Just a quick update today, following on from the theme of the last post, and the horror of the devastation Japan is now experiencing.
With the NZ government announcing a Royal Commission of Enquiry into the building collapses in Christchurch, it has been interesting to observe people’s perceptions, from politicians all the way down (or should that be up?).
The disconnect is partly in trying to understand why there was so much damage in Christchurch for a relatively small 6.3 magnitude quake, as opposed to the massive 9.0 quake seen in Japan.
So the follow on from my last post on ’Buildings are not Designed to be Race Cars’ where I talked about Peak Ground Acceleration (PGA), I thought it would be interesting to make some comparisons.
The Geonet map from the February quake is here and shows central Christchurch had a PGA of between 0.6g and 0.8g with up to 1.88g in the eastern suburbs and an incredible 2.2g at the epicentre.
The PGA maps from the U.S. Geological Survey show that in Sendai (about 130km from the epicentre), the PGA was 0.21g, with surrounding areas experiencing between 0.35g and 0.65g. In Tokyo the PGA was 0.17g. Have a look at the maps here and you can mouse over recording points to see the PGA expressed as a percentage of g.
eg 100% = 1g, 20% = 0.2g.
Even at these levels of acceleration it would appear that some buildings in Japan suffered structural damage as a result of the earthquake rather than the Tsunami.
So here’s the kicker. At the earlier reported magnitude of 8.9 in Japan (it has now been updated to M9.0) the energy released was 8000 times greater than in Christchurch, but the Japanese mainland experienced a significantly lower PGA than Christchurch did.
The Christchurch experience was reported as a short sharp jolt that was extremely violent. The Japan experience has been reported as a very long sway that just continued to build and build in intensity.
Location, proximity, soil types, rupture dynamics and many other factors mean that how each eathquake is expressed (and felt) at the surface is different.
And now back to the design of buildings. How do you best account of these huge differences so that you can structurally design buildings with some certainty? Can a building be made to reliably resist a PGA of say 1.5g (when a Formula 1 car accelerates at 1.4g)? Can older buildings be retrofitted to even remotely approach this? Assuming not, what level is an acceptable level to get older buildings up to? Will we ever experience that sort of PGA again in NZ? So is it worth designing buildings to resist that?
Engineers can now point to real life examples to answer many of those questions and scientists have more data to analyse than they have ever had before .
There are many, many newer buildings in Christchurch that did survive remarkably intact, despite the PGA they experienced (apparently) exceeding their design load state. For instance, a new-ish building around the corner from the CTV building hasn’t even a broken pane of glass.
Again, I must stress that I am not a structural engineer, and these are my thoughts as an Architect. But these are all questions that we have been discussing in our office, with no clear answer. The Department of Building and Housing and the Royal Commission will certainly have their work cut out trying to make sense of it all. And lets hope that sense does prevail.