Fristing the News

Diagnosing building failure is a task that, like diagnosing medical problems, must be performed in person. Having said that, a few basics of earthquake damage from the current news in Christchurch follow.

Some good photos from the Sydney Morning Herald. The cathedral tower collapse (sec0nd picture down) is what happens to unreinforced masonry in quakes: brittle failure which leads to collapse. The tower was most vulnerable because it was tall, slender, and relatively heavy.

The building that has collapsed except its front facade (3rd down, above the thumbnail gallery) appears to have had floor beams supported on an unreinforced brick side wall.

The picture on this page is disturbing, as this is a modern building pancaked from a relatively small (less than full design load) quake. The type of failure of the concrete columns and walls suggests that they were not reinforced sufficiently for ductility, which is the basis of modern design.

Interestingly, NZ has has earthquake codes almost as long as the US: we got them right after after the 1933 Long Beach quake, NZ did after quakes in 1929 and 1935. Buildings built before seismic design can be retrofitted but are almost never as resilient as those designed and built for seismic loading; since our knowledge about how to build for earthquakes increases with each major quake studied, each generation of buildings should be less vulnerable than the one before.

This is also the second quake to hit this area recently. Damage to brittle materials is cumulative, so even though the first quake was bigger, it is not surprising that there is severe damage now.

20 thoughts on “Fristing the News

  1. Today’s quake was smaller than the September one but also a lot closer. Much more liquefaction this time.
    There has also been a steady stream of aftershocks in the intervening 5 months, steadily weakening the masonry.

    The picture on this page is disturbing
    Is that even from Christchurch? I don’t recognise it.

  2. Grant Jacobs has been regularly updating a science-themed post here, with links to geology sources (though not engineering ones).
    “GNS says acceleration was a full 1G, against 0.8G for September, which equates to a 25% higher speed of movement at ground level.”

  3. in any case, it makes me sick to see a city I love be so damaged. I spent a few weeks in New Zealand in 1999-2000 before and after my time in Antarctica and I really enjoyed my time there, especially Christchurch.

      • Discussing the technical issues doesn’t lessen the human impact. If anything, broader knowledge of the technical issues will hopefully lead to improved design, and an effort to retrofit older buildings.

        • Retro-fitting has been in progress for a while, with old buildings sprouting big steel bolts or even an outside lattice of steel girders. It wasn’t happening rapidly, though, because Christchurch was seen as the least geologically-active part of the country. Then after September, retrofitting went into abeyance while damaged buildings were stabilised.

          The technical issues are fairly well understood.

          • The technical issues are understood by engineers and technically-minded laypeople everywhere. But the political will to deal with them is often missing and the fact that retrofitting doesn’t bring a building to the same levels of safety as a new building is often overlooked.

  4. Not my kind of engineering so here’s me talking out my ass:

    That cathedral photo is pretty amazing. The top of the facade just behind the tower base in the photo looks like trouble too. I think it’s kinda neat that there’s all this massive damage that starts shortly above where the buttresses end. Kinda like the stiffened base made the earthquake damage even worse.

    Conclusion: BAN BUTT-TRESSES! ASSES SHOULD NOT BE HAIRY!

    • Changes in stiffness tend to be where problems occur first. So “soft stories” – think a house with a carport at the first floor and a regular wall at the second – are screwed. The buttresses are probably too small to have made a big difference, but the buttress tops probably align with interior structural changes.

      The facade may have disconnected from the roof structure, too.

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