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Posted 9-30-99
CLEVELAND -- Ludwig Figueroa has spent the last decade battling Mother Nature by trying to reduce the prospect of structures toppling during an earthquake.
The civil engineer at Case Western Reserve University uses a geotechnical device called a centrifuge to help mitigate the effects of earthquakes.
Housed in the Structures Laboratory in the Bingham Building, the centrifuge is a rotating circular device that uses centrifugal force to simulate a high-gravity field. It is a powerful tool for studying the behavior of structures under considerable gravity loads by using scaled-down models of structures.
"The centrifuge allows us to model very large structures or soil deposits into small specimens," said Figueroa, CWRU professor of civil engineering.
According to the researcher, if he has a one-foot-by-one-foot specimen in the centrifuge and runs it at 100 times the acceleration of gravity, he can simulate the effects of an earthquake on a 100-foot-by-100-foot structure.
Figueroa noted that while there are only a handful of geotechnical centrifuges in the United States, centrifuge testing is the most realistic way to simulate geotechnical and geoenvironmental events in the laboratory. The Ohio Board of Regents funded CWRU's centrifuge, which costs about $250,000.
CWRU's steel centrifuge is cobalt blue. It is located below ground in an open square chamber and is surrounded by thick, reinforced concrete walls and a support slab. The control room floor level is raised slightly for additional safety. The centrifuge can handle up to 400 pounds at 100 times the acceleration of gravity.
The equipment is driven by a computer-controlled 15 horsepower motor and torque control converter which powers the centrifuge arm through a belt drive. Its maximum speed is about 250 revolutions per minute. A video camera on board and a data acquisition system provide data for researchers.
Figueroa helped build CWRU's centrifuge in 1997 when he was on sabbatical at the University of Colorado, which houses a very large centrifuge.
"I can actually fix our centrifuge myself because I know every part of it," he said.
The centrifuge rotates around an axis at high speed, so it generates a centrifugal force.
"If you fasten a weight on the end of a rope and swing it very fast, the weight gets heavier," Figueroa said. "The action creates a gravitational force."
A centrifuge can simulate large blasts by a very small detonation like a firecracker, he said.
"We can scale the effect to simulate a very large event."
Figueroa noted that it is important to know how the soil responds to an earthquake because this will affect how the soil will support a structure. The centrifuge will help engineers improve structural design by allowing them to determine if the soil will fail during an earthquake.
A phenomenon called soil liquefaction often occurs during earthquakes. When all spaces in the soil are filled with water, there is a temporary increase in pressure inside these void spaces during the seismic event, separating the particles in the soil and weakening it. He referred to another California earthquake as an example.
"The San Francisco marine district is built on reclaimed land from the Bay," Figueroa said. "The soil is loose and prone to liquefaction. During the 1989 earthquake, the soil liquefied and many buildings in the area were damaged. The ground settled unevenly, and the structures collapsed because they lost their support."
Soils that are more porous are more likely to fill with fluid and fail, but there are treatment methods to improve the soil, he explained. Engineers can use a technique called grouting and inject chemicals such as sodium silicate into the soil to fill in the voids and make the soil dense. They can place drains in the soil to allow the water to dissipate the pressure during earthquakes. Engineers can also use a method known as dynamic compaction, which involves dropping a very heavy weight on the soil's surface from about 100 feet pounding the soil and making it more dense.
"Dynamic compaction should be done on an area larger than the footprint of the structure," Figueroa said. "It is also important to study adjacent soil that comes in contact with the structure."
The CWRU civil engineer said his research also applies to dams, transmission towers, bridges, pipelines, water treatment plants, and nuclear power plants, although nuclear power plants are usually built on rock to minimize risk.
Researchers can use the centrifuge to study environmental problems, such as seepage of contaminated waste generated from the production of copper or other minerals; to explore wave forces on marine structures like offshore platforms; and to improve the design of sea-bed anchors for large ships.
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