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Thread: A question for Structural/Earthquake Engineers or other informant ppl

  1. #1
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    Question A question for Structural/Earthquake Engineers or other informant ppl

    Hi,

    I was just looking at some pictures of the chch quake and I noticed some modern/new buildings are also damaged.

    - PGG-Wrightson building

    - Pyne Gould Guinness building

    - car parking building

    I believe new/modern buildings should be probably designed properly against earthquakes. I am really surprised to see the new and modern buildings in the CBD have been damaged but the ordinary houses in the suburbs are safe.

    What is the reason for that? Which buildings are most prone to damage in an earthquakes in NZ? Maybe I am just making a mistake in assuming the aforementioned buildings as new ones? I mean maybe they are only renovated that seem new to me?

    Thank you

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    I don't know the specifics, but believe that the biggest problem this time has been partly the double-dose we seem to have got (the Banks Peninsula reflected its share of the seismic activity back into the city, rather than spreading the love) which in turn meant that the sheer amount of oscillation/acceleration on the ground was more than the building code is set for. Something like that. There's also the sheer number of aftershocks this city has felt since September, each one causing anything from little to lots of gradual damage to those buildings.

    IMHO, there is no such thing as a 100% quake-proof building. We can build to current technologies and current knowledge of what the possible quake might do, and that's about it. Thankfully, it is usually enough.

    If it's any consolation, if NZ didn't have a strict building code... far more would have fallen, and in turn there would have been more deaths

    As one person said to me yesterday - one major purpose of the building code is to absorb kinetic energy and allow occupants enough time to get out safely, much like the crumple zones in a car. The building can be rebuilt. The people within cannot.

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    If I recall correctly from the news coverage here both buildings which collapsed, while modern, were built before the revised building codes came into play. Sophie is also correct in stating that the Eq construction is not for the building necessarily to remain standing, just that if it fails it allows its occupants to leave safely.

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    Quote Originally Posted by Mamee & Co View Post
    both buildings which collapsed, while modern, were built before the revised building codes came into play.
    Well, that explains why they got collapsed.

    But what about the ordinary houses in the suburbs? I have heard many of them fortunately were not collapsed or damaged at all. I am just curious that how ordinary timber houses should remain safe in such an earthquake while they are not reinforced against quake shocks?
    Last edited by Painstaking_Sue; 6th March 2011 at 09:49 AM.

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    Quote Originally Posted by Painstaking_Sue View Post
    Well, that explains why they got collapsed.

    But what about the ordinary houses in the suburbs? I have heard many of them fortunately were not collapsed or damaged at all. I am just curious that how ordinary timber houses should remain safe in such an earthquake while they are not reinforced against quake shocks?
    There are - generally speaking - two different philosophies i the design for stru tures being able to withstand seismic forces: Either they shall be rigid or flexible. Rigid structures have to cope with higher stresses than flexible ones as no energy is transformed into distortion. The requirements for multi-storey buildings to be flexible are very difficult to fulfil. (Reinforced) Concrete is per se quite rigid by comparison, steel framed structures are better and timber framed ones again. However the latter cannot carry to much vertical load so high risers are not done in timber. With steel structures a design allowing for greater flexibilty could be done but very often the purpose / usage of the building leads to rigid design.

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    A definitive answer is currently not possible yet! As usual it will be combination of elements on one side and on the other side it will the specific circumstances.

    Please see http://www.enz.org/forum/showthread....98786#poststop for IPENZ preliminary comment on this earthquake. As soon as known and pulished I will forward further information from our professional organisation of engineers here in NZ.

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    Quote Originally Posted by ralf-nz View Post
    A definitive answer is currently not possible yet! As usual it will be combination of elements on one side and on the other side it will the specific circumstances.

    Please see http://www.enz.org/forum/showthread....98786#poststop for IPENZ preliminary comment on this earthquake. As soon as known and pulished I will forward further information from our professional organisation of engineers here in NZ.
    Thank you ralf

    So are you saying IN GENERAL that Timber structures withstand earthquake forces better, right?


    So you are practically saying that:

    "If it was NOT necessary for us to build high rise buildings in highly populated areas, the best option would be timber one-story structures"

    Is that correct?

    Last edited by Painstaking_Sue; 6th March 2011 at 01:04 PM.

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    Several things in relation to the above.

    1) 'Both modern buidlings' . Depends what you mean by modern, but they were 60s/70s I think, which is a long time ago.
    2) As mentioned, they were both built ahead of the most recent codes, but a code is just a code, a minimum set of forces that a new building needs to be able to withstand. However, that code is based on a balance of risk and practicality. The shallow depth, possible reflection, and the close location to the CBD meant the forces were way above the latest code. The chances of that occuring were considered very low compared to the risks to the city of requireing overengineered buildings.
    3) Large timber buildings are very hard to design and build for fire safety. FIres happen more often than large quakes, een in Christchurch.
    4) Many older buildings were damaged and either demolished or strengthed after September. So you had a 2 stage event that removed many at risk buildings without loss of life.

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    Quote Originally Posted by Painstaking_Sue View Post
    Thank you ralf

    So are you saying IN GENERAL that Timber structures withstand earthquake forces better, right?


    So you are practically saying that:

    "If it was NOT necessary for us to build high rise buildings in highly populated areas, the best option would be timber one-story structures"

    Is that correct?

    I said that there are two philosophies how structures should be designed to withstand seismic loads: Rigid or flexible. Two extremes might be a reinforced concrete bunker and a bamboo scaffold.
    The particular circumstances of a structure will limit the design. E.g. the ground might prevent the construction of a massive (heavy) structure or sound proofness the one of a moving one (as it would also move under wind).

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    re # 2:
    I would think that generally our codes here are sufficient, based on what we know. If new evidence is provided and proven adaptations are quite fast put into force.
    Around 1.5 years ago after the heavy snow falls in Canterbury the regulation for snow loads have been amended within a very sort time!

    However nothing and no one can forbid the owner of a building to do more than the code requires. Depending on the situation I do not consider that overenigineering.
    It is like the tread depth for the tyres on your car where there is a minimum given in the code but you may choose to have more.

    And there it is very similar: If the code required a deeper tread more accidents, injuries and fatalities could (have) been avoided!

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