The Bronze Age saw the rise of several successful civilizations, including a few that managed to build impressive cities with ordered grids and sophisticated plumbing. Now, scientists think that tectonic activity may have contributed to the demise of some of these ancient cultures. For example, research conducted at the city of Megiddo (now part of present-day Israel) suggests that a massive earthquake may have devastated the city, leading to the sandwich-like layers found in excavations. And a series of earthquakes may have brought down the Harappan civilization (in what's now Pakistan), which disappeared suddenly in 1900 B.C.E.
We're just as susceptible today to the aftereffects of powerful earthquakes. When exposed to the sudden lateral forces produced by seismic waves, even modern buildings and bridges can fail completely and collapse, crushing the people in, on and around them. If anything, the problem has become worse as more people live in urban environments and as structures have grown. Luckily, over the last few decades, architects and engineers have devised a number of clever technologies to ensure that houses, multidwelling units and skyscrapers bend but don't break. As a result, the building's inhabitants can walk out unharmed and start picking up the pieces.
On the next few pages, we've assembled 10 of these temblor-thwarting technologies. Some have been around for several years. Others, like the first item in our countdown, are relatively new ideas that are still being tested.
The Levitating Foundation
Engineers and seismologists have favored base isolation for years as a means to protect buildings during an earthquake. As its name suggests, this concept relies on separating the substructure of a building from its superstructure. One such system involves floating a building above its foundation on lead-rubber bearings, which contain a solid lead core wrapped in alternating layers of rubber and steel. Steel plates attach the bearings to the building and its foundation and then, when an earthquake hits, allow the foundation to move without moving the structure above it.
Now some Japanese engineers have taken base isolation to a new level. Their system actually levitates a building on a cushion of air. Here's how it works: Sensors on the building detect the telltale seismic activity of an earthquake. The network of sensors communicates with an air compressor, which, within a half second of being alerted, forces air between the building and its foundation. The cushion of air lifts the structure up to 1.18 inches (3 centimeters) off the ground, isolating it from the forces that could tear it apart. When the earthquake subsides, the compressor turns off, and the building settles back down to its foundation. The only thing missing is the theme song from the "Greatest American Hero."
Another tried-and-true technology to help buildings stand up to earthquakes takes its cue from the auto industry. You're familiar with the shock absorber -- the device that controls unwanted spring motion in your car. Shock absorbers slow down and reduce the magnitude of vibratory motions by turning the kinetic energy of your bouncing suspension into heat energy that can be dissipated through hydraulic fluid. In physics, this is known as damping, which is why some people refer to shock absorbers as dampers.
Turns out dampers can be useful when designing earthquake-resistant buildings. Engineers generally place dampers at each level of a building, with one end attached to a column and the other end attached to a beam. Each damper consists of a piston head that moves inside a cylinder filled with silicone oil. When an earthquake strikes, the horizontal motion of the building causes the piston in each damper to push against the oil, transforming the quake's mechanical energy into heat.
Damping can take many forms. Another solution, especially for skyscrapers, involves suspending an enormous mass near the top of the structure. Steel cables support the mass, while viscous fluid dampers lie between the mass and the building it's trying to protect. When seismic activity causes the building to sway, the pendulum moves in the opposite direction, dissipating the energy.
Engineers refer to such systems as tuned mass dampers because each pendulum is tuned precisely to a structure's natural vibrational frequency. If ground motion causes a building to oscillate at its resonance frequency, the building will vibrate with a large amount of energy and will likely experience damage. The job of a tuned mass damper is to counteract resonance and to minimize the dynamic response of the structure.
Taipei 101, which refers to the number of floors in the 1,667-foot-high (508-meter-high) skyscraper, uses a tuned mass damper to minimize the vibrational effects associated with earthquakes and strong winds. At the heart of the system is a 730-ton (660-metric-ton), gold-colored ball suspended by eight steel cables. It's the largest and heaviest tuned mass damper in the world.
In the world of electricity, a fuse provides protection by failing if the current in a circuit exceeds a certain level. This breaks the flow of electricity and prevents overheating and fires. After the incident, you simply replace the fuse and restore the system to normal.
