all structures
Kalgoorlie, WA 0.10 Moderate Seismic check required for tall structures

Engineering note: The maximum Australian Z value (0.22 at Meckering) is approximately 3.5x the typical capital city value (0.06 average). By comparison, Wellington NZ has Z = 0.40 — about 2x the maximum Australian value. The design spectral acceleration for a standard Australian building is typically 0.05-0.15g, compared to 0.4-1.0g in California.

Probability Factor kp — AS 1170.4 Table 3.1

The probability factor kp adjusts the hazard spectrum for annual exceedance probabilities (AEP) other than the reference 1/500 (10% in 50 years):

Annual Probability of Exceedance Return Period kp
1/100 (serviceability) 100 years 0.50
1/250 250 years 0.75
1/500 (design — Importance 2) 500 years 1.00
1/800 800 years 1.15
1/1000 1,000 years 1.30
1/1500 (Importance 3) 1,500 years 1.50
1/2500 (Importance 4) 2,500 years 2.00

The hazard spectrum for any return period is: C(T) = kp x Z x C_h(T)

Where C_h(T) is the spectral shape factor from AS 1170.4 Table 6.5.

For Importance Level 3 buildings (schools, hospitals): kp = 1.50 (1/1500 AEP). For Importance Level 4 buildings (post-disaster facilities): kp = 2.00 (1/2500 AEP).

Site Sub-Soil Classification — AS 1170.4 Section 4

The site class modifies the bedrock spectrum to account for local soil amplification. The classification is based on the average shear wave velocity in the top 30 m (V_s,30) or the site natural period T_s:

Class Description V_s,30 (m/s) T_s (s) Amplification
Ae Strong rock V_s > 1500 — None
Be Rock 360 < V_s <= 1500 T_s < 0.25 Reference (1.0)
Ce Shallow soil (stiff) 180 < V_s <= 360 T_s < 0.6 Moderate
De Deep or medium soil V_s <= 180 T_s < 1.5 High (up to 2x)
Ee Very soft soil (T_s >= 1.5 s) Special study T_s >= 1.5 Very High (up to 3x)

The site factor F_s modifies the corner periods: T_1 = F_s x 0.25 s (short-period corner) T_2 = F_s x 1.25 s (long-period corner)

Class F_s T_1 (s) T_2 (s)
Ae 0.90 0.225 1.125
Be 1.00 0.250 1.250
Ce 1.25 0.313 1.563
De 1.75 0.438 2.188
Ee 2.50 0.625 3.125

The critical insight: Site Class De and Ee amplify the bedrock hazard by up to 2.5x. A site with Z = 0.08 on rock (Be) becomes equivalent to Z_eff = 0.14 on Class De — pushing the structure from low-seismic into moderate-seismic territory. For Class Ee sites, a site-specific geotechnical investigation and ground response analysis is mandatory per AS 1170.4 Clause 4.3.

Design Spectrum Construction — AS 1170.4 Section 6

The elastic design acceleration spectrum S_a(T) is:

For T = 0: S_a(0) = kp x Z x PGA_factor (approximately 2.5 x kp x Z)

For 0 < T <= T_1: S_a(T) = kp x Z x C_h(0) + (kp x Z x C_h(T_1) - kp x Z x C_h(0)) x T / T_1

For T_1 < T <= T_2: S_a(T) = kp x Z x C_h(T_1) (constant-velocity plateau)

For T > T_2: S_a(T) = kp x Z x C_h(T_1) x T_2 / T (constant-displacement decay)

Where C_h(T_1) = 2.5 (spectral shape factor at the plateau).

Example — Adelaide CBD (Z = 0.11, Class Ce, Importance 2, kp = 1.0):

T_1 = 0.313 s, T_2 = 1.563 s (Class Ce, F_s = 1.25)

S_a(T) for fundamental periods:

For a 4-storey steel moment frame (T_1 ~ 0.8 s), the design spectral acceleration is 0.275g on Class Ce — approximately 30% of the equivalent wind pressure for a 15 m tall building. Wind governs unless the building is highly irregular.

