The new National Construction Code (NCC) released in September 2022 included an update to the Wind Assessment Standards.
From our initial consultation to design and engineering through to material selection and manufacture, Techspan is committed to ensuring that our client projects not only meet the new standards but also deliver quality, practical, efficient and enduring infrastructure that maximises productivity, safety, security and accessibility. Our reputation is unrivalled when it comes to listening to customer needs, attention to detail throughout every stage of a project and standing by our quality workmanship.
What standards apply to your project?
Currently the two standards that apply are:
- AS 4055:2012 (most commonly applied to residential housing — gives a wind rating)
- AS/NZS 1170.2:2011 (used when the geometric limits of AS 4055 are exceeded, e.g. roof height over 8.5m — gives a design wind speed).
Further, the following contributing factors are considered to formulate results to determine what is required with your particular project:
- Wind Region — based on updated standards available for different regions of Australia (see region map below)
- Shielding — what is considered as ‘shielding’ of the structure by adjacent structures and trees (AS 4055 only)
- Terrain — consideration to smoothness or roughness of the terrain due to trees and other structures. For AS 4055 this includes considering terrain within 500 metres in all directions. AS/NZS 1170.2 includes an averaging distance of 500 metres or 40 times average of the project roof height plus a lag distance, whichever is greater.
- Topography — considers any significant cross sections of a hill, any site where it is located on a hill and site slope.
What changes have been made to wind assessment standards?
The new standards will be aligned more closely to each other. Some key changes which may impact the ratings for your projects are as follows:
AS 4055:2021 (Wind loads for housing)
- The geometric limitations of AS 4055 have been clarified with the introduction of the term “averaged ground level”, defined as the natural ground level excluding earthworks where earthworks are minor, or through the modified land where the earthworks are major. This may lead to more projects sitting outside the limitations of AS 4055 and therefore coming under AS/NZS 1170.2:2021.
- Terrain Category 1.5 has been removed. Bodies that fit into this category are now in TC1.
- Some wind region zoning has changed. Most notably for South East Queensland, Toowoomba is now in Region B, not Region A.
- Calculation of topographic class was previously taken as the maximum slope through the closest peak. It now explicitly states that the slope must run through the project site. This brings AS 4055 more in line with AS/NZS 1170.2 and better takes into account conditions at the project site.
- A subsequent revised wind classification table (refer below)
- Equivalent wind ratings from AS/NZS 1170.2 are now allowed for the purpose of specifying building products (eg windows, doors and cladding).
AS/NZS 1170.2:2021 (Structural design actions, Part 2: Wind actions)
- For wind regions B2, C & D, a newly introduced climate change multiplier (Mc) will account for possible changes in climate affecting extreme winds over the design life of buildings. This may be adjusted in future, depending on observed or predicted trends.
- The wind regions map has changed and regions A0, B1 (including Toowoomba) and B2 have been introduced (see updated map below)
- Wind direction multipliers have been revised for all wind regions in Australia.
- Terrain category TC1.5 has been removed (as per AS 4055) and multipliers for TC1 have been slightly reduced.
Design Wind Speed Calculations According to AS1170.2:2021 (Updated Code)
|Based on structure lifespan 25 years / Max av. height 10m
|Terrain Category 2 for all calculations
|AUSTRALIAN WIND REGION
|ULTIMATE REGIONAL WIND SPEED (km/h)
|DESIGN WIND SPEED (km/h)
|Region A1 to A5
Will the changes affect project cost?
There are likely to be variations in results from the old to new standards, some more favourable (costing less for the build), and others not so. The goal of the changes is to improve the system, making wind assessments more accurate (with AS 4055 becoming more consistent with AS/NZS 1170) and therefore safer and more enduring structures.
In short, the requirements and costs related to your project will be determined once your location, wind region, terrain category and importance level are determined.
How Techspan Can Help
When you begin the design consultation at Techspan, location is one of the early questions we ask. We consider your location with respect to Wind Region, Terrain Category and Importance Level and factor this into the design and engineering stages to ensure that your project meets the required standard, in accordance with AS/NZS 1170.2.
In some cases, local councils may also have specific requirements that will also be taken into consideration.
It is extremely important to classify every structure on a case-by-case basis and assess every location individually. If wind is not properly discussed or considered in the design process it could be hazardous and affect the longevity of your structure.
Safety and Structural Integrity are key considerations within our design philosophy here at Techspan and ensuring that all Techspan projects are ‘structured to work’ for many years to come.
With over 20 years’ experience in the industry and hundreds of aviation, rural, agribusiness, mining, commercial, industrial and school infrastructure projects delivered, our team are here to make the process of achieving your new project simple.