Take a look at recent seismic activity, and you might get the impression that Earth, perhaps a bit too overcaffeinated, has a bad case of the shakes. Earthquakes rattled Chile on and off during 2010-11, beginning with a magnitude-8.8 temblor (or earthquake) that struck just off the coast near Concepcion in February 2010. Then, in March 2011, a magnitude-9.0 quake rocked Japan, triggering a tsunami that killed an estimated 29,000 people and damaged nuclear reactors [source: Amazing Planet]. And finally, in August 2011, a magnitude-5.8 quake centered near Mineral, Va., spooked residents up and down the Atlantic seaboard and damaged the Washington Monument.
While those events seem to suggest an ominous future with a shaking, quivering crust, earthquakes have always been common, as has the human resolve to survive them. Over the centuries, engineers have come to know one thing with growing certainty: Earthquakes don't kill people; buildings do. This is a gross oversimplification, of course, because tsunamis also take many lives, but not all earthquakes generate tsunamis. They do, however, cause buildings, bridges and other structures to experience sudden lateral accelerations. All of which leads to a logical question: Is it possible to keep buildings upright and intact during catastrophic earthquakes like those that shook Chile in February 2010 and Japan in March 2011?
Many engineers and architects now believe it's possible to build an earthquake-proof building -- one that would ride the waves of the most fearsome temblor and remain as good as new once the shaking had stopped. The cost of such a building, however, would be staggering. Instead, construction experts strive for something slightly less ambitious -- earthquake-resistant buildings, which are designed to prevent total collapse and preserve life, as well as construction budgets.
In recent years, the science of building earthquake-resistant structures has advanced tremendously, but it's not an entirely new subject. In fact, a few ancient buildings still stand today despite their location in active seismic zones. One of the most notable is the Hagia Sophia, a domed church (now museum) built in Istanbul, Turkey, in A.D. 537. About 20 years after it was completed, the massive dome collapsed after a quake shook the area. Engineers evaluated the situation and decided to rebuild the dome, but on a smaller scale. They also reinforced the whole church from the outside [source: PBS].
Today, the techniques are a bit different, but the basic principles are the same. Before we delve into the nuts and bolts of building earthquake-resistant structures, let's review some basics, namely, what forces are generated during an earthquake and how they affect man-made structures.
You can get the full story on earthquakes in How Earthquakes Work, but a review of the basics will help here. Earthquakes occur when masses of rock in Earth's crust slip and slide against one another. This kind of movement is most common along a fault, a break in a body of rock that can extend for miles or even hundreds of miles. When pieces of crustal rock suddenly slip and move, they release enormous amounts of energy, which then propagates through the crust as seismic waves. At the Earth's surface, these waves cause the ground to shake and vibrate, sometimes violently.
Geologists classify seismic waves into two broad categories: body and surface waves. Body waves, which include P and S waves, travel through the Earth's interior. P waves resemble sound waves, which means they compress and expand material as they pass. S waves resemble water waves, which means they move material up and down. P waves travel through both solids and liquids, while S waves only travel through solids.
After an earthquake strikes, P waves ripple through the planet first, followed by S waves. Then come the slower surface waves -- what geologists refer to as Love and Rayleigh waves. Both kinds move the ground horizontally, but only Rayleigh waves move the ground vertically, too. Surface waves form long wave trains that travel great distances and cause most of the shaking -- and much of the damage -- associated with an earthquake.
If earthquakes only moved the ground vertically, buildings might suffer little damage because all structures are designed to withstand vertical forces -- those associated with gravity -- to some extent. But the rolling waves of an earthquake, especially Love waves, exert extreme horizontal forces on standing structures. These forces cause lateral accelerations, which scientists measure as G-forces. A magnitude-6.7-quake, for example, can produce an acceleration of 1 G and a peak velocity of 40 inches (102 centimeters) per second. Such a sudden movement to the side (almost as if someone violently shoved you) creates enormous stresses for a building's structural elements, including beams, columns, walls and floors, as well as the connectors that hold these elements together. If those stresses are large enough, the building can collapse or suffer crippling damage.
