Why you need to rethink Wind Classifications: 4055 vs 1170.2

Using the correct wind classification is critical to ensuring a building will be designed and constructed in compliance with the applicable Australian building standards – AS 4055 and AS 1170.2. In practice these standards are often not followed strictly (nor scrutinised closely by building inspectors), resulting in buildings with the wrong wind speed rating and potentially costly implications during or after the build. Through site audits, regulators have now identified this as an issue and are starting to more closely scrutinise wind ratings.

The process for selecting and certifying a wind rating goes something like this:

  • An architect or building designer may apply a wind rating to a set of plans, usually according to AS 4055-2012 (Wind loads for housing).
  • A Wind Assessment may be done by an engineer at the same time as a Soil Test, which would allow the above rating to be verified.
  • At the time of Structural Design of the frame, if a wind rating has not been provided by a registered engineer (prior in the project) the design engineer MUST undertake the calculation OR review the already provided calculation – in order to correctly design the frame.
  • The builder would then build to that rating
  • At building approval and practical completion, a building certifier would review the structural design and building products / materials specified and installed for compliance with the chosen rating.

Closer scrutiny of wind ratings by building certifiers

Historically a house may have been outside the limitations set by AS 4055, yet a rating from this standard may have been applied (e.g. an N2) and it likely would not have been picked up on.

In 2019, new guidelines for AS 1170 were released by various building authorities (including QBCC, Building & Energy WA) that require a more stringent application of both Australian Standards for wind load (and seismic) calculations – not just AS 4055. These guidelines highlight the limitations of AS 4055 and how they should be applied.

The result is that building certifiers are now more aware of the distinction between the two standards and correct application is being enforced more strictly. STA Consulting Engineers have been seeing this stricter approach practiced more frequently by certifiers.

The potential implications are significant, in terms of building redesign, materials selection and estimating. For the homeowner and in the spirit of full ongoing disclosure during the design and build cycle, it is important that designers and builders can explain the difference in these standards, as well as when and how build costs may be impacted.

Before getting into the implications more, let’s review the purpose of wind ratings and the two applicable Australian standards for wind loadings.

Cyclone roller coaster ride
Wind classification determines the expected wind speeds on a given building site that a structure will need to endure.

What is a wind classification and when is it needed?

When it comes to building a structure, every site needs a wind classification. It plays a critical role in determining what the likely wind speeds on the site will be, the wind loads that the structure and building products (such as windows) will need to endure and therefore what the final structural design of your building needs to be. 

It is similar to the criticality of a soil test in identifying soil reactivity and the bearing capacity of the soil and ultimately how it determines the type of footings and foundation that can be built on your site.

Wind classification determines various structural design elements of the building:

  • Timber Framing
  • Bracing requirements
  • Tie down and other fixings
  • Roof trusses
  • Cladding
  • Windows

That explains the what and when of wind classification. Now to the how – the Australian standards.

What are the Australian Standards for wind ratings?

There are two standards in Australia that deal with wind speed classification of a site, wind loads for housing, and the subsequent structural design according to that rating.

AS 4055-2012 Wind loading for housing

This is the most commonly used standard applied to residential housing and the one most builders and designers are familiar with. AS 4055 provides a simplified method for determining the wind classification for one or two storey housing only.

The standard considers the following four site characteristics and makes a single wind classification determination based on the worst case for each:

  • Wind Region
  • Topography
  • Shielding
  • Terrain

Class 1 and 10 structures (per the National Construction Code) can be designed using AS 4055 if they fall within the geometric limits set out in Clause 1.2:

  • Maximum external wall height under the eaves ≤ 6.0m
  • Maximum roof height above the natural ground line ≤ 8.5m
  • Maximum house width between external walls across the ridge line ≤ 16m
  • Maximum roof pitch ≤ 35°

The illustration below from the AS 4055 standard details these limitations further.

Geometric limits for designing structure using AS 4055-2012 wind loadings for housing
FIGURE 1.1 GEOMETRY, from AS 4055-2012

For buildings that fall within these limitations, AS 4055 designates either an N (non cyclonic) or C (cyclonic) rating, N1-N6 or C1-C4. These ratings allow – for the most part, though not all – prescriptive designs with building products such as cladding and windows being specified to a residential N or C rating. Each N or C category relates to a maximum limit of design wind speed. Therefore N and C categories effectively identify a range of wind speeds from one category up to the limit of the next higher category. 

Where the proposed house design falls outside the limitations outlined in AS 4055, the alternate standard AS 1170.2 must be used to determine the applicable wind loads. This most commonly occurs when the maximum roof height exceeds 8.5m.

AS 1170.2:2011 (R2016) Structural design actions: Part 2, Wind actions

The AS 1170.2 standard does not have any geometric limitations and in terms of engineering is more precise and complex in application. As such, using AS 1170.2 will generally result in a more accurate wind loading determination than if AS 4055 was used.

Where the highest point of the roof exceeds 8.5m – thus requiring AS 1170.2 to be used instead of AS 4055 – the following assessment is required according to AS 1170.2:

  • The impact of wind, per the specific location and house design proposed.
  • Wind loadings are specified as a nominated design wind speed, e.g. 32m/sec, not a category or range of speeds like AS 4055.

Under AS 1170.2 the wind assessment is completed with a determined relevant loading contained in the engineering report provided for each project. This assessment is then used by engineers, material suppliers and eventually by building certifiers to ensure structural integrity of the structure and building product compliance under the expected conditions. 

