At the risk of sounding Orwellian, no building is 100% earthquake-proof, but some are more earthquake-proof than others.
The number one way that earthquakes damage buildings is they move them back and forth, putting sideways stresses on the walls that are designed for downward pressure. In poor regions, like Haiti, the simplest way to build a building is to make the walls out of brick or concrete block and put a roof over it.
Bricks and concrete have incredible compression strength, meaning you can put a lot of weight on them and they won’t break easily. However they have almost no tensile strength, meaning they can’t bend.
When these buildings are hit by and earthquake, they start moving sideways and all the mass of the building pushes on the middle of the wall, which can’t support the sideways force and crumbles, taking the building down.
To make a building that can survive a moderate quake, the building has to be designed as a box that can take sideways pressure as well as the weight of the roof. In earthquake regions of the US building codes require that houses are built with reinforcements to take the sideways pressure. If the buildings in Haiti were built according to US building codes they still would have collapsed, but the residents would have a minute or two to get out which would have greatly reduced the loss of life.
Even if you make a perfect box that can withstand as much sideways pressure as it can downward pressure a large building won’t be totally earthquake proof as the shaking can literally shake it off its foundation. With a single-family house this is repairable and most people would only suffer minor injuries from being shaken around, with taller building this can cause them to topple over.
To prevent this engineers have been using a foundation technique called base isolation. This means the foundation and the bottom of the building move separately. With smaller buildings this is done by putting large ball bearings between the house and its foundation, with larger building shock absorbers are built into the base of the building to slow the shaking.
Techniques like vibration dampeners, large weights placed on the top of the building that move counter to the vibration, stress points, points designed to fail first, and other methods are being tested and put into practice in earthquake prone regions.
Making buildings that can survive the scale of the earthquake that hit Haiti is nearly impossible because of its loose soil, but there are building methods available that can make buildings withstand an earthquake longer giving the people inside the building more time to get outside.
Hopefully, as Haiti rebuilds from this tragedy some of these building techniques will be used so the next time an earthquake hits the loss of life will be greatly reduced.
Here is a cool little demonstration of the effects of an earthquake to different types of foundations: Discovery Channel Earthquake Simulator.
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4 comments:
Not to be a stickler, but tensile strength, while related to shear strength, is not the same. Tensile stress are those that effectively pull apart, the true opposite of idea compression. Shear stresses are the ones that cause bending and work perpendicularly.
Often, if not always, materials with good tensile strength can handle shear forces well, but they may not do well with compression (because those same malleable properties work against one). Brittle materials, which might have great compressive strength, might be very susceptible to both shear and tensile stresses. But it depends.
Woods, for example, have good shear strength if going perpendicular to the grain (if I remember correctly) but poor going with the grain. They have crappy tensile strength, but good compressive strength. Clever use of wood (like those "paper houses" with their heavy tile roofs used in Japan) can often withstand tremendous shaking and have the added advantage of being readily rebuilt. Very susceptible to fire, however.
Japan has done some incredible things with earthquake engineering as has California and other quake prone areas. Wikipedia has a great article on this that provides some detail.
Yeah, I did mean shear stress.
I was also trying to keep it simple.
As far as Japan they actually made some buildings so strong that an earthquake knocked them off their foundations and they fell over and landed intact.
Kind of neat.
Nice article. Thanks for the Earthquake Simulator link. Only if Haitians could afford better and stronger foundations.
Just like the term "Bullet Proof" the more correct term should be "Bullet Resistant". I really like your style of writing. You have a great way of communicating some relatively difficult topics in a manner that can be easily understood by people that may have never been exposed to that topic. Thank you for visiting my site and commenting on my post regarding the Google Friend Connect. Take care. ;o)
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