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BRANZ Maps – wind zones – how do they work?

BRANZ Maps is an online tool that identifies a range of features for a specific location, including earthquake zone, exposure (corrosion risk) zone, and climate zone. The tool also provides an experimental wind zone for a given location.

The wind zones in BRANZ Maps are an output from a successful 2014 research project at BRANZ. The project aimed to provide automated, calculated wind zones in accordance with NZS 3604:2011 Timber-framed buildings using GIS software, which can now be done for a specific site. A number of approximations were made while creating the map, which are explained below. 

Steps 1 and 2 from Table 1 of NZS 3604:2011 – determining the wind region and if in a lee zone – were completed by digitising the map in Figure 5.1 of NZS 3604:2011. 

Steps 3 and 4 were more challenging, as the NZS 3604:2011 calculation requires counting obstructions (buildings and trees 3 m or higher) around a site. This data doesn’t exist on a nationwide level. As a rough proxy, land use data from Landcare Research was used:

  • For step 3 (ground roughness), urban areas were assumed to be the built-up areas and forests, and these areas were then trimmed by 500 m, as per clause 5.2.3 of NZS 3604:2011. Open areas were deemed to be everything else.
  • For step 4 (site exposure), sheltered areas were assessed using the same built-up areas and forest areas but trimmed by 100 m (representing two rows of obstructions). Steep areas were removed (see step 5). Exposed areas were everything else.
  • Step 5 in NZS 3604:2011 requires calculation of the ground slope, which in turn requires identification of ridges and valleys. Once again, BRANZ looked for a proxy and used a LINZ digital elevation model (DEM) of New Zealand. This splits the country into 80 x 80 m pixels, each pixel representing a particular height. This data was processed to create the ridges and valleys – a task that required some assumptions to be made. Finally, a wind zone was calculated for each 80 x 80 m pixel.

The order of resolution of wind zone calculations at the current time is as follows, with the most precise at the top:

  • calculations made specifically for a site following AS/NZS 1170.2:2011 Structural design actions – Part 2: Wind actions
  • calculations made specifically for a site following NZS 3604:2011
  • council wind maps (if based on AS/NZS 1170.2:2011 or NZS 3604:2011)
  • BRANZ Maps.

More information on the BRANZ Maps wind zones is contained in Build 147 and the about-to-be-released BRANZ Bulletin 588.

From BRANZ Guideline, September 2015

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