Oct. 6, 1997 Research aboard the Space Shuttle is helping to provide a way to protect structures from major vibrations such as those produced by severe earthquakes and high winds.
Dr. Mark S. Whorton, an aerospace engineer at NASA's Marshall Space Flight Center, Huntsville, AL, has been working for several years on resolving vibration problems in a microgravity environment that can affect sensitive science experiments conducted aboard the Shuttle in orbit.
"Movements of the Shuttle, such as attitude corrections and the activities of the crew members aboard the vehicle, create vibrations that can affect delicate experiments being conducted on board," he said. "Solutions to reducing these minor vibrations in space also can be applied to reducing the effect of major vibrations produced by earthquakes and high winds on terrestrial structures such as buildings and bridges."
Whorton has been conducting his research as part of his doctoral studies program under Dr. Anthony J. Calise in the School of Aerospace Engineering at the Georgia Institute of Technology in Atlanta. The research is part of a larger National Science Foundation effort to understand dynamic loads placed on structures by earthquakes and to identify ways of reducing their effect. The Georgia Tech research team for the past eight years has been studying the benefits of using passive and active control strategies to reduce the effects of earthquakes on buildings.
"Technologies we've developed here at Marshall to protect experiments such as those involving crystal growth aboard the Space Shuttle are directly applicable to buildings and bridges during seismic events. Right now we're working on developing technological 'tools' which architects and construction engineers can use when designing more earthquake-tolerant structures and in enabling existing structures to better survive earthquakes," Whorton said.
The National Science Foundation, under its program on Earthquake Hazards Mitigation in the Civil and Mechanical Systems Division, is funding a major research program on structural control. Under this effort, the Georgia Institute of Technology and other universities are involved in various aspects of earthquake engineering such as developing better building materials, passive damping methods, and active vibration control. Research activities at Marshall are focused in the area of active vibration control.
There are several ways to reduce the effect of structural vibrations. A direct approach is to stiffen the structure, which requires changing its mass and therefore its vibration characteristics. While this may be acceptable for many terrestrial applications, the need for strong but lightweight structures in space renders this option infeasible for NASA.
"Clearly, for applications in the space program, lightweight but equally effective vibration-mitigating alternatives were needed. We found that these technologies had down-to-Earth applications as well. One way of countering structural vibrations caused by a strong gust of wind or seismic ground motion is to place sensors and force producing devices called actuators at specific locations on buildings. As sensors in the system measure the motion of the structure, actuators apply forces to counteract the structure's vibrations," Whorton said.
"One such force device would use hydraulic pistons moving counter- weights. Another method involves placing adjustable tendons along the sides of structures. In fractions of a second, sensors in the systems can read the structural vibration patterns caused by earthquakes or high winds and adjust the tension on the appropriate tendons to reduce the excessive forces or motions of the building," Whorton said.
All the actively controlled buildings in operation today are in Japan with the exception of one in Taiwan. A TV tower in Nanjing, China, also is to be retrofitted with active vibration control. Other new construction will incorporate the technology, particularly in seismically active regions; and it may be possible to retrofit the technology to other existing structures.
"This technology -- in part derived from the nation's space program -- is being adapted to meet the needs of the construction industry around the globe," Whorton said. "The active control technology for vibration isolation is mature and is fully capable of doing the job. This is a technology ready now for commercial applications."
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