BOSTON — 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. Separate studies in last week's issue of Science magazine suggested that the region is long overdue for a series of quakes of Northridge-size magnitudes.
Designers have a variety of options for adding earthquake resistance to new or existing buildings, much of it based on construction materials such as steel framing, steel-reinforced concrete, and properly braced and anchored wood framing for homes. While all of these techniques showed some flaws in the Northridge quake, they still can be effective when properly used, engineers say.
In addition, foundations can be mounted on shock absorbing ``base isolaters'' made of springs or alternating layers of rubber and steel plate. The concept has been around for about 15 years, Dr. Buckle says, but it has caught on only within the last five years, as recent quakes have prompted planners to design and retrofit key buildings with isolaters.
Yet isolaters have shortcomings, engineers say. They are most effective on shorter buildings. Even then, buildings can slide off them under some circumstances. And their effectiveness on tall buildings is uncertain: They could actually tip over in a severe quake.
This is prompting researchers to look at active methods for earthquake resistance, particularly for tall buildings.
The principle: add energy to the building to counteract an earthquake's forces, says Tsu-Teh Soong, a civil-engineering professor at the State University of New York at Buffalo.
This can be achieved in two ways, he says: adding braces to the sides of buildings that, through shock-absorbing hydraulics, can change the tension on a building's frame; and adding a movable multiton ``damper'' to the top of a building that counteracts vibrations set up by an earthquake. The braces and dampers are controlled by a computer, which gathers information on the building's movements from strategically placed sensors.
Although the technique shows promise, it shouldn't be oversold, researchers say. First, it has not been tested by strong earthquakes - although a six-story experimental structure in Tokyo performed well in three moderate earthquakes. Second, active measures rely on external power sources that can be vulnerable in a temblor. Moreover, cost remains a factor, although Dr. Soong estimates an active system would add only 3 to 5 percent to a building's construction tab.
At this stage, says Kausel, although active and passive design approaches could be combined, active approaches cannot be relied upon to prevent collapse.
The Japanese have been the most aggressive in exploring active techniques, Soong says. Indeed, in 1990 alone, the top 10 Japanese construction firms spent $500 million on earthquake engineering research, Kausel says. The bulk of the money for earthquake engineering in the United States comes from the National Science Foundation, adds Hall, which spends roughly $15 million a year on engineering research.
``We've got to improve performance without breaking the bank,'' Buckle says.
``We've got research work to do'' in order to achieve that. Over the next eight to 10 years, he says, the Federal Emergency Management Agency plans to spend $40 million to $50 million to help researchers develop better methods of analysis, testing, and simulation to develop performance-based earthquake codes.