Building For A Landscape On The Loose
The earthquake in Northern California reminded all Californians to take earthquake preparedness a lot more seriously. To most of us, that means stocking up on bottled water and flashlights, and learning where to take cover when the shaking starts. But engineers have a different view of earthquake preparedness. After all, they have to make sure that the structures we live and work in can withstand an earthquake's shaking. For engineers, the place to begin earthquake preparedness is where the quake begins — deep in the earth's crust.
Earthquake action begins when two pieces of the crust slip violently past one another. Though the huge rocks may shift only a few inches or feet, they release an enormous mount of energy
Once released, the energy travels in waves to the surface, through layers of soil — a mixture of sand, clay, rock, and water. Since the foundations of buildings, bridges, highways, and other structures rest on or run through these layers, it is here that most earthquake damage originates. Whether or not a foundation will hold a building up during a quake depends in great part on the type of soil on which it rests. And builders know that not all soils are alike.
Loosely packed soils, such as the sandy mixtures in coastal areas, are particularly unstable. Between the soil particles, there is plenty of room for water to become trapped. When an earthquake wave shakes this kind of soil, it changes the soil structure. The spaces between the soil particles close up. This increases the pressure in the water trapped there.
What happens next is that the soil loses its solid consistency and behaves like a liquid. It undergoes liquefaction, turning temporarily into quicksand. When this happens, even the most structurally sound buildings "can get sucked into the earth," says Jose Pires, a professor of civil engineering at the University of California at Irvine. "It's as though, for a few moments, they're only resting on a liquid foundation."
Liquefaction helped bring down dozens of buildings in San Francisco's Marina District during the October quake. But in nearby Foster City, built partially on a similar type of soil, the buildings remained standing. There, engineers had compacted the soil into dense layers before the buildings were constructed. The packed soil had fewer spaces in which water could collect. But even buildings on solid ground face problems in a quake. Though the foundation may not sink, that is no guarantee that the building will stand. Engineers try to make the columns that support the building and the beams that hold the columns together strong enough and flexible enough to shake without breaking.
Like a tuning fork, every structure has a natural vibrating frequency — a rate at which it shakes back and forth. The frequency depends on the structure's height. To understand why, think of a swinging pendulum. If the string attached to the weight is short, the distance the pendulum swings will be less than if the string is long. The short pendulum will also swing more rapidly. Likewise, short buildings vibrate rapidly. Tall buildings, in contrast, vibrate slowly.
The biggest danger to any building short or tall occurs when the frequency, or timing, of the building's natural vibrations matches the frequency of the quake's vibrations. This condition, called resonance, is similar to what happens when you push someone on a swing. If you push in rhythm with the swing's back-and-forth motion, it takes very little energy per push to make the swing climb higher. When a building that's already vibrating gets enough well-timed "pushes" from an earthquake, however, the building can go "over the top": the vibrations rip it apart. Tall buildings don't usually have this problem. For one thing, they are made of strong, flexible materials that can withstand shaking. For another, they are usually built on solid ground. Earthquake waves move rapidly through solid ground. Tall buildings vibrate slowly, so there is less chance that resonance will occur. Even though you might not feel safe in a swaying skyscraper during a quake, chances are you would be.
The tall buildings that collapsed during the Mexico City quake of 1985 were an exception. They were built on soft soils, through which quake waves move slowly. They literally fell victim to resonance because their slow vibrating frequency matched that of the ground. Such resonance may also have helped bring down Oakland's Nimitz Freeway during the recent California quake. To keep short structures from crumbling due to resonance, a group of California earthquake engineers has designed a "seismic isolation system." This system introduces flexibility into the building's design, making it vibrate at a lower frequency-the way tall buildings do. The engineers place a layer of shock-absorbing pads made of rubber, lead, and steel between a building and its foundation. The pads "act like the shock absorbers in a car," says Stephen Weissberg, a structural engineer with Dynamic Isolation Systems in Berkeley, California. "They absorb the energy that would otherwise travel into the building and cause damage."
Some 100 buildings in the world have such a system, but it is very expensive. The 149 rectangular pads that are now being built into a Los Angeles hospital each cost $5,000. Is there a less costly way to quake-proof buildings? Absolutely, says Professor Tsu T. Soong of the National Center for Earthquake Engineering in Buffalo, New York. Computers are at the heart of a system he helped design, known as "active control." Sensors throughout a building continually monitor its movement, the way nerve cells monitor your body's movement. They relay the information to a computer--the building's "brain." If the computer detects excessive vibrations, it figures out the right response--tightening or loosening steel cables to keep the building swaying just the right amount. The active control system is now being tested in Tokyo.
Most engineers believe that any structure can be made earthquake-safe. "The technology exists," says U/Cals's Pires. "We just have to find ways to do it economically."
February 23, 1990