AS/NZS 1170.2:2021 Wind Load Guide — Australian Provisions

Complete reference guide to wind load determination for structural design per AS/NZS 1170.2:2021 (Structural Design Actions, Part 2: Wind Actions). Covers Australian wind regions, regional wind speeds VR, terrain/height multiplier Mz,cat, shielding Ms, topographic factor Mt, aerodynamic shape factor Cfig, dynamic response factor Cdyn, and a worked example for a steel portal frame building in Region A (Sydney).

Related pages: Australian Wind Load | AS 4100 Steel Design | Australian Seismic Design | Wind Load Calculator

AS/NZS 1170.2:2021 Wind Load Framework

Code Reference: AS/NZS 1170.2:2021, Sections 2-6

The Australian wind load standard uses a deterministic approach based on regional 3-second gust wind speeds (500-year return period for ultimate limit state, 1-year for serviceability). The design wind pressure is:

[ p = (0.5 \times \rho*{air}) \times [V*{des,\theta}]^2 \times C*{fig} \times C*{dyn} ]

Where:

• rho_air = 1.2 kg/m^3 (air density, Clause 4.3)
• V_des,theta = design wind speed for the direction theta (Clause 2.2)
• Cfig = aerodynamic shape factor (Clause 5)
• Cdyn = dynamic response factor (Clause 6)

And the design wind speed V_des,theta is:

[ V*{des,\theta} = V_R \times M_d \times (M*{z,cat} \times M_s \times M_t) ]

Where:

• VR = regional gust wind speed (3-s gust at 10 m in Terrain Category 2)
• Md = wind direction multiplier (1.0 unless site-specific)
• Mz,cat = terrain/height multiplier (Clause 4.2)
• Ms = shielding multiplier (Clause 4.3)
• Mt = topographic multiplier (Clause 4.4)

Step 1 — Regional Wind Speed VR (Section 3)

AS/NZS 1170.2 divides Australia into four wind regions based on meteorological data and tropical cyclone risk:

Australian Wind Regions (Figure 3.1(A))

Regional Wind Speeds — Selected Cities (Table 3.1)

For ultimate limit state (ULS): VR = VR,500 from Table 3.1 (500-year return period). For serviceability limit state (SLS): VR = VR,1 (1-year return) = approximately 0.75 x VR,500 for most non-cyclonic regions (Clause 2.5.2), or from published tables.

Step 2 — Wind Direction Multiplier Md (Section 3.4)

The wind direction multiplier accounts for the reduced probability of maximum wind from any particular direction:

For regions C and D, AS/NZS 1170.2 Table 3.2 provides mandatory wind direction multipliers. For regions A and B, Md = 1.0 may be used conservatively, or site-specific values from the wind direction tables may be applied to reduce design forces.

Conservative default: Md = 1.0 (all directions).

Step 3 — Terrain/Height Multiplier Mz,cat (Section 4.2)

Australian Terrain Categories (Clause 4.2.1)

The reference wind speed VR applies in Terrain Category 2 at 10 m height. Mz,cat converts this to the actual height and terrain:

[ M_{z,cat} = \text{Table 4.1 value for the given z and terrain category} ]

Selected Mz,cat Values (AS/NZS 1170.2:2021 Table 4.1)

Note: TC2 at 10 m has Mz,cat = 0.97 (not exactly 1.0 due to rounding and turbulence model refinements in 2021 edition). In the 2011 edition, Mz,cat was calibrated to exactly 1.0 at 10 m in TC2.

Step 4 — Shielding Multiplier Ms and Topographic Multiplier Mt (Sections 4.3–4.4)

Shielding Multiplier Ms

Ms accounts for the reduction in wind speed when the building is shielded by upwind obstructions of similar size:

• Ms = 1.0 for unshielded buildings (no obstructions within 20h upwind)
• Ms = 0.8–0.9 for partially shielded (some obstructions)
• Ms = 0.6–0.8 for fully shielded sites (surrounded by similar or taller buildings)

Ms values require justification by site inspection or wind tunnel study. The conservative default is Ms = 1.0.

Topographic Multiplier Mt

Mt accounts for speed-up over hills, ridges, and escarpments:

• Mt = 1.0 for flat terrain
• Mt = 1.1–1.3 for gentle hills and ridges (height H < 50 m, slope < 0.2)
• Mt = 1.3–1.7 for steep escarpments and ridges
• Mt = 1.7–2.0+ for very steep topography

Mt is calculated per Section 4.4 using: [ M_t = 1 + (M_h - 1) \times \left(1 - \frac{x}{L_2}\right) ]

Where Mh = 1 + (H x s / L1), s = topographic location factor, and L1, L2 are length scale factors from Table 4.2.

For a flat suburban site: Mt = 1.0.

Step 5 — Aerodynamic Shape Factor Cfig (Section 5)

Cfig combines external and internal pressure coefficients with area-reduction and local-pressure effects:

[ C*{fig} = C*{p,e} \times Ka \times K_c \times K_l \times K_p \quad \text{(external)} ] [ C{fig,i} = C_{p,i} \times K_c \quad \text{(internal)} ]

For most building design:

• Ka = area reduction factor (Clause 5.4.1): Ka = 1.0 for tributary area < 10 m^2, decreasing to 0.8 for areas > 100 m^2
• Kc = combination factor (Clause 5.4.2): Kc = 1.0 for individual surfaces, 0.8–0.9 when combining actions
• Kl = local pressure factor (Clause 5.4.3): Kl = 1.0–3.0 for edge/corner zones
• Kp = porous cladding factor (Clause 5.4.4): Kp = 1.0 for solid cladding

External Pressure Coefficients Cp,e (Table 5.2(A)–(C))

Rectangular enclosed building — Windward wall (Table 5.2(A)):

Leeward wall:

Side walls: Cp,e = -0.65 (suction).

Roof — flat or <10 degree pitch (Table 5.2(B)):

Internal Pressure Coefficients Cp,i (Table 5.1(A))

For a typical enclosed industrial building: Cp,i = +/-0.20.

Step 6 — Dynamic Response Factor Cdyn (Section 6)

For rigid structures (fundamental natural frequency >= 1 Hz), Cdyn may be taken as:

[ C_{dyn} = 1.0 ]

Most steel-framed buildings under 30 m in height satisfy the rigid criterion. For taller or more flexible structures, Cdyn must be calculated using the gust factor method in Section 6.2, which accounts for:

• Turbulence intensity at the site
• Size reduction factor (averaging of pressures over the building surface)
• Resonant amplification when n1 is close to the wind gust energy peak

Step 7 — Worked Example: Steel Portal Frame in Western Sydney

Given:

• Location: Penrith, NSW (Region A2)
• VR = 43 m/s (ULS, 500-year return)
• Building: 30 m wide x 18 m deep x 7.5 m eaves height
• Roof pitch: 5 degrees (flat roof provisions apply)
• Terrain Category: TC3 (established industrial area)
• Importance Level: 2 (normal building, Clause 2.5.3)
• Enclosed, uniformly distributed openings
• Flat terrain, unshielded: Ms = 1.0, Mt = 1.0

Design Wind Speed

[ V*{des,\theta} = V_R \times M_d \times (M*{z,cat} \times Ms \times M_t) ] [ V{des} = 43 \times 1.0 \times (M_{z,cat} \times 1.0 \times 1.0) ]

At mean roof height z = h = 7.5 m, TC3:

From Table 4.1: at 7 m, Mz,cat = 0.80; at 10 m, Mz,cat = 0.85. Interpolating at 7.5 m: [ M_{z,cat} = 0.80 + (7.5 - 7.0) \times \frac{0.85 - 0.80}{10 - 7} = 0.80 + 0.5 \times 0.0167 = 0.81 ]

[ V_{des} = 43 \times 0.81 = 34.8 \text{ m/s} ]

Design Wind Pressure

[ p = 0.5 \times 1.2 \times (34.8)^2 \times C*{fig} \times C*{dyn} ] [ p = 0.5 \times 1.2 \times 1,211 \times C*{fig} \times 1.0 = 727 \times C*{fig} \text{ Pa} = 0.727 \times C_{fig} \text{ kPa} ]

Surface Pressures

h/d = 7.5/18 = 0.42.

Windward wall:

Interpolating Cp,e at h/d = 0.42 between 0.70 (h/d = 0.25) and 0.80 (h/d = 1.0): Cp_e = 0.70 + (0.42 - 0.25) x (0.80 - 0.70) / (1.0 - 0.25) = 0.70 + 0.17 x 0.133 = 0.72

With Ka = 1.0, Kc = 1.0, Kl = 1.0, Kp = 1.0: Cfig,e = 0.72

Internal: Cp,i = +/-0.20, Kc = 1.0: Cfig,i = +/-0.20

Net Cfig = 0.72 — (-0.20) = 0.92 (windward + internal suction), or Cfig = 0.72 — 0.20 = 0.52.

