On 22nd February 2011, a powerful earthquake measuring 6.3Rs hit the city of Christchurch in New Zealand. WAN chatted to Ziggy Lubkowski, Arup's seismic business and skills leader for Europe, Africa and the Middle East to get his expert opinion on the structural damage to buildings in the city.
Is the structural damage we are seeing in Christchurch typical for an earthquake of this magnitude?
When we consider earthquakes it’s not just the magnitude of the earthquake it’s the relative location of the source of the earthquake to where the damage is occurring. So what we have in this case is a moderately sized event of magnitude 6.3 very, very close to a built-up area. If we compare this to the event in September last year which was a magnitude 7.1 event but was considerably further away, the motions from the 7.1 event in Christchurch are about three times lower compared to February this year.
There is talk of bringing in new legislation to ensure that all older buildings in the area are made as earthquake resistant as possible. In what ways can this be approached without causing significant damage to the existing architecture?
Well, I think upgrading old buildings to current earthquake standards is a big issue that actually affects most major cities where earthquakes are a problem. So any large cities in the Mediterranean, in Western California, in Japan, they all have large percentages of old building stock that have survived because there hasn’t been a big earthquake since they’ve been built but there will be one around the corner.
The cost of actually replacing or upgrading everything is significant, and often the pragmatic solution is followed so you spend the money on the key public facilities – your older hospitals, bridges, etc. In California they’ve been upgrading all the bridges to meet the latest standards and I know in New Zealand they’ve been doing similar types of things – I don’t know the specific details, but the importance is upgrading those key facilities.
There will always, always be older buildings which are vulnerable until you either demolish everything and replace or spend a lot of money. And often people don’t have the money to spend. Where’s the value? You could spend a lot of money and there’s no earthquake, or you spend a lot of money and there’s an earthquake and you have a good situation.
In truth it’s very, very sad that we’ve had (at the moment) 75 confirmed fatalities, but this number could have been much higher and it actually shows that a lot of things have gone very right; because you compare with something like Haiti last year, similar size of event, close to a major conurbation. Building stock in Haiti a lot, lot worse, population a lot more crammed together, and you’ve got tens of thousands of people dead.
So actually a lot of things have performed well. There are clearly the Pyne Gould Building and the CTV Building – there are questions as to why those have collapsed, and until someone has actually done the forensic examination it’s difficult to judge that. But we do know already that the level of shaking exceeded the design codes in some places by a factor of two, at least. It seems that the level of motion that the earthquake put into the buildings was more than they were designed to stand.
The questions is should they still have collapsed and was there an issue - and we don’t know at the moment - but they were tested very much to the extreme. The way we do designs is that we don’t design for the biggest earthquake, because again there’s a balance of how much money you can spend to create an earthquake-proof building. But ideally you design buildings so that people can get out alive. They may be damaged and there could even be damage beyond repair, but as long as people get out alive you’ve met the design requirement.
You expect to see damage; what you don’t expect to see is collapses as we are seeing in a few cases. Now the television, the media tend to highlight those cases but what you actually need to do is to do a proper review street by street and say: “OK, you had two collapses and there are another 200 buildings that are pretty much unscathed.” So you’ve actually got to look at it without focussing in on the collapses, you’ve got to focus in on the ones that survived and you don’t get that view from watching things on television unfortunately.
We’ve had reports that some of the sub-strata beneath the buildings has been liquefied by groundwater forced to the surface. How is this going to affect the long-term rebuilding operations?
If we have liquefaction, it’s generally something that causes a serviceability issue for buildings. Generally speaking liquefaction won’t cause collapse; you might get settlements, buried structures may float, but it doesn’t cause collapse. There are different ways in which you can deal with that. You can build things to accommodate those kind of effects or you can improve the ground but again that starts to become very expensive.
And so I think that if you’ve got facilities such as hospitals, bridges and other key structures you might wish to focus the money on those areas, but on a more general level you don’t need to. I think it’s fairly well established that Christchurch is underlain by sandy deposits which are susceptible to liquefaction, and we saw quite a lot of liquefaction in the September event.
Do you think that has had a major affect on what has happened in this more recent event?
No. In truth if the ground liquefies, the motions from the earthquake tend to get damped out, reduced by the liquefied layer. It’s like putting a building on roller skates because the liquefied layer turns into a heavy fluid so it’s got no stiffness, it’s got no strength and it’s very difficult to transfer energy through that.
So if a building is sitting on a liquefied layer the structure of the building tends to see less load and therefore will perform better, the foundations may get more of a hit so piles may be damaged or a shallower foundation may settle more, but that tends to be more an issue in terms of the buildings on the tilt slightly as a result of the earthquake, but it’s met its design requirement of life safety. The Television Building and the Pyne Gould Building - I would be very unlikely to say that any liquefaction occurred in those two cases.
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