Question about NZ earthquake zone classification

[emka]

Could some building crack please explain to me how to understand NZ earthquake zones (according to NZS 3604:2011, Figure 5.4)? There are four zones, Zone 1, Zone 2…., but I can’t find any explanation on these zones, such as low, medium, high…

Northland including Auckland and Tauranga and down to Wellington are in Zone 1, as is the coastal slice down from the Banks Peninsula (including ChCh?) to Invercargill in the South Island and Stewart Island.

Wellington and the Banks Peninsula being known high-risk areas, I am tempted to interpret Zone 1 as the highest-risk zone. However, I am not sure because it also includes Auckland and Northland, which do not come to the top of my mind as highly earthquake prone areas.

Does this mean, for bracing purposes in timber-framed buildings, that all areas in Zone 1 are to be braced to the highest possible impact?

[JandM]

My guess is that the stated Zones were declared before the big Christchurch earthquakes - they were a surprise, as mentioned here.

http://www.teara.govt.nz/en/map/4416...ground-shaking

After a further search: oh, yes, it was, as stated here, with what looks like the origin of that same map. http://www.naturalhazards.co.nz/proj...f?projectId=11

[ralf-nz]

These zones are based on a derivation of this factor that corresponds to the peak ground acceleration. As JandM has pointed out indirectly, these are a result of an evaluation from geologists etc. transformed now to be used in structural design. The standard you should actually study is NZS1170 'Structural Design Actions'; its section 5 deals with Earthquake Actions in NZ in general. NZS 3604 Timber Framed Buildings is based on the former in this regard but is rather limited in its application and use; check its scope for details.

When you read the whole section 5.3 of NZS3604 you'll note that these zones as indicated in figure 5.4 are just one element out of several that are used to determine the earthquake bracing demand here. Other elements to be considered are subsoil type, building level, building size, roofing and cladding weights and floor live loads.

When you look a little further in table 5.8, 5.9 & 5.10 you'll note that the derived factors are increasing from EQ zone 1 through to zone 2 and zone 3 to zone 4. Thus zone 1 is the lowest 'risk' zone and zone 4 is the highest; based on the acceleration. In NZS 1170 the maps are more detailed but similar in their contours; in addition to the maps that standard also has a table with stated figures. Based on the Canterbury earthquakes these figures have been increased for Canterbury in 2011. (The map has not been redrawn yet, but I would think that'll happen in future.)

Your interpretation of the map is somewhat wrong - did you use a bad copy?! North Island is spread over three zones, increasing from the northwest to the southeast; Wellington being in zone 3. Christchurch is in zone 2 and Akaroa in zone 1; both ought to be now in zone 3 too, since these changes.

Another factor that NZS1170 takes directly into account is a Near Fault - factor - for the distance to fault lines IF they are known. The Canterbury earthquakes showed the geologists some new ones...

[emka]

Thank you, Ralf. This was indeed helpful. You are right: most likely the map I saw was old and might have been a bad copy (scan). I found it somewhere on the net without any explanations of the zones. I don't actually have a full copy of NZS 3604, just some comments on the changes made to the previous, 1999 version. Therefore I can't read the whole of it or look at other tables that might also be interesting to me. I don't have a copy of NZS 1170 either.
I realise that other factors, such as exposure zones (wind force, also salty sea spray that's corrosive to metal although the latter is not relevant to bracing) and the ones you mention are also taken into consideration to determine bracing demands. Anyway, now that I know that zone 1 is the lowest earthquake risk zone, my main question is answered. It always pays to ask ;-))

[JandM]

http://freedownloadb.com/pdf/timber-...4-7370823.html
http://www.worldoffiles.net/link-f-a...-free-download

[ralf-nz]

You're welcome

I realise that other factors, such as exposure zones (wind force, also salty sea spray that's corrosive to metal although the latter is not relevant to bracing)

Sorry, but that is not quite right, steel is frequently used for bracing - even exposed at sea!

[Super_BQ]

If you're building in Christchurch you will need to pay particular detail to the geo 'Technical Category" of the land. http://cera.govt.nz/maps/technical-categories

You will find Chch city council will only give building consents based on what ground tests made on site in reference to the TC categories and not based on the NZS standard. What we are finding is that sections that are in the TC1 category sell faster and at a premium over TC3 land. This is because TC3 has a major requirement in foundation build which adds an extra $40,000 to $60,000 on an average size house. Few days ago I was talking to a builder down the street (during his open home house) and the discussion about using 'rib raft' foundation proved to be an expensive requirement because that kind of $ could buy a lot of thing ; meaning it could be the difference of a cheaply fitted low budget house vs a very high end fitted house with an elaborate kitchen design and home central heating system, PDL Modena electrical fittings, more heat pumps, basically the whole works. In another way, it could be an extra bedroom + a study.

[emka]

I am not building in Christchurch, so these geotechnical considerations are not relevant to me. I am building in Tauranga after having kissed my original plan to buy or build in Auckland good-bye. Real estate prices there have ballooned out of my price range.

The soil reports for the section are fine, it is suitable for a standard concrete base, and the house design I have chosen is pretty simple (single storey, simple shape of envelope, brick & tile with a cavity). I was just unsure what earthquake zone 1 was, as there was no explanation on the drawings that went to council for consent. Now that I know it's the lowest risk classification, this has put my mind at ease.

[jawnbc]

I wouldn't use bricks for structural purposes in any region with significant earthquake risk. I'm not a builder, mind you. But bricks and earthquakes are, from what learned on my (Canadian) body corporate executive, a terrible mix.

[Mamee & Co]

Emma, Tauranga is still in an area. My OH, as an architect, says he would not use brick these days preferring a cladding of weatherboard or zincalume being lighter and more flexible. I insulation can still be achieved as it is the cavity that matters.

Having seen the damage to many brick/block structures in Christchurch it is something we would avoid...as well as roof tiles!