Wind loading is an essential factor in structural design. Aspec Engineering student mechanical engineer Drishti Patel discusses the key changes introduced in the 2021 version of AS/NZS 1170.21:2021 and their implications.
In structural engineering, wind load standards are crucial in ensuring that buildings and infrastructure can withstand wind forces. These standards underpin the safety, performance, and longevity of buildings and infrastructure.
The release of the AS/NZS 1170.21:2021 standard provides updates to the guidelines governing wind action. The 2021 revision supersedes the previous AS/NZS 1170.2:2011 version and its amendments.
Why the change?
Several severe weather events and ongoing wind tunnel testing and research prompted the changes to the standard.
Tropical Cyclone Seroja caused vast damage to buildings in coastal and inland areas in April 2011. The tropical cyclone crossed the mid-west coast of Western Australia near Port Gregory. It caused significant structural damage to buildings due to non-conservative internal suction factors in AS/NZS 1170.2:2011.
Tropical Cyclone Seroja saw maximum wind speeds over land in Kalbarri of 166 to 184km/h, which is 80 to 90 per cent of the design wind speed for houses.
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In November 2014, Brisbane experienced a similar event: severe hail, flooding, and strong winds of up to 141km/h gusts. While the wind speeds were below design values, the damage was severe due to outdated building standards that did not account for pressure changes from openings on the windward wall.
Impacts
The factors prompted a revision to the standard, which was released in 2021 and included several changes and improvements.
The impact of these code changes on determining wind action can be summarised in table two. This is an example of determining the wind actions on a hypothetical industrial structure with the results showing a significant difference in results obtained between the two code revisions.
The changes are further demonstrated by the difference between the multipliers from the 2011 and 2021 versions of the standards. Figure two shows the percentage difference between the terrain/height multiplier and the 2011 code values.
On show in WA
One of the best examples of the impact of these revisions is showcased in Western Australia.
Wind Region D is unique to Western Australia as it stretches 50 kilometres wide along the northwest coast, north of the Tropic of Capricorn.
The current version of AS/NZS 1170.2 reduces the wind speed in Wind Region D, prompting a Western Australia variation to the National Construction Code (NCC). The national code had sought to retain wind speeds similar to those in the 2011 standard for the region. Now, the NCC refers to a table of annual probability of exceedance, which has increased for houses in Wind Region D. By understanding the rationale behind these changes to the standard, it becomes easier to understand their use in a building and infrastructure context.
While there are likely to be future amendments and further improvements made to the AS/NZS 1170.2:2021 standard, the standard maintains a key role in underpinning the safety, performance, effectiveness and longevity of buildings and infrastructure.
