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Cyclone Cook vs NZS 1170.2 (NZ wind load standard) (NZBS7)

Shawn McIsaac

Cyclone Cook is exactly why I get up every morning. 

If this “mid-latitude low” lives up to the hype, this type of storm is what engineers design building enclosures to withstand. This gives us a great chance to put NZS 1170.2 into some very real context.

For those of you who design buildings for a living, you should be familiar with the concept of serviceability limit states (SLS) and ultimate limit states (ULS). The SLS is a 1 in 25 year storm. Components are expected to require repairs but not experience catastrophic failure; bending, cracking, and leaking would all be permissible. A ULS event, is a 1 in 500 year storm at which complete failure would occur. These are roof tear-offs, cladding detachments, blown-out windows etc.

The 140 km/hr winds (39m/s) expected for Auckland today will exceed the SLS (~37m/s) for many buildings in the area but be less than the ULS (~45m/s) for most buildings. This means damage but not complete failure will occur to many buildings in Auckland today and tomorrow. This type of wind (with rain) exceeds the design standards for nearly every window in New Zealand (~>800 kPa) so you should expect your windows to leak today; there is nothing wrong with them, it’s just going to be more storm than they are designed to handle.

In other parts of the country in the direct path of Cyclone Cook, the outlook isn’t so good. Wind at 165 km/hr (46 m/s) start to approach or exceed the ULS of many buildings. Some detachment of cladding, roof tear offs, detached balustrades, blown over signs are all very real possibilities today in those areas, even for modern designed buildings. Stay safe, inside, and away from windows.

Combine wind at 39 m/s and 50 mm of rain per hour and you have a facade engineer’s dream storm, also known as AAMA 501.1 Testing. This is (supposed to be) the “perfect storm”.

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