Another critical factor is the substrate of a house or skyscraper. Buildings constructed on bedrock often perform well because the ground is firm. Structures that sit atop soft or filled-in soil often fail completely. The greatest risk in this situation is a phenomenon known as liquefaction, which occurs when loosely packed, waterlogged soils temporarily behave like liquids, causing the ground to sink or slide and the buildings along with it.
Clearly, engineers must choose their sites carefully. Up next, we'll discover how engineers plan for and design earthquake-resistant buildings.
Before a major construction project begins, engineers must first evaluate the seismic activity of the building site. In the U.S., they have access to a resource to aid in this process -- National Seismic Hazard Maps prepared by the U.S. Geological Survey (USGS). These maps show the probability that ground motions will exceed a certain value in the next 50 years. To calculate the value at a specific location, geologists take historical earthquake data and then make an educated guess about ground motions from all future possible earthquake magnitudes at all possible distances from that location. The result is a colored contour map that shows which areas of the country have the highest earthquake hazard. As you might expect, the entire coast of California is an area of high hazard. Other earthquake hot spots in the U.S. include Alaska, Hawaii, South Carolina and a region encompassing southeast Missouri, southern Illinois, western Kentucky and Tennessee, and northeast Arkansas.
Building codes, such as the International Building Code used throughout most of the U.S., establish seismic design provisions based on the USGS Seismic Hazard Maps. In high-hazard areas, engineers and architects must adhere to more rigorous standards when designing buildings, bridges and highways to make sure these structures withstand earthquake shaking. At the same time, in low-hazard areas, engineers are spared from overdesigning buildings that have a low probability of experiencing severe ground motion as the result of an earthquake.
Once engineers determine the seismic risks of a site, they must propose an appropriate building design. In general, they avoid irregular or asymmetrical designs at all costs. These include L- or T-shaped buildings or split-level structures. Although such designs increase visual interest, they're also more susceptible to torsion, or twisting about their longitudinal axes. Instead, seismic engineers prefer to keep buildings symmetrical so that forces are distributed equally throughout the structure. They also limit ornamentation, such as cornices, vertical or horizontal cantilever projections or fascia stones because earthquakes can easily dislodge these architectural elements and send them crashing to the ground.
Symmetry alone won't save a building. We'll talk more about what can -- next.
Even symmetrical buildings must be able to withstand significant lateral forces. Engineers counteract these forces in both the horizontal and vertical structural systems of a building. Diaphragms are a key component of the horizontal structure. They include the floors of a building, as well as its roof. Engineers generally place each diaphragm on its own deck and strengthen it horizontally so it can share sideways forces with the vertical structural members. On the roof, where a strong deck isn't always possible, engineers strengthen the diaphragm with trusses, which are diagonal structural members inserted into the rectangular areas of the frame.
The vertical structural system of a building consists of columns, beams and bracing, and functions to transfer seismic forces to the ground. Engineers have several options when building the vertical structure. They often build walls using braced frames, which rely on trusses to resist sideways motion. Cross-bracing, which uses two diagonal members in an X-shape, is a popular way to build wall trusses. Instead of braced frames or in addition to them, engineers may use shear walls -- vertical walls that stiffen the structural frame of a building and help resist rocking forces. Engineers often place them on walls with no openings, such as those around elevator shafts or stairwells.
Shear walls do, however, limit the flexibility of the building design. To overcome this downfall, some designers opt for moment-resisting frames. In these structures, the columns and beams are allowed to bend, but the joints or connectors between them are rigid. As a result, the whole frame moves in response to a lateral force and yet provides an edifice that's less obstructed internally than shear-wall structures. This gives the designer more flexibility in placing architectural elements, such as exterior walls, partitions and ceilings, as well as building contents, such as furniture and loose equipment.
Of course, the structural members of a building rest on its foundation. On the next page, we'll look at how engineers are improving building foundations to make them more resilient in strong earthquakes.
If a building's foundation sits on soft or filled-in soil, the whole building may fail in an earthquake regardless of the advanced engineering techniques employed. Assuming, however, that the soil beneath a structure is firm and solid, engineers can greatly improve how the building-foundation system will respond to seismic waves. For example, earthquakes often knock buildings from their foundations. One solution involves tying the foundation to the building so the whole structure moves as a unit.