Structural Ductility Factor mu — AS 1170.4 Section 5

The structural ductility factor mu reduces the elastic design forces to account for inelastic energy dissipation. The design base shear is:

V = C_d(T_1) x W_t / mu

Where:

Ductility Factors for Steel Structures per AS 4100 / AS 1170.4 Table 5.5:

Lateral Force Resisting System mu Sp Notes
Ordinary moment frame (OMF) 2 0.67 Limited ductility, no special detailing
Intermediate moment frame (IMF) 3 0.50 Moderate ductility, AS 4100 Clause 13.2 detailing
Special moment frame (SMF) 4 0.42 High ductility, full Clause 13.3 detailing
Limited ductility concentrically braced 3 0.50 CBF with moderate ductility
Moderately ductile concentrically braced 3.5 0.46 Enhanced CBF detailing
Eccentrically braced frame (EBF) 4 0.42 Link beam yielding mechanism
Buckling-restrained braced frame (BRBF) 5 0.37 Requires project-specific testing

The Sp/mu relationship is key: For mu = 2, Sp/mu = 0.67/2 = 0.335. For mu = 5, Sp/mu = 0.37/5 = 0.074. The effective force reduction is substantial at higher ductility — but comes with stringent detailing requirements per AS 4100 Section 13.

AS 4100 Seismic Steel Design — Key Provisions

When seismic design governs, AS 4100 Section 13 supplements the normal design rules:

Clause 13.2 — Ordinary ductility (mu <= 2):

Clause 13.3 — Moderate ductility (2 < mu <= 3.5):

Clause 13.4 — High ductility (mu > 3.5, up to 6):

Critical Rule: Never mix ductility levels in the same direction of the same storey. The lowest-ductility element at a given storey determines the system ductility factor.

Worked Example — Seismic Design Category for a 6-Storey Office Building

Problem: Determine the seismic design category for a 6-storey steel-framed office building in Newcastle, NSW (20 m height). Site investigation indicates stiff clay over rock (V_s,30 = 280 m/s). Building is regular in plan and elevation, Importance Level 2.

Step 1 — Hazard factor Z: Newcastle: Z = 0.11 (from Table 3.2)

Step 2 — Probability factor kp: Importance Level 2, design life 50 years: kp = 1.0 (1/500 AEP)

Step 3 — Site sub-soil class: V_s,30 = 280 m/s. From Table 4.1: 180 < V_s <= 360 -> Site Class Ce. F_s = 1.25 for Class Ce.

Step 4 — Design spectrum: Corner periods: T_1 = 0.313 s, T_2 = 1.563 s. S_a(T_1) = kp x Z x C_h(T_1) = 1.0 x 0.11 x 2.5 = 0.275g

Step 5 — Fundamental period estimate: For a steel moment frame: T_1 = 0.085 x h_n^0.75 = 0.085 x 20^0.75 = 0.085 x 9.46 = 0.80 s

Since T_1 = 0.80 s is in the constant-velocity plateau (0.313 < 0.80 < 1.563), S_a(T_1) = 0.275g.

Step 6 — Structural system selection: Option A: Ordinary moment frame (OMF), mu = 2, Sp = 0.67 V_A = (0.275 x W_t) / 2 x 0.67 = 0.092 x W_t

Option B: Intermediate moment frame (IMF), mu = 3, Sp = 0.50 V_B = (0.275 x W_t) / 3 x 0.50 = 0.046 x W_t

Step 7 — Compare with wind: Wind base shear for a 20 m tall building in Region A2 (Newcastle, non-cyclonic): C_fig x C_dyn x p_z x A_proj — typically V_wind ~ 0.08-0.12 x W_t

Result: Wind base shear (0.08-0.12W) is comparable to the seismic base shear for OMF (0.092W) but significantly higher than for IMF (0.046W). For an OMF system, seismic may govern some elements; for IMF, wind governs. The seismic design category per Table 2.1 for Z = 0.11 and Importance Level 2 is Category D (moderate seismic). A static analysis per Section 7 is permitted for this regular structure under 50 m height in Category D.

