In structural engineering, understanding and accurately calculating wind loads is crucial for designing buildings and structures that can withstand the forces imposed by wind. The Australian Standard and New Zealand Standard AS/NZ 1170.2 (2021) provide guidelines for determining wind loads on structures.

This article aims to provide structural engineers with a detailed guide on how to calculate wind loads as per the latest standards, including a step-by-step calculation example.

Understanding AS/NZ 1170.2 (2021)

AS/NZ 1170.2 (2021) is the Australian and New Zealand Standard that specifically addresses wind loads on structures. It outlines the procedures and methodologies for determining the wind loads on various structures. The standard provides a comprehensive approach that considers factors such as terrain, topography, and the characteristics of the structure itself.

This Standard aims to provide wind actions for use in the design of structures subject to wind action. It provides a detailed procedure for determining wind actions on structures, ranging from those less sensitive to wind action to those for which dynamic responses are to be taken into consideration.

AS/NZS 1170.2:2021 is the latest iteration of this crucial standard, superseding the 2011 amendments. The 2021 revision’s objectives are to remove ambiguities and incorporate recent research and experiences from recent severe wind events in Australia and New Zealand.

This Standard is Part 2 of the Structural design actions series, which comprises the following parts:

• AS/NZS 1170.0, Structural design actions, Part 0: General principles
• AS/NZS 1170.1, Structural design actions, Part 1: Permanent, imposed, and other actions
• AS/NZS 1170.2, Structural design actions, Part 2: Wind actions
• AS/NZS 1170.3, Structural design actions, Part 3: Snow and ice actions
• AS 1170.4, Structural design actions, Part 4: Earthquake actions in Australia
• NZS 1170.5, Structural design actions, Part 5: Earthquake actions — New Zealand

The Standard covers structures within the following criteria:

1. Buildings and towers less than or equal to 200 m high.
2. Structures with unsupported roof spans of less than 100 m.
3. Offshore structures within 30 km from the nearest coastline.
4. Other structures apart from: offshore structures more than 30 km from the nearest coastline, bridges, windfarm structures, and power transmission and distribution structures, including supporting towers and poles.

It’s important to note that AS/NZS 1170.2:2021 is referenced by the National Construction Code (NCC) 2022, specifically within Volume 2 (Building Code of Australia). This code forms the regulatory backbone for building design and construction across Australia.

By adhering to the wind load calculation methods outlined in AS/NZS 1170.2:2021, designers and engineers ensure their projects meet the minimum safety and performance requirements stipulated by the NCC 2023. This connection emphasizes the critical link between accurate wind load assessment and compliance with national building regulations.

Procedure for Calculating Wind Actions

The procedure for determining wind actions (W) on structures and elements of structures or buildings shall be as follows:

a) Determine site wind speeds (see AS/NZS 1170.2 Clause 2.2).

The site wind speeds (Vsit,β) defined for the 8 cardinal directions (β) at the reference height (z) above ground (see Figure 2.1) shall be calculated from Equation 2.2:

$$V_{sit,\beta}=V_RM_cM_d(M_{z,cat}M_sM_t)$$

Where:

• $V_R$ = regional gust wind speed, in metres per second, for average recurrence interval of R years, as given in AS/NZS 1170.2 Section 3
• $M_c$ = climate change multiplier, as given in AS/NZS 1170.2 Section 3
• $M_d$ = wind directional multipliers for the 8 cardinal directions (β) as given in Section 3
• $M_{z,cat}$ = terrain/height multiplier, as given in AS/NZS 1170.2 Section 4
• $M_s$ = shielding multiplier, as given in AS/NZS 1170.2 Section 4
• $M_t$ = topographic multiplier, as given in AS/NZS 1170.2 Section 4

Generally, the wind speed is determined at the average roof height (h). In some cases, this varies according to the structure.

b) Determine design wind speed from the site wind speeds (see AS/NZS 1170.2 Clause 2.3).
c) Determine design wind pressures and distributed forces (see AS/NZS 1170.2 Clause 2.4).
d) Calculate wind actions (see AS/NZS 1170.2 Clause 2.5).

