How Engineers Are Making Buildings More Quake-Proof
Peter N. Spotts, writer of The Christian Science Monitor, The Christian Science Monitor
EDUARDO KAUSEL sets a model of a four-story building on his desk, adds two weights, and slides it slowly back and forth. The plywood-and-steel structure sways smoothly.
As he shortens and intensifies motions to mimic an earthquake, the model wriggles like molded jello, each floor moving differently from the one below it.
Such complex motions challenge designers as they try to improve earthquake-resistant structures. Yet engineers are no longer satisfied with buildings that avoid collapse during an earthquake - the basis of current "life safety" earthquake building codes. They now want to design buildings that require only minor repairs and remain usable while repairs are made.
One of the more promising techniques, say some engineers, involves computerized machinery that adjusts a building's structure hundreds of times a second to offset the effects of ground vibrations - so-called active designs for earthquake resistance.
Until now, "designing to prevent catastrophic failure is the job engineers have been striving for," says Dr. Kausel, a professor of civil engineering at the Massachusetts Institute of Technology in Cambridge, Mass.
Even if a building later had to be razed, the engineering was usually deemed successful if it held up long enough for people to escape unharmed.
"But preventing collapse is no longer enough," he adds. "The damage associated with a major earthquake could disrupt the life of a city. We need to be able to prevent large economic losses."
Ian Buckle, deputy director of the National Center for Earthquake Engineering in Buffalo, N.Y., cites the example of the Hyatt Regency Hotel at San Francisco's airport. During the Loma Prieta quake in 1989, he says, several of the hotel's load-bearing walls cracked. "From an engineering standpoint, it was a success," he says. "But the hotel had to close during repairs. The hotel lost more money than it cost to build it in the first place."
Then, too, even if a structure remains sound, severe shaking can demolish the contents, threatening people inside. According to a report released last week by the US Department of Housing and Urban Development, if the Northridge quake had happened later in the day "thousands of children would have been injured or killed by falling debris, furniture, and lighting fixtures."
Even in Japan, with its frequent strong temblors, 1971 building-code revisions only require that structures resist sudden collapse, according to Shizuo Hayashi, an engineering professor at the Tokyo Institute of Technology.
Two factors are prompting the shift toward "performance based" designs:
* The high economic cost of strong earthquakes. With a magnitude of 6.7, "the Northridge quake was not a large one," says John Hall, a civil-engineering professor at the California Institute of Technology in Pasadena, Calif. "Yet we're still trying to deal with it a year later." It caused $20 billion in damage. Damage estimates from the Kobe disaster range from $50 billion to $100 billion.
* The likelihood of more frequent quakes in economically significant regions. Last Friday, the Southern California Earthquake Center released a report on earthquake probabilities for the region. The authors estimate that Southern California faces an 80 to 90 percent probability that an earthquake with a magnitude of 7 or larger will strike by 2024.
A similar estimate in 1988 put the probability at 60 percent by 2018, and only along the San Andreas and San Jacinto faults. …