Why Buildings Respond Differently to Earthquakes

[ricktee]

There was only one earthquake with such a magnitude of 7.1. The second one was of less magnitude but more damaging. This is evidence that magnitude is not all there is.

BTW our code not only uses the hazard factor "Z" as mentioned in an earlier post but there are more factors e.g. for "site subsoil class", being "near [a] fault", "importance of a structure".


As also written earlier there is currently no evidence from the latest major earthquakes that the Japanese building code is indeed of a "higher standard" than our NZ one!

Magnitude is used to measure earthquakes. When you have low magnitude earthquakes, it is likely there will be minimal or no damage in buildings (even taking to account other factors such as "site subsoil class", being "near [a] fault", "importance of a structure" ).

As the magnitude goes up , then people will be taking account other factors.

There is not much we can do about the other factors like subsoil class when the tall buildings (like the ones in Christchurch CBD) are already there. Like I said before, engineers should make sure buildings can withstand at least an earthquake with the magnitude of previous quakes.


If you look this article : http://www.bbc.co.uk/news/science-environment-12540504

It says that:"

In fact, a magnitude 6 earthquake possesses 32 times more energy than a magnitude 5 quake, as seismologists use a logarithmic scale to record these natural disasters.

a gap of two steps, from 5 to 7, represents an earthquake nearly 1,000 times stronger."

Japan had a 9.0 and NZ had a 7.1. Their buildings held up better. Is it because of other factors? Like I said, it could be. It is my opinion that it is probably the code.

[ricktee]

Well, the current code appears to be pretty good IMO. I mean, how many lives were lost in a building built under current code? If it shows that many lives were lost in a newly built building, then yes, there should be an upgrade. But you have to consider that downtown Christchurch had many old heritage like buildings and by efforts, most of them didn't meet the 67% current standard. Let me reiterate, taking old buildings and upgrade strengthening them to 67% of the current standard. So right off the start, you could have 80% of the buildings not meeting the current standard and you suggest to increase the standard again? What future would this have for old buildings that can't even make 67% today? We may end up with a lot of empty parking lots downtown if things get too tough.

80% of the buildings not meeting the current standard and you suggest to increase the standard again?

That is news for me. I did not know engineers approve safety evaluations of buildings not meeting the standard. If so, I suggest not raising the standard of the code. But they have to strengthen buildings to (whatever % )of the current standard to make it withstand a magnitude of past quakes. Not doing so might have consequences similar what happened on 22/02/2011.

[ralf-nz]

Magnitude is used to measure earthquakes.

Magnitude is one possibility to measure earthquakes.

And for the general public it might seem that is the only one. As it might be that the general public is ignoring other factors that are as relevant as the Hazard Factor for a regions.

[Mamee & Co]

Hi Ricktee

I would suggest that you widen your research upon earthquakes, damage and building codes. Your fixation with Richter magnitude is blinding you to the other scales of measurement involved in natural disaster such as this. I have seen no mention of PGA, ground velocity, horizontal and vertical acceleration etc. etc. and the myriad of scales that are used to perform an indepth measurement. Also ignored is the pretty basic distance from the epicentre measurement.

From experience, I live in a house in which two biggerr earthquakes have been felt...one measured a 7.1, the other a 6.3. The 6.3 caused substantially more damage to the building and its contents than the 7.1...where all I lost was a jar of pasta sauce on the edge of a shelf. Yet by your argument all that matters is that it was a 7.1 and therefore larger......

There are far more things to consider than magnitude on the Richter Scale, there is also direction and the amplification effects of the natural geological structures that surround the area of rupture.

You have to take into account the variety of other factors. I am not well versed on building codes, but am well aware that most buildings here stood up to the quake sufficiently to allow the majority to escape with their lives. That, in my opinion, is a good outcome.

My suggestion is that you look both into more information upon the nature of earthquakes and the factors other than Richter scale magnitude and also the building codes in both countries as at today's date, rather than relating what you think is the building code to buildings built years ago and declaring that NZ does not have stringent enough building codes.

