Australian Wind Load — AS 1170.2:2021 Wind Actions for Steel Design

Complete reference for wind load calculation on steel structures per AS 1170.2:2021 (Structural Design Actions — Wind Actions). Regional wind speeds V_R for all Australian regions, terrain and height multipliers, shielding and topographic effects, net pressure coefficients C_p,n for steel portal frames, and a full worked example for a steel building in cyclonic and non-cyclonic regions.

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AS 1170.2 Wind Load Framework

AS 1170.2:2021 (the third edition, replacing AS 1170.2:2011 and AS 1170.2:2002) provides the wind load methodology for all Australian and New Zealand structural design. The standard defines:

Design wind speed: V_des,θ = V_R × M_d × M_z,cat × M_s × M_t

Design wind pressure (dynamic): p = 0.5 × ρ_air × V_des,θ²

Net pressure coefficient: C_p,n (accounts for both external and internal pressures)

Aerodynamic shape factor: C_fig = C_p,n × K_a (where K_a is the area reduction factor for large surfaces)

Design wind action: F_design = p × C_fig × A_ref (the net wind force on the structural element)

Wind Regions of Australia

AS 1170.2 divides Australia into four wind regions based on tropical cyclone risk:

Region Classification Geographical Extent V_R at 1:1000 years (m/s)
A1 Non-cyclonic — inland Most of mainland Australia (south of 26°S inland) 28-33 m/s
A2 Non-cyclonic — coastal Coastal fringe south of 26°S 33-37 m/s
A3 Non-cyclonic — Tasmania Tasmania and Bass Strait 33-37 m/s
A4 Non-cyclonic — far south Southern coastal fringe 33-36 m/s
B Intermediate Coastal Queensland north of 26°S to Rockhampton 45 m/s
C Tropical cyclone Queensland coast Rockhampton to Cooktown, NT coast 50 m/s
D Severe tropical cyclone Pilbara and Kimberley coast (WA) 55 m/s

Sydney and Melbourne are in Region A2 (non-cyclonic coastal) with V_R ≈ 33-37 m/s. Brisbane is in Region B (intermediate) with V_R = 45 m/s. Darwin is in Region C with V_R = 50 m/s. Port Hedland is in Region D with V_R = 55 m/s.

Important: AS 1170.2 uses the 1:1000-year return period for ultimate limit state (ULS) wind speeds, unlike ASCE 7 which uses 1:700 for risk category II or EN 1991-1-4 which uses 1:50-year basic speeds with a 1.5 load factor.

Wind Speed Multipliers

Terrain/Height Multiplier (M_z,cat)

The M_z,cat factor accounts for the variation of wind speed with height above ground and terrain roughness:

Height (m) TC1 (Open water, grassland) TC2 (Suburban, scattered trees) TC3 (Urban, dense vegetation) TC4 (City centre)
3 0.91 0.77 0.63 0.49
5 0.97 0.83 0.69 0.56
10 1.00 0.91 0.83 0.66
15 1.02 0.98 0.89 0.73
20 1.04 1.01 0.93 0.79
30 1.07 1.05 0.99 0.86
50 1.10 1.09 1.05 0.96
100 1.14 1.14 1.12 1.06

For a typical 6 m high steel portal frame building in suburban terrain (TC2): M_z,cat ≈ 0.83-0.87 at eaves height, increasing to approximately 1.0 at the ridge.

Directional Multiplier (M_d)

M_d accounts for the reduced probability of extreme wind from any given direction:

Region M_d
A (non-cyclonic) 0.95
B (intermediate) 0.95
C (cyclonic) 0.95
D (severe cyclonic) 0.95

The constant value of 0.95 reflects the statistical analysis of Australian wind direction data.

Shielding Multiplier (M_s)

M_s accounts for the reduction in wind speed due to upwind buildings and terrain features:

Shielding Category Obstruction M_s
No shielding Open flat terrain, no significant upwind obstructions 1.0
Light shielding Scattered low-rise buildings, trees 0.95
Moderate shielding Dense suburban development 0.90
Heavy shielding City centre, dense high-rise 0.85

For isolated steel buildings in open terrain, M_s = 1.0 (conservative — no credit taken). For buildings within industrial estates or suburban developments, M_s = 0.90-0.95 may be used where justified.