Key Components of Site Wind Speed Calculation

Regional Wind Speed ($V_R$)

Determining the regional wind speed ($V_R$) is the first step in calculating wind load.

AS/NZ 1170.2 provides wind speed maps based on the structure’s location. Engineers must identify the site’s wind region and terrain category to obtain the corresponding regional wind speed.

The AS/NZS 1170.2:2021 standard classifies wind regions as cyclonic and non-cyclonic based on wind event likelihood and intensity. Cyclonic regions in Australia experience higher wind speeds and complex patterns due to tropical cyclones, while non-cyclonic regions have lower, more consistent wind speeds influenced by other meteorological systems. This classification is important for determining the appropriate design wind speed for structures in different locations.

As per Clause 3.2 of AS/NZS 1170.2, Regional wind speeds ($V_R$) for all directions based on peak gust wind data shall be as given in Table 3.1(A) or Table 3.1(B) for the regions shown in Figure 3.1(A) and Figure 3.1(B) where $R$ (average recurrence interval) is the average time interval between exceedances of the wind speed listed.

In Region C, ($V_R$) values shall be obtained by linear interpolation between the value given for Region C (maximum) and the value given for Region B2 for the same $R$, according to the distance from the smoothed coastline.

In Region D, ($V_R$) values shall be obtained by linear interpolation between the value given for Region D (maximum) and the value given for Region C (maximum) for the same $R$, according to the distance from the smoothed coastline.

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Wind Direction Multiplier ($M_d$)

The wind direction multiplier ($M_d$) for all regions shall be as given in Table 3.2(A) or Table 3.2(B) as per AS/NZS 1170.2. However, for the following cases, ($M_d$) shall be taken as 1.0:
a) structures such as chimneys, tanks, and poles with circular or polygonal cross-sections
b) cladding and immediate supporting structures (as defined in Clause 5.4.4) on buildings in Regions B2, C, and D.

In regions where the prevailing wind directions vary with wind speed, wind direction multipliers have been calculated for the higher wind gusts (i.e., those associated with ultimate limit states design).

Climate Change Multiplier ($M_c$)

The climate change multiplier ($M_c$) shall be as given in Table 3.3 for regions shown in Figure 3.1(A) and Figure 3.1(B).

Terrain/height multiplier ($M_{z,cat}$)

As per Clause 4.2.1 in AS/NZS 1170.2, terrain over which the approach wind flows towards a structure shall be assessed on the basis of the following category descriptions:

• Terrain Category 1 (TC1) — Very exposed open terrain with very few or no obstructions and all water surfaces (e.g., flat, treeless, poorly grassed plains; open ocean, rivers, canals, bays, and lakes).
• Terrain Category 2 (TC2) — Open terrain, including grassland, with well-scattered obstructions having heights generally from 1.5 m to 5 m, with no more than two obstructions per hectare (e.g., farmland and cleared subdivisions with isolated trees and uncut grass).
• Terrain Category 2.5 (TC2.5) — Terrain with some trees or isolated obstructions, terrain in developing outer urban areas with scattered houses, or large acreage developments with more than two and less than 10 buildings per hectare.
• Terrain Category 3 (TC3) — Terrain with numerous closely spaced obstructions having heights generally from 3 m to 10 m. The minimum density of obstructions shall be at least the equivalent of 10 house-size obstructions per hectare (e.g., suburban housing, light industrial estates, or dense forests).
• Terrain Category 4 (TC4) — Terrain with numerous large, high (10 m to 30 m tall) and closely spaced constructions, such as large city centres and well-developed industrial complexes.

Selection of the terrain category shall be made with due regard to the permanence of the obstructions that constitute the surface roughness.

The variation with height ($z$) of the effect of terrain roughness on wind speed (terrain and structure height multiplier, $M_{z,cat}$, shall be taken from the values for fully developed profiles given in Table 4.1.

When the upwind terrain varies for any wind direction, an averaging of terrain-height multipliers shall be adopted. The terrain-height multiplier, $M_{z,cat}$, shall be taken as a weighted average over an averaging distance, xa, depending on the height, z. For more information on this, see Clause 4.2.3 of AS 1170.2 (2021).

