September 19, 2010
Busy times for a structural engineer, especially one with a focus on earthquake design. For those who care not for the Southern Hemisphere, on Saturday, September 4th, Christchurch was rocked by an earthquake. While NZ is known for its proximity to the Ring of Fire and an active seismic history, this doesn’t normally apply to the whole country. Auckland and the Northlands and the East Coast of the South Island are low-seismic regions. Christchurch is squarely on the East Coast of the South Island and was caught off guard. It was the equivalent of the Haiti earthquake happening in Tempe, Arizona. Phoenix would be pretty torn up. So what happened, what does this mean, and how does this affect me? I’m glad you asked.
First, what happened. At 4:35am on a Saturday morning, a 40 second long earthquake magnitude 7.1 earthquake emanated from a previously unknown fault roughly 25 miles (40km) from Christchurch. Richter Scale projections give very little useful information to the scientific community but are a simple way of describing magnitude to laypeople. It is supposed to describe the total energy released in on an exponential scale (where a 7 is 10 times greater than a 6 and 100 times greater than a 5, etc.). In reality, it doesn’t accurately estimate energy and it says nothing for the truly destructive and highly measurable parameters that are better indicators for predicting damage. What engineers, geologists and seismologists look for is ground acceleration. The Christchurch earthquake (known internationally as the Canterbury Earthquake) had peak ground accelerations measured at 1.25g or 1.25 times the speed of gravity. That’s a lot. Especially lasting for 40 seconds. If that was purely vertical motion, the ground would move away faster than you could fall. An unanchored house would be picking up downward speed while the ground was coming back up at it. Think Wile E. Coyote. Not good. Even as horizontal motion, that’s like Bill Murray yanking out the table cloth in Ghostbusters.
That brings us to the second major issue in this earthquake: soil stability. A foundation is only as good as the soil into which it is embedded. Hard bedrock is the best material for stability and clay is the worst, but silty sand is not much better than clay. In an earthquake, silty sand has the added detriment of liquefaction. In technical terms, the soil particles lose their shear strength. As the particles experience the pulses of energy from an earthquake, they dance and vibrate, turning an apparently dry material into a quivering jelly akin to quicksand. Imagine you’re standing at the beach down by the waterline. If you stand still there’s barely a footprint but if you wiggle your feet back and forth water draws to the surface and your feet begin to sink. Foundations in liquefied soil can sink or buckle, knocking a house off its foundation or sending cracks up the walls. This is also particularly damaging to utility lines and empty submerged tanks which become buoyant and can pop to the surface like balloons. Christchurch, unfortunately, was built on a giant alluvial fan of river washout, reclaimed salt marshes, tidal flats and coastal swampland—all silty sands and clays and highly susceptible to liquefaction.
The bad news for Christchurch does not stop there. Because the city is older, and was built in what was believed to be a low-seismic zone, there are tons of brick buildings and brick facades. Brick may have worked for the 3 Little Pigs but they were only worried about huffing and puffing. The weight of brick is great for resisting wind which is why they use it in tornado-prone areas like the Midwest. In an earthquake the mass of brick absorbs lots of energy but because of its brittle nature, it cannot dissipate it without failing. Brick is the single worst building material in earthquake regions. While wood houses shake, steel buildings sway and concrete tries to hold firm, brick peels like corn off the cob. Buildings in earthquake zones need to have a balance of strength and ductility, and the ability to resist tension and compression. If you have enough strength you don’t have to bend; if you have enough ductility, you can bend without breaking. The balance of tension and compression is more due to material properties. Timber and steel are good for tension but not as good with compression. This can be dealt with by the configuration of the structural members. Concrete and cinder block are good in compression but terrible in tension, which is why steel rebar is used. Brick is solid and therefore has no place to put rebar.
As you can tell, Christchurch is a bad place for an earthquake. Given the magnitude of the quake, the lack of casualties and relatively minimal damage is a testament to NZ building code, structural engineers and builders of the past. There were no deaths. Compare that to Haiti which had a similar earthquake but with mostly newer buildings and very little brick and they had 230,000 deaths and untold numbers injured and displaced. Cantabrians also banded together as neighbors and took care of each other and allowed the Civil Defense to maintain order. Some of the news media outlets tried to drum up stories about looting and rioting but everything went unconfirmed and was later recanted. The closest anyone could come to a shocking headline was that one man died of a heart attack close to the time of the earthquake but there’s no proof they were related. The government was measured in its response and aid providers were brought in only as they could be used. Senior engineers from my office were dispatched quickly, but only after damaged buildings could be catalogued and coordinated were mid-level engineers like myself sent in.
I was flown down last Wednesday to perform building investigations for an insurance assessor. It was very exciting. I have likened it to being a doctor but only getting to work on patients once a decade and then a whole bunch of patients all at once. I don’t mean to compare myself to doctors because they invented HMOs and those are the basest form of evil. My work is far, far more benevolent. Like Ghandi. But with more sex appeal. I’m headed back again this week to do more assessments and I’ll put together a big photo gallery. Here’s what I’ve got so far.
http://picasaweb.google.com/mfontanesi/ChristchurchEarthquake#
