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
"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
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"
* 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
* 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. …