AS/NZ 1170.2:2021, Understanding Wind Load Calculations

In Australia and New Zealand, AS/NZ 1170.2 is the Standard that sets out the procedures for determining wind speeds and their resulting wind loads. It is to be used in the structural design of any structures that are subjected to wind actions (other than those caused by tornadoes).

This article will examine the scope of this Standard in structural design and how it relates to the broader AS/NZ 1170 series. Correct calculation of AS 1170.2 wind loads is the key to safe structural design in Australia and New Zealand, and this Standard is a must-have for engineers and construction professionals.

What is AS/NZ 1170.2:2021?

The full title of the current Standard AS/NZ 1170.2:2021 is “Structural design actions, Part 2: Wind actions” and is part of the broader AS 1170 “Structural design actions” series. The 2021 revision superseded AS/NZ 1170.2:2011 (Structural design actions: Wind actions​) by introducing changes to the Australian and New Zealand wind regions and allowing for more extreme weather by using a Climate Change Multiplication Factor. The 2021 Standard also includes changes to terrain categories, roof zoning and other factors.

The scope of the AS/NZ 1170.2 Standard guides wind action assessments and covers structures within the following criteria:

• Buildings and towers less than or equal to 200 m high.
• Structures with unsupported roof spans of less than 100 m.
• Offshore structures within 30 km of the nearest coastline.
• 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.

How AS/NZ 1170.2 Integrates with AS/NZ 1170.0:2002

Exactly how does this Standard tie in with AS/NZ 1170.0:2002 “Structural design actions, Part 0: General principles”? According to the 1170.2 Standard, its application “shall be read in conjunction with AS/NZS 1170.0”.

The 1170.0 Standard applies to the structural design of whole buildings or structures and their elements. It covers the actions of dead loads, live loads, wind loading, snow loads, earthquake loads, liquid pressure, ground water, rainwater ponding and earth pressure. Because both 1170.0 and 1170.2 cover wind action, they are interdependent. The obvious difference is that 1170.0 covers many other loads in addition to wind. In practice, the wind loads are calculated in accordance with AS/NZ 1170.2, and these are then incorporated into the load combinations (dead loads, live loads, wind loads, etc.) covered in AS/NZ 1170.0.

Key Wind Load Parameters in AS 1170.2

To properly calculate AS 1170.2 wind loads, some of the key variables to consider are:

• Wind Regions: These regional wind speed maps cover all areas of Australia and New Zealand. They include cyclonic and non-cyclonic (more consistent and lower speed) regions.
• Terrain Categories (TCs):
• TC1 — Very exposed open terrain with very few or no obstructions, and all water surfaces.
• 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.
• TC2.5 — This covers terrain such as in developing outer urban areas with scattered houses.
• TC3 — This covers terrain such as suburban housing, light industrial estates or dense forests.
• 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.
• Shielding Multipliers: These multipliers are outlined in the Standard, which states, “Shielding may be provided by upwind buildings or other structures. Shielding shall not be provided by trees or vegetation.” Of course, a shielding building must have an elevation that exceeds the building being shielded.
• Topographic Multipliers: These take into account the impact caused by terrain features such as slopes, hills, etc.
• Climate Change Multiplication Factor: This factor takes into account the increases in extreme weather due to climate change.

What's New in the 2021 Revision Compared to AS/NZ 1170.2:2011?

The current Standard (replacing AS/NZ 1170.2:2011 (Structural design actions: Wind actions) had the main objective of removing ambiguities and incorporating recent research and experiences from recent severe wind events in Australia and New Zealand.

There are 27 major changes in the 2021 edition, including Climate Change Multiplier, windborne debris test speeds, new wind region boundaries for Australia and New Zealand, revised wind direction multipliers, changes to terrain categories, reduction of certain terrain height multiplers (to reflect observed values of gust factors), a new method for the combination of along-winds and crosswinds, and many more!

Application Examples for Residential and Commercial Structures

The basic procedure for calculating wind actions is:

• Calculate the site wind speeds by determining the regional wind speed (from the regional wind speed maps) and factor in multipliers such as wind direction, climate change, terrain/height, shielding, topography, hill-shape and lee.
• Use this site wind speed calculation to determine your design wind pressure, taking into account the aerodynamic shape factor of the structure and the dynamic response factor.
• The wind actions can then be determined using the formulas given in the Standard.

As a practical example, the most significant difference in wind action between a low-rise home and a high-rise building is usually the terrain category. The low-rise home would have the Terrain Category 3 (for suburban housing, light industrial estates or dense forests) factor applied to it, while the high-rise building should be in Terrain Category 4 (for large city centres and well-developed industrial complexes).

With coastal vs inland structures, the relevant wind region zone (as outlined in the Standard) would most likely provide the biggest difference in wind action.

Common Challenges in Applying AS 1170 Standards

There are many variables in the AS 1170 Standard concerning wind action. And engineers and designers can often make mistakes or face confusion when trying to select the correct category for each of these variables. For example:

• Choosing the correct terrain category: Is it Terrain Category 2.5 (developing outer urban areas with scattered houses) or Terrain Category 3 (suburban housing)?
• Is the structure close to a borderline on the regional wind speed maps, causing confusion about where it really belongs?
• What about the aerodynamic shape factor? An unusual shape can cause headaches.
• And what about the site exposure multipliers (which include terrain/height, shielding and topographic multipliers)? 

A thorough study of the finer points of AS/NZ 1170.2 can help alleviate many of these difficulties.

Why Compliance with AS/NZ 1170.2 is Critical

Weather can be a wild variable, especially in this part of the world. And calculating wind action consists of a much more complex set of factors than, say, a dead load. That’s why it’s essential to fully adhere to the AS/NZ 1170.2 Standard, which was introduced to prevent structural failure due to wind loads. Safety, legal, and insurance implications require all designers to strictly follow this Standard.

Frequently Asked Questions

What’s the difference between AS/NZ 1170.2 and AS/NZ 1170.0?


The difference is that AS/NZ 1170.2 specifically deals with wind action, while AS/NZ 1170.0 considers all actions on the structure (wind load, dead load, live load, earthquake load, etc.).

How often is AS/NZ 1170.2 updated?


AS/NZ 1170.2 was issued in 2002 and then revised in 2011 and 2021. So, it’s currently being updated every 9 to 10 years.

Is AS/NZ 1170.2 mandatory in all building projects?

Absolutely! AS/NZ 1170.2 is part of the National Construction Code (NCC), which is a part of every state’s legislation. So you are legally required to follow this Standard for all building design in Australia.