Shielding Multiplier ($M_s$)

Shielding may be provided by upwind buildings or other structures. The shielding multiplier accounts for the influence of nearby structures on wind flow. It considers the size, shape, and distance of adjacent buildings and structures. Engineers need to assess the surroundings to determine the appropriate shielding multiplier.

Shielding shall not be provided by trees or vegetation. An upwind building shall not be used to provide shielding on a slope with a gradient that is greater than 0.2, unless its overall height above a common datum, such as mean sea level, exceeds that of the subject building.

‍The shielding multiplier ($M_s$) that is appropriate to a particular direction shall be as given in Table 4.2 as per AS1170.2 for structures with h ≤ 25 m in height (h is defined in Figure 2.1).

The shielding multiplier shall be 1.0 for structures with h greater than 25 m, where the effects of shielding are not applicable for a particular wind direction, or are ignored. Only buildings within a 45° sector of radius 20h (symmetrically positioned about the directions being considered) with a height greater than or equal to h shall be used to provide shielding.

Where the average upwind ground gradient between the structure in question and the upwind structure is greater than 0.2, the upwind building shall not be treated as a shielding building.

The shielding parameter is determined below as per Clause 4.3.3 in AS1170.2:

$$S = \sqrt{\frac{l_s}{h_sb_s}}$$

Where:

• $l_s$ = average spacing of shielding buildings, given by Equation 4.3(2) AS1170.2: $l_s=h(\frac{10}{n_s}+5)$
• $h_s$ = average roof height of shielding buildings
• $b_s$ = average breadth of shielding buildings, normal to the wind stream
• $h$ = average roof height, above ground, of the structure being shielded
• $n_s$ = number of upwind shielding buildings within a 45° sector of radius 20h and with hs ≥ h

Topographic Multiplier ($M_t$)

Topography can significantly influence wind flow patterns. The topographic multiplier considers the impact of hills, slopes, and other terrain features. Engineers must assess the site’s topography to determine the appropriate topographic factor.

As per AS1170.2 (2021) Clause 4.4.1, The topographic multiplier (Mt) shall be taken as follows:
For sites in Regions A4, NZ1, NZ2, NZ3 and NZ4 over 500 m above sea level, use Equation 4.4(1):
$$M_t=M_hM_{lee}(1+0.00015E)$$
Where:

• $M_h$ = hill shape multiplier
• $M_{lee}$ = lee (effect) multiplier (taken as 1.0, except in New Zealand lee zones, see Clause 4.4.3)
• $E$ = site elevation above mean sea level, in metres

For sites in Region A0, use Equation 4.4(2):
$$M_t=0.5+0.5M_h$$

Elsewhere, the larger value of the following:
$$M_t = M_h$$
$$M_t = M_{lee}$$

Hill-shape Multiplier ($M_h$)

The hill-shape multiplier shall be taken as 1.0 outside of the local topographic zones shown in
Figures 4.3 to 4.5, and for H < 10 m.

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Within the local topographic zones, the hill shape multiplier ($M_h$) shall be assessed for each cardinal direction considered, taking into account the most adverse topographic cross-section that occurs within the range of directions within 22.5° on either side of the cardinal direction being considered.

The values shall be as follows:

a) For $\frac{H}{2L_u} < 0.05, Mh = 1.0$

b) For $0.05 ≤ \frac{H}{2L_u} ≤ 0.45$ (see Figures 4.3 and 4.4), use Equation 4.4(3):
$$Mh=1+\frac{H}{3.5(z+L_1)}(1-\frac{|x|}{L_2})$$

c) For$\frac{H}{2L_u} > 0.45$ (see Figure 4.5):

Within the rectangular peak zone (see Figure 4.5), use Equation 4.4(4):
$$M_h=1+0.71(1-\frac{|x|}{L_2})$$

Elsewhere within the local topographic zone (see Figures 4.3 and 4.4), Mh shall be as given in Equation 4.4(3).
$$M_h=1+\frac{H}{3.5(z+L_1)}(1-\frac{|x|}{L_2})$$

where

• $H$ = height of the hill, ridge or escarpment
• $L_u$ = horizontal distance upwind from the crest of the hill, ridge or escarpment to a level half the height below the crest
• $x$ = horizontal distance upwind or downwind of the structure to the crest of the hill, ridge, or escarpment
• $L_1$ = length scale, to determine the vertical variation of Mh, to be taken as the greater of 0.36 Lu or 0.4 H
• $L_2$ = length scale, to determine the horizontal variation of Mh, to be taken as 4 L1 upwind for all types, and downwind for hills and ridges, or 10 L1 downwind for escarpments
• $z$ = reference height on the structure above the average local ground level

For the case where x and z are zero, the value of $M_h$ is given in Table 4.3.

