Australian Wind Load Design — AS/NZS 1170.2:2021 Complete Guide

Comprehensive reference for wind load calculation on steel structures per AS/NZS 1170.2:2021 (Structural Design Actions — Wind Actions). Covers regional wind speeds V_R for all Australian wind regions A1 through D, terrain and height multipliers M_z,cat, shielding multiplier M_s, topographic multiplier M_t, net pressure coefficients C_p,n for steel portal frames, aerodynamic shape factor C_fig, dynamic response factor C_dyn, and a fully worked example for a steel industrial building in Region A2.

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Wind Load Framework

AS/NZS 1170.2:2021 (third edition, replacing AS 1170.2:2011 and AS/NZS 1170.2:2002) provides the definitive wind load methodology for all Australian and New Zealand structural design. The standard defines a multi-step calculation sequence that converts the regional gust wind speed into design wind pressures and forces on structural elements.

Core Wind Load Equations

Design wind speed (gust, at height z):

V_des,theta = V_R x M_d x M_z,cat x M_s x M_t

Where:

• V_R = regional 3-second gust wind speed at 10 m height in open terrain, annual probability of exceedance 1/R (m/s)
• M_d = wind direction multiplier (typically M_d = 1.0 for all directions, or 0.85 to 1.0 for specific directions where lower wind speeds are justified)
• M_z,cat = terrain and height multiplier accounting for terrain roughness and height above ground (dimensionless)
• M_s = shielding multiplier — accounts for reduced wind speeds due to upwind buildings (typically M_s = 1.0 for conservative design, may be less than 1.0 where permanent shielding can be demonstrated)
• M_t = topographic multiplier — accounts for speed-up over hills, ridges, and escarpments (M_t >= 1.0)

Design wind pressure (dynamic):

p = 0.5 x rho_air x (V_des,theta)^2 x C_fig x C_dyn

Where:

• rho_air = density of air = 1.2 kg/m^3
• C_fig = aerodynamic shape factor (accounts for both external and internal pressures)
• C_dyn = dynamic response factor (typically C_dyn = 1.0 for portal frames and low-rise buildings; > 1.0 for tall slender structures)

Aerodynamic shape factor:

C_fig = C_p,n x K_a x K_c x K_l x K_p

Where:

• C_p,n = net pressure coefficient (difference between external pressure C_p,e and internal pressure C_p,i)
• K_a = area reduction factor (reduces pressure for large tributary areas > 10 m^2)
• K_c = combination factor (accounts for non-simultaneous peak pressures on different surfaces, typically K_c = 0.90 for portal frames)
• K_l = local pressure factor (increases pressure for cladding and small areas, K_l typically 1.5 for edge zones)
• K_p = porous cladding factor (for permeable cladding, K_p = 1.0 for solid cladding)

Design wind force on a surface or member:

F_design = p x A_ref

Where A_ref is the reference area — typically the projected area normal to the wind direction for the structural element being designed.

Wind Regions of Australia

AS/NZS 1170.2 divides Australia into four principal wind regions based on the meteorological mechanism producing extreme winds:

For Importance Level 2 structures (normal buildings, the default category), the design wind speed corresponds to an annual probability of exceedance of 1/500 (i.e., 10% probability of exceedance in a 50-year design life). For Importance Level 3 (schools, hospitals, major assembly buildings) use 1/1,000. For Importance Level 4 (post-disaster facilities) use 1/2,000.

Terrain and Height Multiplier M_z,cat

The terrain-height multiplier accounts for the reduction in wind speed near the ground due to surface roughness (buildings, trees, topography) and the increase with height above ground. AS/NZS 1170.2 Table 4.1 provides M_z,cat values for four terrain categories:

For a typical steel portal frame industrial building with eave height h_e = 7.5 m in a suburban industrial estate (TC3), M_z,cat at the roof height (mean roof height z = 8.5 m for a 10-degree pitch) is approximately 0.83. This means the design wind speed at roof height is only 83% of the regional gust speed V_R, significantly reducing design wind pressures compared with open-terrain conditions.

Terrain Category Averaging Distance

The terrain category is determined by the upwind fetch in each wind direction. The averaging distance (typically 500 m to 1,000 m upwind) defines the terrain classification. Where upwind terrain changes within this distance (e.g., transitioning from suburban to open), an intermediate M_z,cat may be calculated by linear interpolation or by using the more conservative (lower) M_z,cat from the rougher terrain category.

Shielding Multiplier M_s

The shielding multiplier accounts for the reduction in wind speed experienced by a structure when it is sheltered by adjacent buildings of similar or greater size. Per AS/NZS 1170.2 Clause 4.4:

M_s = 1.0 (default — no shielding assumed)

M_s = 0.85 to 0.95 where permanent upwind obstructions exist within 20 obstruction-heights of the structure. The shielding buildings must exist at the time of construction and be expected to remain for the design life of the structure.

