Earthquakes destroy buildings by generating waves that propagate through soil and create movement at a building’s foundation. This energy transfers into the building’s structure; if the structure cannot properly absorb it—through a combination of strength, flexibility, and ductility (the ability to bend without breaking)—the building will fail. “You have to build in a way that allows the earthquake energy to be absorbed. Our objective as engineers is to increase the absorption,” says Professor of Civil and Environmental Engineering Oral Buyukozturk.
The first step is making a location-specific estimate of how much “demand” an earthquake can be expected to apply to a building. The next step is designing or upgrading the building’s “performance,” or energy-absorbing capacity. Varying levels of protection are possible, depending on economics and quake probability. Some large, well-financed buildings (the San Francisco Airport, for example) make use of sophisticated roller systems that isolate the building from ground motion, or internal counterweights that can offset the energy of even large quakes.
Other, less-advanced tactics vary with building type. Reinforced concrete structures, for example, need the ability to deform under stress. “If the building can deform and rotate at critical locations, it can accommodate the earthquake force; if not, it can result in the failure of building elements—beams, columns, joints, and eventually the whole building,” explains Buyukozturk. Brick or block structures fail quickly when their alignment is disturbed; they can benefit from the addition of lightweight sheet materials like aluminum, or carbon fiber reinforced plastics (polymers). “Certain configurations can result in really effective solutions that keep the walls in alignment and effectively transfer in-plane forces,” he says.
One of the simplest solutions, applicable to many types of building, is the addition of internal shear walls, starting in the basement on strong footings and running continuously to upper floors. These distribute stress and limit movement; as few as two perpendicular shear walls can greatly bolster a building.
But, Buyukozturk notes, even the best design offers no protection if not executed faithfully. “I’ve seen entire towns wiped out because all the concrete has become like sand - there’s not enough cement or reinforcement in concrete, or not enough anchorage and confinement in critical elements like columns and connections. It’s heartbreaking,” he says. “After a disaster, earthquake protection gets attention, but then interest fades. That’s where regulations and codes come into play, by transferring experience into practice. But there’s a big problem in many countries with economics, enforcement, and lack of application experience.” —Peter Dunn
Thanks to Carlie from Port-au-Prince, Haiti, for this question.