What I would say though, is that planning in both countries, with building on reclaimed land and siting of nuclear power plants next to the sea in a tsunami risk area do seem, to my uneducated mind, a little short-sighted!! But such is the power of the capitalist society in search of the dollar or the yen!!! Free Market Economics rule....yeah..right... *tone of cynicism* :-(

[ralf-nz]

Our weekly magazine EngineeringDirect (ISSUE 391 • 7 APRIL 2011):

"As I indicated last week, the distribution of our Fact Sheets garnered many responses. You may be interested to know they were quoted by Japanese television and newspapers, used for high school lessons, distributed at a branch of Mitre 10 and recommended through an insurance company."

[ricktee]

You have to take into account the variety of other factors. I am not well versed on building codes, but am well aware that most buildings here stood up to the quake sufficiently to allow the majority to escape with their lives. That, in my opinion, is a good outcome.
:-(

I agree it is a good outcome. What is not a good outcome is 300 buildings will have to be demolished. See article:

http://www.nzherald.co.nz/nz/news/ar...ectid=10719274

According to the article :[ "engineers are calling for a rethink on building design in the wake of the devastation in Christchurch after the February 22 quake."

"Associate Professor Greg MacRae, an earthquake engineer at Canterbury University, told the Science Media Centre the most important factor to consider when rebuilding the city was seismic sustainability."

"People have a choice. They can design and rebuild using 1980s technology, and they'll get the kind of results we've seen. Or they can use modern technology and get very little damage."]

After reading the article, I think the future buildings constructed will respond better if they use "modern technology"

[Mamee & Co]

There is no doubt though Rick that they will use modern technology if they rebuild now. I can't understand why you would think that this country would allow buildings to be erected now with codes used between 100 and 30 years ago. The codes have already changed and may be strengthened further because of the levels of ground acceleration that were experienced.

But buildings did suffer in Japan too. The quake was 80km off shore. If it had been directly under the buildings or within 10k of the main CBDs you might have had a more direct comparison as to comparative building reactions to earthquakes. Unfortunately the tsunami proved that codes are not all that there is when considering natural disasters...planning has a lot to do with it too!!

If the quake had been 80km away there would have been less of a problem, if it had been felt much at all!!!!!

[ralf-nz]

Not always is the new better than the old. E.g. the old timber structures built at first in the wilderness are very much seismic resistant; not very comfortable by modern standards though !

However in another thread I have very recently posted:

'... we received a paper CHRISTCHURCH SEISMIC DESIGN LOAD LEVELS - INTERIM ADVICE from SESOC (Structural Engineering Society NZ) on this. Apparently this paper can currently not be accessed on the internet.

Amongst others it reads:
'SESOC has recommended to the Department of Building and Housing that the hazard factor in the Canterbury region is increased to a minimum of:
Z = 0.3 ...

The revised z factor is intended only for use for the design and assessment of buildings and structures, pending further research.'



http://www.enz.org/forum/showthread.php?t=33695

[Super_BQ]

There is no doubt though Rick that they will use modern technology if they rebuild now. I can't understand why you would think that this country would allow buildings to be erected now with codes used between 100 and 30 years ago. The codes have already changed and may be strengthened further because of the levels of ground acceleration that were experienced.

Yes but at what cost? Everyone loves to stand around and say the standards will improve without having a proper assessment. Of the 300 buildings that have to come down, how many of them were damaged based on recent designs vs. old designs? I'm sure the majority of these buildings were very old buildings built pre-centuries ago. The real question is how good was the 1980s methods and how many of these buildings had to be taken down. It may be that there's nothing wrong with the current building code but rather, a problem being, landlords were slow at upgrading the buildings to 33% earthquake rating.

The other aspect is there comes to a point where no amount of engineering can prevent a failure from an earthquake.

Then we've not even touched the issue on what will happen to residential home construction? What kind of building improvement will they see?

[JandM]

I'm sure the majority of these buildings were very old buildings built pre-centuries ago.

Christchurch was founded around 1850.