M_s = 1.0 must be used when: the upwind obstructions could be removed (vacant lots, car parks), the structure is taller than the shielding buildings, or the spacing between buildings exceeds 6 obstruction-heights.

For most industrial buildings on greenfield sites or isolated lots, M_s = 1.0 is the correct and conservative choice. Shielding should only be claimed where it is demonstrably permanent and documented in the design basis.

Topographic Multiplier M_t

The topographic multiplier accounts for the acceleration of wind flow over hills, ridges, and escarpments. Wind speed increases as flow lines converge over convex topography.

M_t = 1.0 for flat terrain or sites more than 5x the hill height from the crest.

M_t = 1.0 + 0.71 x (H / (2 x L_u)) x s for the crest of a two-dimensional ridge or escarpment, where H is the hill height, L_u is the horizontal distance upwind from the crest to the half-height point, and s is a position factor (s = 1.0 at the crest, decreasing downwind).

For a building located on a ridge with H = 30 m and L_u = 100 m, M_t = 1 + 0.71 x (30/200) x 1.0 = 1 + 0.107 = 1.11 at the crest. This 11% increase in design wind speed translates to a 23% increase in wind pressure (p is proportional to V^2), making topographic effects significant for hilltop sites. Site-specific M_t calculation is required for any site where the upwind slope exceeds 1:20 (5%).

Aerodynamic Shape Factor C_fig and Pressure Coefficients

Net Pressure Coefficient C_p,n for Portal Frames

For a steel portal frame building, AS/NZS 1170.2 Table 5.0(A) provides net pressure coefficients for the external surfaces. Combined with internal pressure from Clause 5.3, the net pressure coefficient for the frame is:

C_p,n = C_p,e (external) - C_p,i (internal)

Where C_p,i depends on building permeability:

• +0.2 for buildings with dominant openings on the windward wall (internal pressure)
• -0.3 for buildings with dominant openings on the leeward or side walls (internal suction)
• 0.0 or +/-0.2 for buildings with uniformly distributed permeability (the default assumption)

For a typical enclosed industrial building without dominant openings, use C_p,i = +0.2 and C_p,i = -0.3 (check both cases, as the worst case depends on the combination of external and internal pressures).

External Pressure Coefficients for Rectangular Enclosed Buildings

For a 10-degree roof pitch portal frame with h/d = 0.5, net pressure on the windward roof: C_p,n = -0.55 (external) — (+0.2) = -0.75 (internal pressure case, uplift) or C_p,n = -0.55 — (-0.3) = -0.25 (internal suction case). The uplift case with internal pressure governs roof design.

Area Reduction Factor K_a

Wind pressures reduce over large tributary areas because peak gusts do not act simultaneously over the entire surface. Per Table 5.2:

For a portal frame at 6 m bay spacing and 8 m eave height, the tributary area per frame is approximately 6 x 8 = 48 m^2 for the windward wall. K_a = 0.91 for the structural frame. For individual purlins (tributary area ≈ 6 x 1.8 = 10.8 m^2), K_a = 0.99, essentially no reduction.

Complete Worked Example: Steel Portal Frame in Region A2

Building Description

• Location: Western Sydney industrial estate (Region A2)
• Building dimensions: 24 m span x 42 m long x 8.0 m eave height, 10-degree roof pitch
• Bay spacing: 6.0 m (7 bays)
• Cladding: Insulated sandwich panel, solid, no dominant openings
• Importance Level: 2 (normal building)
• Terrain: Suburban industrial estate, TC3, upwind fetch > 500 m in all directions

Step 1 — Regional Wind Speed

Region A2, Importance Level 2: V_R = 37 m/s (annual probability of exceedance 1/500)

Step 2 — Direction Multiplier

M_d = 1.0 (conservative for all wind directions)

Step 3 — Terrain-Height Multiplier

Mean roof height: z = 8.0 + (tan 10° x 24/2) x 0.5 = 8.0 + (0.176 x 12) x 0.5 = 9.06 m ≈ 9.1 m

From AS/NZS 1170.2 Table 4.1 (interpolation), for TC3 at z = 9.1 m: M_z,cat = 0.84

Step 4 — Shielding and Topography

M_s = 1.0 (no permanent shielding upwind) M_t = 1.0 (flat site, gradient < 1:20)

Step 5 — Design Wind Speed

V_des,theta = 37 x 1.0 x 0.84 x 1.0 x 1.0 = 31.1 m/s

Step 6 — Design Wind Pressure

p = 0.5 x 1.2 x (31.1)^2 = 0.5 x 1.2 x 967 = 580 Pa = 0.58 kPa (base pressure)

Step 7 — Pressure Coefficients for the Frame

Building h/d ratio: height/depth = 8.0 / 24 = 0.33 (use h/d <= 0.5 values)

External pressures at 10° roof pitch:

Internal pressures: C_p,i = +0.2 (worst uplift case for roof), and C_p,i = -0.3 (check windward wall inward + leeward wall outward)

Net pressure coefficients C_p,n = C_p,e - C_p,i:

Step 8 — Area Reduction Factor

Tributary area per frame for the windward wall: A = 6.0 x 8.0 = 48 m^2. From Table 5.2, K_a = 0.91 for structural frames.