Researchers from Stanford University and the University of Illinois have been experimenting with a similar concept in the quest to build an earthquake-resistant building. They call their idea a controlled rocking system because the steel frames that make up the structure are elastic and allowed to rock on top of the foundation. But that by itself wouldn't be an ideal solution.
In addition to the steel frames, the researchers introduced vertical cables that anchor the top of each frame to the foundation and limit the rocking motion. Not only that, the cables have a self-centering ability, which means they can pull the entire structure upright when the shaking stops. The final components are the replaceable steel fuses placed between two frames or at the bases of columns. The metal teeth of the fuses absorb seismic energy as the building rocks. If they "blow" during an earthquake, they can be replaced relatively quickly and cost-effectively to restore the building to its original, ribbon-cutting form.
In many modern high-rise buildings, engineers use core-wall construction to increase seismic performance at lower cost. In this design, a reinforced concrete core runs through the heart of the structure, surrounding the elevator banks. For extremely tall buildings, the core wall can be quite substantial -- at least 30 feet in each plan direction and 18 to 30 inches thick.
While core-wall construction helps buildings stand up to earthquakes, it's not a perfect technology. Researchers have found that fixed-base buildings with core-walls can still experience significant inelastic deformations, large shear forces and damaging floor accelerations. One solution, as we've already discussed, involves base isolation -- floating the building on lead-rubber bearings. This design reduces floor accelerations and shear forces but doesn't prevent deformation at the base of the core-wall.
A better solution for structures in earthquake zones calls for a rocking-core wall combined with base isolation. A rocking core-wall rocks at the ground level to prevent the concrete in the wall from being permanently deformed. To accomplish this, engineers reinforce the lower two levels of the building with steel and incorporate post-tensioning along the entire height. In post-tensioning systems, steel tendons are threaded through the core wall. The tendons act like rubber bands, which can be tightly stretched by hydraulic jacks to increase the tensile strength of the core-wall.
Seismic Invisibility Cloak
You may think of water or sound when considering the topic of waves, but earthquakes also produce waves, classified by geologists as body and surface waves. The former travel rapidly through Earth's interior. The latter travel more slowly through the upper crust and include a subset of waves -- known as Rayleigh waves -- that move the ground vertically. This up-and-down motion causes most of the shaking and damage associated with an earthquake.
Now imagine if you could interrupt the transmission of some seismic waves. Might it be possible to deflect the energy or reroute it around urban areas? Some scientists think so, and they've dubbed their solution the "seismic invisibility cloak" for its ability to render a building invisible to surface waves. Engineers believe they can fashion the "cloak" out of 100 concentric plastic rings buried beneath the foundation of a building [source: Barras]. As seismic waves approach, they enter the rings at one end and become contained within the system. Harnessed within the "cloak," the waves can't impart their energy to the structure above. They simply pass around the building's foundation and emerge on the other side, where they exit the rings and resume their long-distance journey. A French team tested the concept in 2013.
Shape Memory Alloys
As we discussed earlier in the countdown, the plasticity of materials presents a major challenge to engineers trying to build earthquake-resistant structures. Plasticity describes the deformation that occurs in any material when forces are applied to it. If the forces are strong enough, the material's shape can be altered permanently, which compromises its ability to function properly. Steel can experience plastic deformation, but so can concrete. And yet both of these materials are widely used in almost all commercial construction projects.
Enter the shape memory alloy, which can endure heavy strains and still return to its original shape. Many engineers are experimenting with these so-called smart materials as replacements for traditional steel-and-concrete construction. One promising alloy is nickel titanium, or nitinol, which offers 10 to 30 percent more elasticity than steel [source: Raffiee]. In one 2012 study, researchers at the University of Nevada, Reno, compared the seismic performance of bridge columns made of steel and concrete with columns made of nitinol and concrete. The shape memory alloy outperformed the traditional materials on all levels and experienced far less damage [source: Raffiee].