Irrespective of the provisions of this Clause, the influence of any peak may be ignored, provided the crest is distant from the site of the structure by more than 10 times its crest elevation above sea level, and any intervening valley is more than 10 times the distance of the valley floor below the crest.

Lee Multiplier ($M_{lee}$)

The lee (effect) multiplier (Mlee) shall be evaluated for New Zealand sites in the lee zones, as shown in Figure 4.6. For all other sites, the lee multiplier shall be 1.0. Within the lee zones, the lee multiplier shall apply only to wind from the cardinal directions nominated in Table 4.4.

Each lee zone shall extend by the distance as specified in Table 4.4, this distance is measured from the leeward crest of the initiating range, downwind in the direction of the wind nominated.

The lee zone comprises:

• a shadow lee zone, which extends from the crest of the initiating range (the upwind boundary of the lee zone);
• an outer lee zone over the remainder of the lee zone; and
• lateral transition zones, which extend from the lateral edges of the shadow zone by x/4, where x is the distance from the initiating crest along the edge of the shadow zone and by R/4 for the lateral edges of the outer zone, where R is the distance from the initiating crest to the leeward edge of the shadow zone. The coordinates of the initiating crests are shown in Table 4.5.

The lee multiplier for shadow zones shall be as specified in Table 4.4. Within the outer lee zone, the lee multiplier shall be determined by linear interpolation with horizontal distance, from the shadow/outer zone boundary (where Mlee is from Table 4.4) to the downwind boundary of the outer zone (where Mlee = 1.0). Within the lateral transition zone, the lee multiplier shall be determined by linear interpolation along a line parallel to the crest from the value at the point at the lateral edge of the shadow and outer zones to a value of 1.0 at the far edge of the lateral zone.

Design Wind Speed Calculation ($V_{des,\theta}$)

As per Section 2.3 of AS/NZS 1170.2, the building orthogonal design wind speeds ($V_{des,\theta}$) shall be taken as the maximum cardinal direction site wind speed ($V_{sit,\beta}$) linearly interpolated between cardinal points within a sector ± 45° to the orthogonal direction being considered (see Figures 2.2 and 2.3).

That is, Vdes,θ equals the maximum value of site wind speed ($V_{sit,\beta}$) in the range [β = θ ± 45°] where $\beta$ is the cardinal direction clockwise from true North and $θ$ is the angle to the building orthogonal axes. In cases such as walls and hoardings and lattice towers, where an incident angle of 45° is considered, $V_{des,\theta}$ shall be the maximum value of $V_{sit,\beta}$ in a sector ± 22.5° from the 45° direction being considered. For ultimate limit states design, $V_{des,\theta}$ shall not be less than 30 m/s.

A conservative and common approach is to design the structure using the wind speed and multipliers for the worst direction. For example, for a building on an escarpment, it may be easily checked whether the $V_R M_c M_d (M_{z,cat} M_s M_t)$ on the exposed face (towards the escarpment) is the worst case. To simplify design, this value could then be used as the design wind speed for all directions on the building.

Design Wind Pressure Calculation ($p$)

The design wind pressures §, in pascals, shall be determined for structures and parts of structures as per the below.
$$p=0.5p_{air}{V_{des,\theta}}^{2}C_{shp}C_{dyn}$$

where

• $p$ = design wind pressure in pascals = pe, pi or pn where the sign is given by the Cp values used to evaluate Cshp
• $ρ_{air}$ = density of air, which shall be taken as 1.2 kg/m3
• $V_{des,\theta}$ = building orthogonal design wind speeds (usually, θ = 0°, 90°, 180°, and 270°), as given in Clause 2.3 of AS/NZS 1170.2
• $C_{shp}$ = aerodynamic shape factor, as given in Section 5 of AS/NZS 1170.2
• $C_{dyn}$ = dynamic response factor, as given in Section 6 of AS/NZS 1170.2 (the value is 1.0 except where the structure is dynamically wind sensitive [see Section 6])

Aerodynamic Shape Factor ($C_{shp}$)

Section 5 of AS/NZS 1170.2 shall be used to calculate the aerodynamic shape factor ($C_{shp}$) for structures or parts of structures. Values of $C_{shp}$ shall be used to determine the pressures applied to each surface.