Topographic Multiplier (M_t)

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

Slope H/(2L_u) Crest Multiplier Lee-Slope Multiplier
0.0-0.1 1.0 1.0
0.2 1.08 0.95
0.3 1.15 0.90
0.4 1.20 0.85
0.5 1.25 0.80

For steel structures on level ground (most urban and industrial sites), M_t = 1.0. For buildings on exposed hilltops (e.g., communication towers on ranges, wind farms), M_t can be as high as 1.2-1.25, significantly increasing the design wind pressure.

Net Pressure Coefficients for Steel Portal Frames

External Pressure Coefficients (C_pe)

For rectangular enclosed buildings with pitched roofs (steel portal frames):

Walls (C_p,e):

Surface C_p,e (windward) C_p,e (leeward)
h/w = 0.25 0.7 -0.3
h/w = 0.5 0.7 -0.3
h/w = 1.0 0.7 -0.3
h/w ≥ 2.0 0.7 -0.3

Note: The leeward wall pressure for building with depth d (parallel to wind):

d/b C_p,e (leeward)
≤ 1 -0.3
2 -0.2
≥ 4 -0.1

Pitched Roofs (C_p,e):

Roof Pitch Windward Slope Leeward Slope
0° (flat) -0.65, -0.65 -0.65, -0.65
5° -0.9, -0.4 -0.5, -0.5
10° -1.0, -0.3 -0.5, -0.5
15° -0.7, +0.2 -0.5, -0.5
20° -0.3, +0.4 -0.5, -0.5
30° 0.0, +0.5 -0.4, -0.4
45° +0.3, +0.5 -0.3, -0.3

The two values for each condition represent the worst-case positive (pressure) and negative (suction) coefficients. For steel portal frames with typical 10-15° roof pitch, the windward roof experiences significant suction (-0.7 to -1.0) while the leeward roof experiences moderate suction (-0.5).

Internal Pressure Coefficients (C_p,i)

Building Porosity C_p,i (dominant openings windward) C_p,i (dominant openings leeward)
Enclosed (porosity < 0.5%) ±0.2 ±0.2
Partially enclosed (porosity 0.5-5%) +0.6 -0.3
Open (porosity > 5%) — —

For enclosed steel buildings with minimal openings (standard industrial portal frames), C_p,i = ±0.2. For buildings with large roller doors on the windward side during a storm, C_p,i = +0.6 (worst case internal pressure adding to roof uplift).

Local Pressure Factors

AS 1170.2 requires local pressure factors (K_l) for cladding and connections:

Zone Description K_l
General Main building surface 1.0
Edge Within 1 m of building edge 1.5
Corner Within 1 m of building corner 2.0
Ridge Ridge line of roof 2.0
Eaves Eaves line 1.5

Cladding elements and their fixings must be designed for the higher local pressures. For steel purlins and girts within 1 m of the edge or ridge, the local pressure factor of 1.5-2.0 significantly increases the design load.

Worked Example: Steel Portal Frame Wind Load

Problem: Calculate the wind load on a steel portal frame building in Brisbane (Region B).

Building data:

Step 1 — Wind speed multipliers:

M_d = 0.95 (Region B)

M_z,cat at eaves height (6 m) for TC2: Using linear interpolation between 5 m (0.83) and 10 m (0.91): M_z,cat(6 m) = 0.83 + (6-5)/(10-5) × (0.91-0.83) = 0.83 + 0.2 × 0.08 = 0.846

M_z,cat at ridge height (6 + 24/2 × tan(15°) = 6 + 3.22 = 9.22 m): M_z,cat(9.22 m) = 0.83 + (9.22-5)/(10-5) × (0.91-0.83) = 0.83 + 0.844 × 0.08 = 0.898

M_s = 0.95 (light shielding) M_t = 1.0 (level ground)

Step 2 — Design wind speed:

V_des,θ(eaves) = 45 × 0.95 × 0.846 × 0.95 × 1.0 = 34.3 m/s (123 km/h)

V_des,θ(ridge) = 45 × 0.95 × 0.898 × 0.95 × 1.0 = 36.4 m/s (131 km/h)

Step 3 — Design wind pressure:

Using ρ = 1.2 kg/m³ at 20°C:

p(eaves) = 0.5 × 1.2 × 34.3² = 0.5 × 1.2 × 1176 = 706 Pa = 0.71 kPa

p(ridge) = 0.5 × 1.2 × 36.4² = 0.5 × 1.2 × 1325 = 795 Pa = 0.80 kPa

Step 4 — Net pressure coefficients for 15° roof:

Windward roof: C_p,e = -0.7 (suction governs for structural design) Leeward roof: C_p,e = -0.5 (suction) Windward wall: C_p,e = +0.7 (pressure) Leeward wall: C_p,e = -0.3 (suction)

