Kyle Conway
Learn how to calculate wind loads accurately to AS/NZ 1170.2:2021. This guide covers steps, formulas & software for simplified compliance.
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.
Table of Contents
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:
The Standard covers structures within the following criteria:
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.
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:
Where:
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).
Determining the regional wind speed () 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 () 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 (average recurrence interval) is the average time interval between exceedances of the wind speed listed.
In Region C, () 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 , according to the distance from the smoothed coastline.
In Region D, () 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 , according to the distance from the smoothed coastline.
Figure 1: Wind regions in Australia per Figure 3.1 (A) in Clause 3.2 of AS/NZS 1170.2 (Reference)
Figure 2: Wind regions in New Zealand per Figure 3.1 (B) in Clause 3.2 of AS/NZS 1170.2 (Reference)
The wind direction multiplier () 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, () 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).
The climate change multiplier () shall be as given in Table 3.3 for regions shown in Figure 3.1(A) and Figure 3.1(B).
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:
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 () of the effect of terrain roughness on wind speed (terrain and structure height multiplier, , 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, , 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 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.
Figure 3: Upwind buildings on a slope per Figure 4.2 in Clause 4.3 of AS/NZS 1170.2 (Reference)
The shielding multiplier () 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:
Where:
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): Where:
For sites in Region A0, use Equation 4.4(2):
Elsewhere, the larger value of the following:
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.
Figure 4: Hill-shape multiplier for hills and ridges per Figure 4.3 in Clause 4 of AS/NZS 1170.2 (Reference)
Figure 5: Hill-shape multiplier for escarpments per Figure 4.4 in Clause 4 of AS/NZS 1170.2 (Reference)
Figure 6: Hill-shape multiplier for hills and escarpments with upwind slope greater than 0.45 per Figure 4.5 in Clause 4 of AS/NZS 1170.2 (Reference)
Within the local topographic zones, the hill shape multiplier () 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
b) For (see Figures 4.3 and 4.4), use Equation 4.4(3):
c) For (see Figure 4.5):
Within the rectangular peak zone (see Figure 4.5), use Equation 4.4(4):
Elsewhere within the local topographic zone (see Figures 4.3 and 4.4), Mh shall be as given in Equation 4.4(3).
where
For the case where x and z are zero, the value of 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.
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:
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.
Figure 7: Location of New Zealand lee zones per Figure 4.6 in Clause 4 of AS/NZS 1170.2 (Reference)
As per Section 2.3 of AS/NZS 1170.2, the building orthogonal design wind speeds () shall be taken as the maximum cardinal direction site wind speed () 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 () in the range [β = θ ± 45°] where is the cardinal direction clockwise from true North and 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, shall be the maximum value of in a sector ± 22.5° from the 45° direction being considered. For ultimate limit states design, 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 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.
The design wind pressures (p), in pascals, shall be determined for structures and parts of structures as per the below.
where
Section 5 of AS/NZS 1170.2 shall be used to calculate the aerodynamic shape factor () for structures or parts of structures. Values of shall be used to determine the pressures applied to each surface.
For calculating pressures, the sign of 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.
Figure 8: Sign conventions for per Figure 5.1 (A) in Clause 5 of AS/NZS 1170.2 (Reference)
As per Section 5.2 of AS1170.2 (2021), the aerodynamic shape factor () shall be determined for specific surfaces or parts of surfaces as for enclosed buildings as follows: for internal pressures for external pressures for frictional drag forces
Where the parameters are found as per Tables 5.1-5.9 in AS1170.2 (2021)
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).
A dynamic response factor () shall be taken to equal 1.0 for the following cases:
Most basic building structures have a of 1.0. For more complex structures, refer to Section 6 of AS/NZS 1170.2.
As previously discussed, the design wind pressures (), in pascals, shall be determined for structures and parts of structures as per the below.
We have discussed how to find each of these factors.
To determine wind actions, the forces () 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 ().
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:
Where:
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).
where
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:
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.
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.
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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