As a side note, when the highest point of the roof exceeds 8.5m, this also triggers the requirement for a seismic assessment according to Part 4 of AS 1170. The assessment determines impact of seismic actions, under static lateral loads, given the specific location and house design proposed. AS 1170.4 seismic assessments are presented as an Engineering Design class (EDC) and must be considered when specifying building products and providing a bracing design.

Aerial view of a house in a heavily wooded area
Factors such as shielding of a structure from wind due to trees is not taken into account with AS 1170.2

Why are wind loading results different: AS 4055 vs AS 1170.2

The two standards do not provide the same guidelines by which a site is assessed. Site conditions that may lead to a difference in wind speeds when being assessed to AS 1170.2 include the following:

Shielding

This relates to shielding of the house against wind due to trees or structures.

  • Heavily wooded areas adjacent to a house site can be considered shielding in AS 4055, in Regions A and B only.
  • AS 1170.2 allows no consideration for trees in shielding calculations.
  • Shielding structures must be of equal or greater height than the shielded house.

Therefore houses adjacent to wooded areas such as bushland or parkland, or houses adjacent to single story dwellings may have a higher wind speed when assessed as per AS 1170.2.

Terrain

This assesses the smoothness or roughness of the terrain in all directions, due to trees and other structures.

  • AS 4055 considers smoothest terrain within 500m radius of the house site
  • AS 1170.2 considers average terrain within an ‘averaging distance’ + ‘lag distance’ dependant on height of structure e.g. an 8.5m structure = 510m; 9.5m structure = 570m

Therefore houses with a variety of terrains within the relevant radius could see an improvement in wind speed.  If a worse terrain exists just outside of the 500m radius stated in AS 4055, the wind speed may increase.

Topography 

Topography looks at the site with respect to a hill, ridge, or escarpment and assesses its position relative to the top of the hill as well as the slope of the hill

  • AS 4055 considers the nearest peak on a hill and the site is categorised based on its position on that hill and the slope of the top half of that hill.
  • AS 1170.2 calculation is undertaken also considering the peak of the hill the site is located on. However, the slope considered must be taken through the site itself.  This may lead to a different peak being selected than would have been using AS 4055.

Therefore, for houses in hilly locations where no obvious local peak is evident, results may vary. For flat sites or sites clearly within the lower third of an obvious hill, results should not vary greatly between the two standards.

Aerial view of suburban Australian streets
The ready availability of geospatial data has made accurate wind ratings easier.

Technology has improved wind rating accuracy – and auditing

In the last 5 years the capability of software systems has improved, such that building designers and engineers (and building certifiers) now have access to large volumes of geospatial data for any building site in the country, including terrain, wind conditions, topography and shielding. Some systems provide detection and analysis of site specific information, assisting greatly in accurate identification of a valid wind classification.

The same technology has no doubt made it possible for building certifiers and regulatory bodies to audit wind loading classifications with greater ease and effectiveness. Consequently identifying inconsistencies in the application of AS 4055.

The widespread availability of these software packages has likely also contributed to an increase in expectations of governing bodies (and certifiers) – that designers and engineers should now be able to apply accurate wind speed ratings to every site.

The implications for getting wind classification wrong

In an environment where building certifiers are cracking down more stringently on wind ratings, there are significant implications for all involved in the building process – the architect or designer, the engineer, the builder and of course the client paying for the structure to be built. 

If an incorrect wind classification is not identified until the structural engineering design review (by an engineer now applying a strict interpretation of the standards) then a number of problems could result:

  • A redesign is required that results in a higher wind speed – resulting in more expensive materials or building products being required
  • A redesign is required that results in a lower wind speed – resulting in potential savings in the cost of the build
  • Fixed price contracts may be signed prematurely
  • If building materials have been purchased in advance, these may need to be replaced
  • If the build has already started, it may require demolition and rebuild of certain parts of the structure.
  • Potentially lengthy litigation
  • And for all of the above, a difficult conversation with the client about what could amount to tens of thousands of dollars increase in the build cost.

The solution: Get a wind assessment early on

A Wind Assessment from an engineering company (like us) is a low cost item. Especially when you order it at the same time as your soil test. 

For the client and building designer, this ensures that an accurate wind classification is confirmed as early as possible in the build process, thus avoiding the aforementioned implications.

It also means we conduct a single site visit for both the soil test and wind assessment requirements. And then complete the remaining assessment in our engineering office (and our NATA certified lab for the soil test).

It is a small cost that delivers important clarity for the building project and significant risk mitigation. Find out exactly how small the cost is by getting a quote using our simple online quote tool.

Compliance and cost optimisation of wind classification

While the australian standards for wind loadings haven’t changed, the application and scrutiny of the standards has. Government departments and building certifiers have been alerted to the inconsistencies and now expect building designers and their engineers to adhere strictly to the standard.

We – engineers, builders, designers and clients alike – all need to be conscious that there may be differences in building costs that result from the correct assessment of design and site specifications as determined by the relevant Australian Standard. By applying the standards correctly, the build can be optimised for compliance and building costs, to the benefit of all.

It is therefore crucial that an accurate wind assessment is conducted as early as possible in the building project, on every site. Commonly the best time for this is when the soil test is done. If you are concerned about your specific project, please get in touch with our engineering team for clear advice on this matter.

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