Internal pressure: C_p,i = ±0.2 (enclosed building)

Net pressure coefficients:

Step 5 — Design wind actions on frame:

Area per frame = 6 m bay spacing × 1 m height = 6 m²/m

Windward wall (at eaves): F = 0.71 kPa × 0.9 × 6 m = 3.83 kN/m horizontal Leeward wall (at eaves): F = 0.71 kPa × (-0.5) × 6 m = -2.13 kN/m horizontal Windward roof (at ridge): F = 0.80 kPa × (-0.9) × 6 m = -4.32 kN/m uplift normal to roof

Step 6 — Load combination for ULS (AS 1170.0 Combination 4a):

1.2G + ψ_l × Q + W_u

Total wind uplift on rafter ≈ 4.32 kN/m × cos(15°) ≈ 4.17 kN/m vertical uplift

This uplift must be combined with the reduced dead load (0.9G per AS 1170.0 Combination 5a for the worst-case net effect) and used as the design load for the rafter in uplift, bottom flange bracing, and column hold-down bolts.

Component and Cladding Wind Loads

For steel purlins, girts, and cladding, AS 1170.2 applies higher local pressures with area reduction factors:

p_net = 0.5 × ρ × V_des,θ² × C_p,n × K_a × K_l

Where K_a is the area reduction factor:

Tributary Area K_a
≤ 1.0 m² 1.0
10.0 m² 0.8
100.0 m² 0.6

For a roof purlin in Zone 2 (edge zone) with a tributary area of 12 m² (6 m span × 2 m spacing):

p_net = 0.71 × (-1.5 × 0.78) ≈ 0.83 kPa suction (local factor K_l = 1.5, area factor K_a ≈ 0.78)

This is significantly higher than the main frame pressure of 0.71 kPa — cladding element design must account for these local effects.

Frequently Asked Questions

What is the difference between AS 1170.2 wind regions A, B, C, and D?

Region A (non-cyclonic) covers most of Australia south of 26°S latitude and includes Sydney, Melbourne, Adelaide, and Perth. Region B (intermediate) covers coastal Queensland from just south of Rockhampton northward and the remainder of the NT coast. Region C (tropical cyclone) covers the Queensland coast from Rockhampton to Cooktown. Region D (severe tropical cyclone) covers the Pilbara and Kimberley coastline of Western Australia. The key parameter V_R (regional wind speed at 1:1000-year return period) ranges from 28 m/s (Region A1 inland) to 55 m/s (Region D).

How does AS 1170.2 wind load compare with ASCE 7?

The fundamental difference is the return period. AS 1170.2 uses a 1:1000-year return period for ULS wind speeds, while ASCE 7-22 uses 1:700-year for risk category II (700 mph gust). The Australian standard also uses 3-second gust speeds (not hourly mean like NBCC), making V_R directly comparable to ASCE 7 basic wind speeds. A Sydney V_R = 33-37 m/s (74-83 mph) maps to approximately ASCE 7 category II wind speeds of 110-120 mph (after accounting for the different terrain and topographic models). The pressure equation format is also different: AS 1170.2 uses dynamic pressure directly (0.5ρV² × C_fig) while ASCE 7 uses velocity pressure with gust effect factors.

When is a dynamic wind analysis required for Australian steel buildings?

AS 1170.2 requires dynamic analysis for buildings with a fundamental natural frequency below 1.0 Hz (period > 1.0 second). For typical steel portal frames and low-rise buildings up to 15 m height, the natural frequency is well above 1.0 Hz and static analysis is adequate. For tall steel buildings over 50 m, long-span roofs, or any structure with unusual flexibility, dynamic analysis should be considered. AS 1170.2 Appendix B provides the dynamic response factor for along-wind response, and Appendix C for cross-wind response of tall buildings.

What are the local pressure factors and when do they apply?

AS 1170.2 requires local pressure factors K_l = 1.5 (edge zones within 1 m of building edge), K_l = 2.0 (corner and ridge zones within 1 m of building corner or ridge), and K_l = 1.0 (general zones). These apply to cladding, roofing, and their fixings — but NOT to the main structural frame. For steel purlins and girts near building edges and ridges, the local factor of 1.5-2.0 typically governs the design. This is why edge-zone roof sheeting and its fasteners are often specified at closer centres than general-zone areas.

Related Pages

Educational reference only. Wind load methodology per AS 1170.2:2021. Verify regional wind speed, terrain category, and topographic conditions for your specific site. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification.