AS/NZS 1170.2 - Lee Zone for NZ

[m_struct]

Is looking at holiday home in Castle Hill and trying to determine if it is the lee zone.

Figure 3.1(B) shows a wind regions map for NZ. It has the LEE ZONE regions shown on it. That map is lacking detail. Is there a google overlay of this map?

The BRANZ Wind Zone map shows it in LEE ZONE, while another reference shows the wind zone starting at the crest a bit further east. I am told that map is to the original 1170.2:2011 and has not been updated for the new amendment.

 

[rowingengineer]

WSP-OPUS did the mapping of the lee zone for the 2011 code, and this mapping is used by Checkwind and most wind mapping softwares for lee zone areas. The Branz's info is out of date, however the local council do rely heavily on this data and expect an RFI on the matter if your m/s is less than Branz. The lee zones will change again when AS/NZS1170-2021 is published, however you are welcome to enter the office pool on that one, to guess the date, as it was meant to be march and is now July and will probably be nov before we see it.

Not knowing the exact location in Castle Hill Canterbury, and not knowing the exact map of castle hill area, I am un-willing to give you 100% guarantee that you are out of the lee zone, but I would suggest you are more than likely out of the lee zone, as the lee zone in my reckoning starts on the outer edge of springfield.

 

[m_struct]

Checkwind says it is out of the lee zone.

Great point about BRANZ and RFI from council. I was looking for a reference to back that up as the wind pressures are driving the design and are significantly higher if it is in the lee zone. I have emailed BRANZ and the local council.

Castle Hill is west of Springfield and Porters Pass. It it between 2 crests. It looks like BRANZ wind map takes the LEE ZONE starting at the crest west of Castle Hill while CheckWind takes it from the crest east of Castle Hill.

 

[Agent666]

Does anyone know what assumptions were made by BRAMZ-MAP to determine the NZS3604 wind zone, like what height structure is assumed for determining Mz,cat, is shielding considered in 'residential' type areas?

Often wondered this, as often a site-specific check for even 2 storey structures ends up being one zone worse for a few directions. Seen a few weird ones as well where they claim a low/medium wind zone, but you check it in accordance with AS/NZS1170.2 and back calc out the wind speed and it seems well out.

https://engineervsheep.com

 

[Agent666]

Answered my own question....

https://www.buildmagazine.org.nz/index.php/articles/show/branz-maps-goes-mobile

Seems to cover how it was implemented. Complete with warnings about how inaccurate it can be....
linked from this

https://www.branz.co.nz/branz-maps-zones/

If using NZS3604 approach then the wind zone is only based on 2 storeys. Otherwise I guess you should do a site specific assessment. Similarly they based the terrain assessment on 80x80m pixels....

m_strut, they cover in the BUILD article that they determined the lee zones for BRANZ-MAP by simply tracing over a map with a digital pen.... so thats how accurate it is!

https://engineervsheep.com

 

[m_struct]

Thank you for the response Agent666.

I have a similar experience for calculating winds loads for 2 story residential buildings in suburban areas, that the wind speeds from 1170.2 equations are generally a 3604 classification higher, such as medium zone vs low zone. 1170.2 commentary suggests using a shielding multiplier of 0.8-0.9 in suburban areas. Using the 1170.2 shielding equation, that might work for single story houses, but not for two story houses surrounded by single story ones. I thought my spreadsheets were off, so I compared it with Checkwind, and it was getting similar wind speeds for two storey in suburban areas.

 

[Agent666]

Yeah I've seen a few people get caught out with the building height also affecting the averaging and lag distances, and the terrain being considered as contributing to the wind speed. I can only surmise BRANZ-MAP only considers max two storeys at whatever the basis of NZS3604 building height was considered to be.

For the recently popular 3+ storey type of buildings, you can have quite a different wind zone if you're close to water for example and you're dominated by TC1/TC1.5 when council maps are based on TC3 for shorter structures....

You're left trying to convince the architect it's high wind instead of low.... but they just don't get it sometimes.

https://engineervsheep.com

 

[m_struct]

I sent an enquiry to BRANZ.

The response said that wind maps in BRANZ are specific to NZS 3604. If you are dealing with "wind zones" (which is specific to 3604) and timber buildings, then the BRANZ Maps is most appropriate to use. If you are dealing with site specific wind speed and pressure calculation or buildings out the scope of NZS 3604, then NZS 1170.2:2011 is the right one

It continued to say "in BRANZ maps, the wind regions and lee zones layer was created to support the subsequent wind zone layer. ‘Updating’ the lee zones to NZS 1170:2011 therefore doesn’t really makes sense because the subsequent zone calculation would be invalid."

We could, in theory, add the NZS 1170.2011 lee zones as an additional layer, but having two maps with lee zones in BRANZ maps would probably confuse things even more!!"

So BRANZ maps is an overlay of multiple maps and not just the current NZS 1170.2:2011 lee zone.

Do you know of a map overlay showing the lee zone of the current 1170.2:2011?

 

[Agent666]

No I'm not aware of anything like that.

Why can't they just update the entire calculation.... Otherwise get rid of it if its not correct.

https://engineervsheep.com

 

[Agent666]

FYI m_struct, 2021 version of AS/NZS1170.2 has just been released. There is an actual table of coordinates mapping out the new lee zone boundaries.

https://engineervsheep.com