After the massive earthquake near Japan this morning one wonders if it’s possible to build an earthquake-proof building? The answer is yes and no. There are of course, engineering techniques that can be used to create a very sound structure that will endure a modest or even strong quake. However, during a very strong earthquake, even the best engineered building may suffer severe damage. Engineers design buildings to withstand as much sideways motion as possible in order to minimize damage to the structure and give the occupants time to get out safely.
Buildings are basically designed to support a vertical load in order to support the walls, roof and all the stuff inside to keep them standing. Earthquakes present a lateral, or sideways, load to the building structure that is a bit more complicated to account for. One way to to make a simple structure more resistant to these lateral forces is to tie the walls, floor, roof, and foundations into a rigid box that holds together when shaken by a quake.
The most dangerous building construction, from an earthquake point of view, is unreinforced brick or concrete block. Generally, this type of construction has walls that are made of bricks stacked on top of each other and held together with mortar. The roof is laid across the top. The weight of the roof is carried straight down through the wall to the foundation. When this type of construction is subject to a lateral force from an earthquake the walls tip over or crumble and the roof falls in like a house of cards.
Construction techniques can have a huge impact on the death toll from earthquakes. An 8.8-magnitude earthquake in Chile in 2010 killed more than 700 people. On January 12, 2010, a less powerful earthquake, measuring 7.0, killed more than 200,000 in Haiti.
The difference in those death tolls comes from building construction and technology. In Haiti, the buildings were constructed quickly and cheaply. Chile, a richer and more industrialized nation, adheres to more stringent building codes.
As the buildings get bigger and taller other techniques are employed such as “base isolation.” During the past 30 years, engineers have constructed skyscrapers that float on systems of ball bearings, springs and padded cylinders. Acting like shock absorbers in a car, these systems allow the building to be decoupled from the shaking of the ground.
Watch the video below to see these system in action. These buildings don’t sit directly on the ground, so they’re protected from some earthquake shocks. In the event of a major earthquake, they can sway up to a few feet. The buildings are surrounded by “moats,” or buffer zones, so they don’t swing into other structures.
Another technique to dampen the swaying of a tall building is to build in a large (several tons) mass that can sway at the top of the building in opposition to the building sway. Known as “tuned mass dampers”, these devices can reduce the sway of a building up to 30 to 40 percent. The Taipei 101, formerly known as the Taipei World Financial Center, has just such a giant pendulum mounted between the 88th and 92nd floors. Weighing in at 730 tons and capable of moving 5ft in any direction, it takes the prize as the worlds largest and heaviest building damper. In fact, it is so heavy that it had to be constructed on site since it is to heavy to be lifted by a crane.
At Imagination Station we have several shake tables in our Engineer It! exhibit that give visitors the chance to build various model structures and then test them on a shaking “earthquake” platform. Our Earthquake platform is large enough to stand on, but not nearly as big as the shake tables at some engineering test facilities. These things are really called shake platforms because they are much, much larger than a “table”.
Here is a great video from WIRED that shows how a large shake platform can be used to test a full-scale structure in response to the motion of an earthquake. I love the crazy shot at 1:17 into this video of a ball bearing base isolator scooting around to stabilize a structure!
For more information about earthquakes in general check out the United States Geological Survey’s earthquake website.
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