Governing net Cfig (windward): = 0.92

[ p_w = 0.727 \times 0.92 = 0.67 \text{ kPa} ]

Leeward wall:

h/d = 0.42 → Cp,e = -0.31 (interpolated between -0.30 and -0.50)

Net Cfig (with +Cpi): -0.31 — 0.20 = -0.51 Net Cfig (with -Cpi): -0.31 — (-0.20) = -0.11

Governing: -0.51 [ p_l = 0.727 \times (-0.51) = -0.37 \text{ kPa} ]

Side wall: Cp,e = -0.65 Net Cfig (with +Cpi): -0.65 — 0.20 = -0.85 [ p_s = 0.727 \times (-0.85) = -0.62 \text{ kPa} ]

Windward roof: Cp,e = -0.90 Net Cfig: -0.90 — 0.20 = -1.10 [ p_{r} = 0.727 \times (-1.10) = -0.80 \text{ kPa} ]

Total Frame Shear (Portal Frame at 6 m Centres)

Windward wall: Fw = 0.67 x 7.5 x 6 = 30.2 kN Leeward wall: Fl = 0.37 x 7.5 x 6 = 16.7 kN Total frame base shear: 46.9 kN per portal frame.

Roof uplift tributary per frame: Fu = 0.80 x 9 m (roof half-width) x 6 = 43.2 kN per frame (uplift).

Comparison: AS/NZS 1170.2 vs ASCE 7-22 and EN 1991-1-4

Frequently Asked Questions

When should I use Terrain Category 2.5 instead of TC3?

TC2.5 was introduced in AS/NZS 1170.2:2021 to fill the gap between open farmland (TC2) and established suburban (TC3). Use TC2.5 for low-density residential areas with scattered trees, fringe urban development, or a mix of open land with isolated 1-2 storey buildings within a ~500 m upwind fetch. TC3 should be used only when the upwind terrain is consistently built-up with obstructions 3-5 m high for at least 500 m. Misclassifying a TC2.5 site as TC3 unconservatively reduces Mz,cat by 5–8%, which can materially affect lateral bracing design.

How do I handle wind directionality for a rectangular building in Region A (non-cyclonic)?

For non-cyclonic regions, AS/NZS 1170.2 Table 3.2 provides wind direction multipliers Md by 45-degree sector. For Sydney (Region A2), Md = 0.80 for westerly winds and 0.95 for southerly winds, with the maximum Md in any sector = 1.00. The conservative approach is Md = 1.0 applied to all directions, which eliminates the need to check 8 wind directions separately. However, for a building where the long face is normal to the direction with the lowest Md, applying direction-specific Md can reduce structural demand by 15–20%. This is permissible but must be documented in the design basis.

What is the effect of shielding on portal frame design?

Shielding from upwind buildings can reduce the effective wind speed by 10–40% (Ms = 0.6–0.9). However, AS/NZS 1170.2 Clause 4.3.2 places strict conditions on shielding: the upwind shielding buildings must be of similar or greater height and must remain for the design life of the building, and the shielding must be effective for the full width of the subject building. For industrial estates where upwind buildings may be demolished, the conservative Ms = 1.0 is standard practice unless a covenant or plan of subdivision guarantees permanent shielding. Shielding is rarely relied upon in structural design unless documented with a long-term site condition report.

Why is the Australian standard wind speed for Sydney (43 m/s) apparently lower than the ASCE 7 value?

The numbers are not directly comparable because they use different reference periods: VR,Sydney = 43 m/s is a 500-year return period 3-second gust, while V_ASCE7 for an equivalent location might be 54 m/s (3,000-year return period, ultimate). When adjusted to the same return period, the underlying wind climate is similar. Furthermore, the Australian standard uses an explicit importance factor (Clause 2.5.3) and return-period factor that is embedded differently in the ASCE 7 wind map. The design wind pressure p — not the reference wind speed — is the correct point of comparison between codes.

What changed in the 2021 edition compared to AS/NZS 1170.2:2011?

Key changes in the 2021 edition include: (a) introduction of Terrain Category 2.5 for intermediate terrain, (b) updated wind speed maps based on an additional 10 years of meteorological data (2011–2020), raising VR in some Region B coastal areas by 2-3 m/s, (c) revised Mz,cat values including a small reduction in TC2 at 10 m (from 1.00 to 0.97 due to improved turbulence modelling), (d) introduction of the porous cladding factor Kp, (e) enhanced provisions for solar panels and rooftop equipment, and (f) revised dynamic response procedure Cdyn Section 6 with updated turbulence spectral models. Always confirm the governing edition with the local building certifier or authority.

Reference only. Verify all values against the current edition of AS/NZS 1170.2:2021 Structural Design Actions — Wind Actions and any state/territory variations. This guide does not constitute professional engineering advice and must be independently verified by a licensed Professional Engineer for the specific project location, terrain, and exposure conditions.