Another solution -- known as base isolation -- involves floating a building above its foundation on a system of bearings, springs or padded cylinders. Engineers use a variety of bearing pad designs, but they often choose lead-rubber bearings, which contain a solid lead core wrapped in alternating layers of rubber and steel. The lead core makes the bearing stiff and strong in the vertical direction, while the rubber and steel bands make the bearing flexible in the horizontal direction. Bearings attach to the building and foundation via steel plates and then, when an earthquake hits, allow the foundation to move without moving the structure above it. As a result, the building's horizontal acceleration is reduced and suffers far less deformation and damage.
Even with a base-isolation system in place, a building still receives a certain amount of vibrational energy during an earthquake. The building itself can dissipate, or damp, this energy to some extent, although its capacity to do this is directly related to the ductility of the material used in the construction. Ductility refers to the ability of the material to undergo large plastic deformations. Brick and concrete buildings have low ductility and therefore absorb very little energy. This makes them especially vulnerable in even minor earthquakes. Buildings constructed of steel-reinforced concrete, on the other hand, perform much better because the embedded steel increases the ductility of the material. And buildings made of structural steel -- steel components that come in a variety of preformed shapes, such as beams, angles and plates -- offer the highest ductility, allowing buildings to bend considerably without breaking.
Ideally, engineers don't have to rely solely on a structure's inherent ability to dissipate energy. In increasingly more earthquake-resistant buildings, designers are installing damping systems. Active mass damping, for example, relies on a heavy mass mounted to the top of a building and connected to viscous dampers that act like shock absorbers. When the building begins to oscillate, the mass moves in the opposite direction, which reduces the amplitude of mechanical vibrations. It's also possible to use smaller damping devices in a building's brace system.
Even with extensive testing on laboratory shake tables, any seismic engineering design concept remains a prototype until it experiences an actual earthquake. Only then can the larger scientific community evaluate its performance and use what it learns to drive innovation. In the next section, we'll examine some of those innovations, as well as what the future may hold for seismic engineering.
The goal of earthquake-resistant buildings is to preserve life. That means a building that doesn't collapse and allows its inhabitants to escape is considered a success -- even if it ends up being demolished. But what if a building could experience deformation during a quake, then return to its original shape? For some researchers, such as Greg Deierlein of Stanford University and Jerome Hajjar of Northeastern University, that's the future of seismic engineering.
Deierlein and Hajjar have teamed up to develop an innovative technology known as the rocking frame, which consists of three basic components -- steel frames, steel cables and steel fuses. Here's how it works: When an earthquake strikes, the steel frames rock up and down to their heart's content. All of the energy gets directed downward to a fitting that houses several toothlike fuses. The teeth of the fuses gnash together and may even fail, but the frame itself remains intact. Once the shaking has stopped, the steel cables in the frame pull the building back into an upright position. Workers then inspect the fuses and replace any that are damaged. The result is a building that can be reoccupied quickly after an earthquake.
Another innovation is something that's been dubbed the seismic invisibility cloak, suggesting a building could be made transparent to the surface waves produced by an earthquake. To accomplish this, engineers would bury a series of up to 100 concentric plastic rings beneath the foundation of a building. When waves encounter the rings, they enter and then become compressed as they are forced into a bottleneck. The waves basically zip by, just beneath the building's foundation, and exit the rings on the other side, where they resume their original speed and amplitude.
Interestingly, much of the future of seismic engineering involves looking back, not forward. That's because retrofitting old buildings with improved designs and materials is just as important as constructing new buildings from scratch. Engineers have found that adding base-isolation systems to structures is both feasible and economically attractive. According to the National Earthquake Hazards Reduction Program, more than 200 buildings in the United States, including many city government and fire and emergency buildings, now feature isolation systems. After the 1989 Loma Prieta quake alone, engineers retrofitted several buildings, including the city halls of San Francisco, Oakland and Los Angeles. The earthquake-resistant structures in these buildings will most certainly face a test in the form of a serious seismic event. The only question is when and to what extent.
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