For calculating pressures, the sign of $C_{shp}$ indicates the direction of the pressure on the surface or element (see Figure 5.1), positive values indicate pressure acting towards the surface, and negative values indicate pressure acting away from the surface (less than ambient pressure, i.e., suction). The wind action effects used for design shall be the sum of values determined for different pressure effects, such as the combination of internal and external pressure on enclosed buildings.

Clauses 5.3, 5.4, and 5.5 provide values for enclosed rectangular buildings. Methods for other types of enclosed buildings, exposed members, lattice towers, free walls, free roofs, and other structures are given in Appendices A to E of AS/NZS 1170.2.

As per Section 5.2 of AS1170.2 (2021), the aerodynamic shape factor ($C_{shp}$) shall be determined for specific surfaces or parts of surfaces as for enclosed buildings as follows:
$C_{shp}=C_{p,i}K_{c,i}K_v$ for internal pressures
$C_{shp}=C_{p,e}K_aK_{c,i}K_lK_p$ for external pressures
$C_{shp}=C_fK_aK_{c,e}$ for frictional drag forces

Where the parameters are found as per Tables 5.1-5.9 in AS1170.2 (2021)

• $C_{p,e}$ = external pressure coefficient
• $C_{p,i}$ = internal pressure coefficient
• $C_f$ = frictional drag force coefficient
• $C_{p,n}$ = net pressure coefficient acting normal to the surface for canopies, freestanding roofs, walls, and the like
• $K_a$ = area reduction factor
• $K_c$ = combination factor
• $K_{c,e}$ = combination factor applied to external pressures
• $K_{c,i}$ = combination factor applied to internal pressures
• $K_ℓ$ = local pressure factor
• $K_v$ = open area/internal volume factor for internal pressures
• $K_p$ = porous cladding reduction factor

Aerodynamic shape factors for circular bins, silos, and tanks, as well as freestanding walls, hoardings, or roofs can be found in Section 5.2 of AS1170.2 (2021).

Dynamic Response Factor ($C_{dyn}$)

A dynamic response factor ($C_{dyn}$) shall be taken to equal 1.0 for the following cases:

• Buildings and free-standing towers, where the natural frequency of the first mode of vibration is greater than 1 Hz.
• Poles and chimneys with height to average diameter aspect ratio less than 5.
• Ground-mounted solar panels with natural frequencies greater than 5 Hz.

Most basic building structures have a $C_{dyn}$ of 1.0. For more complex structures, refer to Section 6 of AS/NZS 1170.2.

Wind Actions ($F$)

As previously discussed, the design wind pressures ($p$), in pascals, shall be determined for structures and parts of structures as per the below.

$$p=0.5p_{air}{V_{des,\theta}}^{2}C_{shp}C_{dyn}$$

We have discussed how to find each of these factors.

To determine wind actions, the forces ($F$) in newtons, on surfaces or structural elements, such as a wall or a roof, shall be the vector sum of the forces calculated from the design wind pressures applicable to the assumed areas ($A$).

Example Calculating Wind Loads By Hand

Let’s assume we have a residential building in the south-eastern suburbs of Melbourne that is 25m x 15m in plan view and 15m high, aligned lengthways with north, with a 22.5 degree pitched gable roof, with the largest opening being the garage at the front of the house. To be conservative we will consider there are no shielding buildings or hills. Let’s calculate the external wind force.