It makes sense to consider earthquake resistance when you're building a new structure, but retrofitting old buildings to improve their seismic performance is just as important. Engineers have found that adding base-isolation systems to structures is both feasible and economically attractive. Another promising solution, much easier to implement, requires a technology known as fiber-reinforced plastic wrap, or FRP. Manufacturers produce these wraps by mixing carbon fibers with binding polymers, such as epoxy, polyester, vinyl ester or nylon, to create a lightweight, but incredibly strong, composite material.
In retrofitting applications, engineers simply wrap the material around concrete support columns of bridges or buildings and then pump pressurized epoxy into the gap between the column and the material. Based on the design requirements, engineers may repeat this process six or eight times, creating a mummy-wrapped beam with significantly higher strength and ductility. Amazingly, even earthquake-damaged columns can be repaired with carbon-fiber wraps. In one study, researchers found that weakened highway bridge columns cocooned with the composite material were 24 to 38 percent stronger than unwrapped columns [source: Saadatmanesh].
While engineers make do with shape memory alloys and carbon-fiber wraps, they anticipate a future in which even better materials may be available for earthquake-resistant construction. And inspiration for these materials may likely come from the animal kingdom. Consider the lowly mussel, a bivalve mollusk found attached to ocean rocks or, after it's been removed and steamed in wine, on our dinner plate. To stay attached to their precarious perches, mussels secrete sticky fibers known as byssal threads. Some of these threads are stiff and rigid, while others are flexible and elastic. When a wave crashes on a mussel, it stays put because the flexible strands absorb the shock and dissipate the energy. Researchers have even calculated the exact ratio of stiff-to-flexible fibers -- 80:20 -- that gives the mussel its stickiness [source: Qin]. Now it's a matter of developing construction materials that mimic the mussel and its uncanny ability to stay put.
Another interesting thread comes from the south end of spiders. We all know that, pound for pound, spider silk is stronger than steel (just ask Peter Parker), but MIT scientists believe that it's the dynamic response of the natural material under heavy strain that makes it so unique. When researchers tugged and pulled on individual strands of spider silk, they found the threads were initially stiff, then stretchy, then stiff again. It's this complex, nonlinear response that makes spider webs so resilient and spider thread such a tantalizing material to mimic in the next generation of earthquake-resistant construction.
And what about developing countries, where it's not economically feasible to incorporate anti-earthquake technologies into houses and office buildings? Are they doomed to suffer thousands of casualties every time the earth shakes? Not necessarily. Teams of engineers are working all over the world to design earthquake-resistant structures using locally available or easily obtainable materials. For example, in Peru, researchers have made traditional adobe structures much stronger by reinforcing walls with plastic mesh. In India, engineers have successfully used bamboo to strengthen concrete. And in Indonesia, some homes now stand on easy-to-make bearings fashioned from old tires filled with sand or stone.
Even cardboard can become a sturdy, durable construction material. Japanese architect Shigeru Ban has designed several structures that incorporate cardboard tubes coated with polyurethane as the primary framing elements. In 2013, Ban unveiled one of his designs -- the Transitional Cathedral -- in Christchurch, New Zealand. The church uses 98 giant cardboard tubes reinforced with wooden beams [source: Slezak]. Because the cardboard-and-wood structure is extremely light and flexible, it performs much better than concrete during seismic events. And if it does collapse, it's far less likely to crush people gathered inside. All in all, it makes you want to treat the cardboard tubes nestled in your toilet paper roll with a little more respect.
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Author's Note: 10 Technologies That Help Buildings Resist Earthquakes
When the 2011 Virginia earthquake struck, I was about 55 miles (89 kilometers) from the epicenter. It produced a locomotive-like rumbling and moved the earth in an unsettling way that's hard to describe. In the small towns of Louisa and Mineral, near my mother's house, a couple of structures collapsed, and many more experienced significant damage. While the quake itself was frightening, what was more disturbing was our collective sense that, being so far from the Ring of Fire and the constant threat of tectonic activity, we were somehow insulated from these kinds of events. Makes me wonder if the building codes in Virginia have been updated to incorporate some of these earthquake-resistant technologies.
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