E4 BLOCKHOUSE  – A Case Study on Seismicity for POROTHERM in Martinborough

Eboss Detailed Blog published HERE

The ground floor is up, stresscrete mid-span floor going on next week – this is the long-awaited Martinborough e4 BLOCKHOUSE going up at last. 

As seismicity is first & foremost on everyone’s minds, especially in NZ’s 2nd highest seismic zone, Martinborough - we wanted to be ahead of our game so our Engineer Ted Dowdall, SENZ suggested we should consult with one of the world’s highest authorities for seismic design of both reinforced and pre-stressed masonry buildings, Professor Jason Ingham. Dr Ingham is Head of Department at Auckland University of Civil & Environmental Engineering. One of his many research groups is the University of Auckland Centre for Earthquake Engineering Research(UACEER).

Dr Ingham worked with Ted prior to lodging building consent, to produce a Peer Review we were confident would stand up to very tough scrutiny. It was a very positive journey with a great outcome for us all. 

In Ted’s own words…
Currently, New Zealand codes (which don't have confined masonry design within them) limits ULS (Ultimate Limit States) deflections to 2.5% × the storey height, or "such lesser limit as may be prescribed in the appropriate material Standard". So were there to be a code for confined masonry, this figure could easily be less. Basically it allows "flexible" buildings to drift 2.5% either way in a severe earthquake. So for a 2-storey building say 6.0 m high the limit would be approximately 150 mm.
 
Eurocode 8 limits ULS inter-storey to 0.8% i.e. 48 mm for a 6.0 m high building, a considerable difference. I suspect were there to be a New Zealand code, it would restrict allowable deflections to something more like this, the limits in the New Zealand code are clearly written for timber or steel buildings, or reinforced concrete buildings with moment-resisting frames, all of which are designed to be flexible. Reinforced concrete buildings with shear walls are much more stiff.
 
For the Martinborough e4, under ULS loadings, the building structure only deflected a certain amount in any particular direction under any particular earthquake (there were 16 different types of analyses) before the walls failed due to bending or shear rather than displacement. These deflections varied between 1.3 mm and 2.6 mm but they are not in any way a reflection of the building's capacity, because the buildings failed due to other reasons than reaching the deflection limits.
 
Essentially were we to strengthen the building, then it would be able to take higher forces, and under those forces it would deflect more, so you can't read too much into the actual figures. I hope that makes sense.
 
For instance the Taupo project analysis currently reports between 7.4 mm and 16.5 mm deflection depending upon which earthquake direction and loading pattern is considered. This is mainly due to the large amount of glazing all along the front wall of the building. It is much more flexible than the e4, but it is still nowhere near the deflection limits which would be something like 48 mm.
 
So you can't generalise for POROTHERM construction, each individual building will have its own characteristics. All you can say is something like this:
 
"Under current New Zealand codes & standards, deflection limits for buildings under ULS loadings are approximately 3 × those required by the Eurocode method of design, and generally houses constructed from POROTHERM do not come anywhere near these limits when analysed. So houses constructed from POROTHERM are extremely stiff and much less liable to cracking under earthquake loadings, than traditional timber-framed structures, or indeed most other types of construction currently used in New Zealand”.