We first determine the site windspeed to be:

$$V_{sit,\beta}=V_RM_cM_d(M_{z,cat}M_sM_t)=451.01.00.891.0*1.0=40.05m/s$$

Where:

• $V_R$ = 45m/s regional gust wind speed, for an average recurrence interval of 500 years for region A5 (Melbourne), as given in AS/NZS 1170.2 Section 3
• $M_c$ = 1.0 climate change multiplier for region A5 (Melbourne), as given in AS/NZS 1170.2 Section 3 and Appendix F of AS1170.0
• $M_d$ = 1.0 most severe wind directional multipliers for the 8 cardinal directions (β) as given in Section 3
• $M_{z,cat}$ = 0.89 terrain/height multiplier, for Terrain Category 3 (closely spaced buildings) for a 15m high building as given in AS/NZS 1170.2 Section 4
• $M_s$ = 1.0 shielding multiplier, most conservative value as given in AS/NZS 1170.2 Section 4
• $M_t$ = 1.0 topographic multiplier (no nearby hills), as given in AS/NZS 1170.2 Section 4

Next, we will determine the design wind speed from the site wind speeds (see AS/NZS 1170.2 Clause 2.3).

To be conservative we assume the design wind speed to equal the site wind speed.

Determine design wind pressures and distributed forces (see AS/NZS 1170.2 Clause 2.4).

$$p=0.5p_{air}{V_{des,\theta}}^{2}C_{shp}C_{dyn} =0.51.240.05^20.761.0=731.43pa$$

where

• $p$ = design wind pressure in pascals = pe, pi or pn where the sign is given by the Cp values used to evaluate Cshp
• $p_{air}$ = density of air, which shall be taken as 1.2 kg/m3
• $V_{des,\theta}$ = building orthogonal design wind speeds (usually, θ = 0°, 90°, 180°, and 270°), as given in Clause 2.3 of AS/NZS 1170.2
• $C_{shp}$ = aerodynamic shape factor, factors found as given in Section 5 of AS/NZS 1170.2Cshp=Cp,eKaKc,iKlKp for external pressuresCshp=Cp,eKaKc,iKlKp =0.80.951.01.01.0=0.76
• $C_{dyn}$ = 1.0 dynamic response factor, as given in Section 6 of AS/NZS 1170.2 (the value is 1.0 except where the structure is dynamically wind sensitive [see Section 6])

Next, we will determine the wind actions (see AS/NZS 1170.2 Clause 2.5).

The area of the largest wall is 15m x 25m = 375sqm. Therefore, the external wind load on the building is:

$$F=pA=731.43pa*375sqm=274,284N=274.29kN$$

It’s important to note that this example only considers the external wind force on the largest wall. There are many other wind calculations that need to be performed considering internal pressure, the roof, and the other walls, which can all be found by following this same procedure.

Example Calculating Wind Loads Using ClearCalcs

Using the Wind Loads Calculator to AS/NZS 1170.2:2021, we can input the key parameters such as the structure importance level, wind region, terrain category, and building direction from the above example:

To compute the internal wind pressure we input all of the areas of openings in the building.

Clause 5 of AS1170.2 (2021) should be referred to for the wind load factors for each surface. ClearCalcs provides reference to the standard. The base case for each factor is set to 1.0.

ClearCalcs then outputs a summary of the design wind speed based on direction.

The most useful output for design is the wind load table, which summarises all the design pressures for each surface.

The best thing about ClearCalcs is that it immediately updates these wind pressures if design inputs are tweaked (e.g., if building dimensions are adjusted or if windows are added or removed).

Rather than performing an entirely new iteration of calculations, ClearCalcs provides instant feedback, saving engineers an incredible amount of time during design development.

Conclusion

Calculating wind loads accurately is crucial for structural safety and AS/NZ 1170.2:2021 compliance. This guide explained key aspects of wind load calculation, including determining site wind speed, factoring in topography and shielding, and calculating design wind speed and pressure. The examples provided demonstrate how to determine wind loads for a building, emphasizing the importance of accuracy for effective structural design.

Engineers are encouraged to stay updated with the latest standards and continuously refine their skills in wind load calculations to contribute to creating resilient and safe structures. Structural analysis software like ClearCalcs provides a powerful and user-friendly platform to simplify this complex process, giving you confidence in your designs.

Ready to streamline your wind load calculations and ensure code compliance? Try ClearCalcs free today!.

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