Wind classifications for steel framing in Queensland
Comparing options · 6 min read · Published
Short answer
AS 4055 gives houses a site wind classification: N1 to N6 in non-cyclonic regions and C1 to C4 in cyclonic regions. The classification combines the wind region with terrain, shielding and topography, so it is set per site by an engineer or certifier. Queensland Government guidance places coastal areas north of Bundaberg in cyclonic Wind Region C.
Region vs classification
The wind region comes from the map in AS/NZS 1170.2 and describes the broad wind climate. The site wind classification, from AS 4055 for housing, narrows that down to the individual lot by looking at the terrain around it, whether nearby buildings or trees shield it, and whether it sits on a hill or ridge.
| Classification | Applies in |
|---|---|
| N1 to N6 | Non-cyclonic regions (most of South East Queensland and northern NSW) |
| C1 to C4 | Cyclonic regions (including coastal Queensland north of Bundaberg) |
Where cyclonic design starts
The Queensland Reconstruction Authority's guidance for homes says coastal areas north of Bundaberg are classified as Wind Region C, and homes designed to AS 4055 there have a classification of C1, C2, C3 or C4 depending on the site. For the towns we service, that includes the Gladstone region: Tannum Sands, Boyne Island, Calliope, Agnes Water and surrounds.
Why neighbouring lots can differ
A house on an exposed ridge and a house in a sheltered street a few hundred metres away can have different classifications. That is why a frame supplier asks for the classification for your site rather than using a suburb-wide figure.
What it changes in the frame
- Bracing: how much and where
- Tie-down from roof to walls to floor or slab
- Truss design and connections
- Member sizes and fixing schedules
AS 4055 covers houses within its geometric limits. Larger or taller buildings are designed to AS/NZS 1170.2 by the structural engineer.