Seismic Design Actions on Non-Structural Components — AS 1170.4 Section 8

Parts and components (facades, cladding, ceilings, services) must be designed for seismic actions even when the primary structure is wind-governed. The design horizontal force on a component:

F_ph = C_p(T_p) x a_p x W_p

Where:

For a rigid facade panel (T_p < 0.1 s) in an Adelaide building: C_p = 2.5 x kp x Z = 2.5 x 1.0 x 0.11 = 0.275 a_p = 2.5 (rigid component on a multi-storey building) F_ph = 0.275 x 2.5 x W_p = 0.688 x W_p — significant.

This is often the controlling load for lightweight cladding connections, even in low-seismic zones, and is frequently overlooked in Australian practice.

Frequently Asked Questions

What is the seismic hazard factor Z for Sydney, Melbourne, and Brisbane?

Per AS 1170.4:2007 Table 3.2: Sydney Z = 0.08 (Region 3), Melbourne Z = 0.08 (Region 3), Brisbane Z = 0.05 (Region 1). These correspond to a 500-year return period hazard on rock (Site Class Be). For comparison, Wellington NZ has Z = 0.40 — five times higher than Sydney. Most Australian capital cities except Adelaide, Canberra, and Newcastle fall into low-seismic categories, meaning seismic design rarely governs for buildings under 50 m height. However, the 1989 Newcastle earthquake (M5.6, 13 fatalities) demonstrated that even moderate-magnitude intraplate events can cause significant damage to unreinforced masonry and older structures.

When does AS 1170.4 seismic design govern over wind design in Australia?

Seismic design governs over wind design in Australia only for specific conditions: (1) buildings in high-hazard zones (Z > 0.15) — typically Meckering, Tennant Creek, or the Eastern Highlands seismic zone; (2) buildings exceeding 50 m height in moderate-hazard zones (Z > 0.09) — including Adelaide, Newcastle, and Canberra; (3) irregular structures with significant torsional response where the static wind analysis underestimates corner column forces; (4) structures on Site Class De or Ee (deep soft soils) where site amplification doubles or triples the bedrock hazard. In most Australian capital cities (Sydney, Melbourne, Brisbane, Perth), wind loads govern the lateral force-resisting system for buildings up to approximately 80 m height.

How does the AS 1170.4 site classification affect the seismic base shear?

Site classification directly scales the corner periods of the design spectrum. On Site Class De (F_s = 1.75), the constant-velocity plateau extends to T_2 = 2.188 s, compared to T_2 = 1.250 s on Class Be. For buildings with fundamental periods between 1.25 s and 2.19 s, the design spectral acceleration is effectively multiplied by 1.75 relative to the bedrock value. For Site Class Ee (F_s = 2.50), the amplification can reach 2.5x. A site with Z = 0.08 on rock (Be) becomes equivalent to Z_eff = 0.14 on Class De and Z_eff = 0.20 on Class Ee — moving the structure from low-seismic (Category E) to moderate-seismic (Category D) or even high-seismic (Category C) territory based on soil conditions alone. This is why site classification is the single most important parameter in Australian seismic design after the hazard factor Z.

What is the difference between the structural ductility factor mu and the response modification factor R in ASCE 7?

The AS 1170.4 ductility factor mu and ASCE 7 response modification factor R serve the same purpose — reducing elastic design forces to account for inelastic energy dissipation — but the numerical values differ. The approximate relationship is R = 1.5 x mu for steel systems. Typical comparisons: AS 1170.4 Ordinary Moment Frame mu = 2 (equivalent R ~ 3) vs ASCE 7 OMF R = 3.5; AS 1170.4 Intermediate Moment Frame mu = 3 (R ~ 4.5) vs ASCE 7 IMF R = 4.5; AS 1170.4 Special Moment Frame mu = 4 (R ~ 6) vs ASCE 7 SMF R = 8. The key difference is that AS 1170.4 applies an additional structural performance factor Sp/mu, which further reduces forces beyond the 1/mu reduction — this makes the Australian force levels somewhat lower than US practice for the same nominal ductility category, offset by more conservative detailing requirements per AS 4100 Section 13.

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This page is for educational reference only. Seismic hazard factors per AS 1170.4:2007 (Amdt 2:2018). Verify site-specific hazard and site classification with a qualified geotechnical engineer. All structural designs must be independently verified and certified by a licensed Professional Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.