Roof tributary area per frame: A ≈ 6.0 x (12.0 / cos 10°) = 6.0 x 12.2 = 73 m^2. K_a = 0.89.

Dynamic response factor C_dyn = 1.0 (low-rise portal frame, natural frequency > 1 Hz). Combination factor K_c = 0.90 for portal frames. K_l = 1.0 (structural frame, not cladding).

Step 9 — Net Design Pressures on Frame

Step 10 — Member Forces and Frame Design

Windward wall line load on frame (column): w = 0.48 kPa x 6.0 m bay spacing = 2.88 kN/m vertical line load on the windward column. Total base shear per frame V = 2.88 x 8.0 = 23.0 kN per column.

Roof uplift line load: w_roof = 0.30 kPa (windward) x 6.0 m = 1.80 kN/m along the rafter (uplift). Combined with dead load (typically 0.15–0.25 kPa downward for roof sheeting + purlins), net rafter load may be upward in the wind load case. AS 4100 load combination 1.0G + Wu (wind uplift) must be checked.

Rafter design check: A 310UB32.0 rafter at 12.2 m slope length (from eave to ridge) subject to uplift of 1.80 - 0.9 x 0.20 (dead load, with phi = 0.9) = 1.62 kN/m net uplift. The rafter experiences combined bending (from wind uplift) and axial compression (from the portal frame thrust). The interaction check per AS 4100 Clause 8.4 verifies that the rafter capacity is adequate.

Internal Pressure Scenario: Dominant Opening

If the building has a large roller door on the windward wall that is likely to be open during a storm (or fail under wind pressure), a dominant opening scenario must be checked. Per Clause 5.3.2:

• Dominant opening on windward wall: C_p,i = +0.60 (max internal pressure)
• Net windward roof C_p,n = -0.45 - 0.60 = -1.05 (uplift nearly doubles)
• Net leeward wall C_p,n = -0.30 - 0.60 = -0.90 (suction on leeward wall increases significantly)

For buildings with large door openings facing the prevailing wind, internal pressure can be the governing load case for roof uplift and leeward wall cladding. Australian industrial buildings commonly have roller doors on multiple sides — check all door positions in both open and closed configurations.

Wind Load for Secondary Members (Purlins and Girts)

Secondary members (purlins, girts) are designed for wind loads using local pressure coefficients (K_l > 1.0) that account for higher pressures at edges, ridges, and corners:

Local pressure factor K_l for cladding:

• General area: K_l = 1.0
• Edge zones (within distance 'a' from edges, where a = min(0.2b, 0.2d, h)): K_l = 1.5
• Corner zones (within distance 'a' from corners on both edges): K_l = 2.0

For a building with a = min(0.2 x 42, 0.2 x 24, 8.0) = 4.8 m, purlins within 4.8 m of the gable ends are designed for edge zone pressures (50% higher than general area), and corner purlins at the gable-eave intersection for double the general area pressure. This edge-zone amplification is the reason gable-end purlins and their connections are often heavier than the general roof purlins.

Wind Load Combinations for AS 4100

Per AS/NZS 1170.0 Clause 4.2.2, the following factored wind load combinations must be checked per AS 4100:

1. 1.2G + Wu — Wind uplift with reduced dead load (Wu = 1.0 x wind action)
2. 1.2G + 1.5Q + Ws — Wind serviceability with live load (Ws = 0.6 x wind action)
3. 0.9G + Wu — Minimum dead load resisting uplift (stability check for foundations and hold-down bolts)
4. 1.2G + Wu + psi_c x Q — Wind with companion live load reduction (psi_c typically 0.4 for office/retail, 0.6 for storage)

The base plate hold-down bolt tension is typically governed by combination (3): 0.9G + Wu. The rafter mid-span bending may be governed by combination (1) when the wind uplift exceeds the dead load, producing upward bending opposite to the gravity load case.

Key Design Checks — Summary

Before finalising any Australian wind load design, verify:

• Regional wind speed V_R correctly selected from AS/NZS 1170.2 wind region map for the site.
• Terrain category M_z,cat verified by site inspection or aerial photography of upwind fetch.
• Shielding M_s only claimed where permanent and documented.
• Topographic M_t > 1.0 calculated if site slope exceeds 1:20 within 500 m upwind.
• Internal pressure C_p,i checked for both +0.2 and -0.3 cases (and dominant opening if applicable).
• Area reduction K_a applied correctly for structural frames vs cladding elements.
• Edge and corner zone local pressure factors K_l applied to cladding and purlins.
• Wind directionality M_d considered for sites where critical wind only occurs from certain sectors.
• Load combinations per AS/NZS 1170.0 applied with correct psi factors for wind.
• Foundation hold-down checked for the 0.9G + Wu combination (minimum dead load with full wind uplift).