Space Research Provides Earthshaking Revelations

Dr. Mark S. Whorton, an aerospace engineer at NASA's Marshall SpaceFlight Center, Huntsville, AL, has been working for several years onresolving vibration problems in a microgravity environment that can affectsensitive science experiments conducted aboard the Shuttle in orbit.

"Movements of the Shuttle, such as attitude corrections and theactivities of the crew members aboard the vehicle, create vibrations thatcan affect delicate experiments being conducted on board," he said."Solutions to reducing these minor vibrations in space also can be appliedto 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 doctoralstudies program under Dr. Anthony J. Calise in the School of AerospaceEngineering at the Georgia Institute of Technology in Atlanta. The researchis part of a larger National Science Foundation effort to understand dynamicloads placed on structures by earthquakes and to identify ways of reducingtheir effect. The Georgia Tech research team for the past eight years hasbeen 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 experimentssuch as those involving crystal growth aboard the Space Shuttle are directlyapplicable to buildings and bridges during seismic events. Right now we'reworking on developing technological 'tools' which architects and constructionengineers can use when designing more earthquake-tolerant structures and inenabling existing structures to better survive earthquakes," Whorton said.

The National Science Foundation, under its program on EarthquakeHazards Mitigation in the Civil and Mechanical Systems Division, is funding amajor research program on structural control.  Under this effort, the GeorgiaInstitute of Technology and other universities are involved in various aspectsof earthquake engineering such as developing better building materials, passivedamping methods, and active vibration control. Research activities at Marshallare 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 massand therefore its vibration characteristics.  While this may be acceptable formany terrestrial applications, the need for strong but lightweight structuresin space renders this option infeasible for NASA. 

"Clearly, for applications in the space program, lightweight butequally effective vibration-mitigating alternatives were needed.  We foundthat these technologies had down-to-Earth applications as well.  One way ofcountering structural vibrations caused by a strong gust of wind or seismicground motion is to place sensors and force producing devices called actuatorsat specific locations on buildings.  As sensors in the system measure themotion of the structure, actuators apply forces to counteract the structure'svibrations," Whorton said.

"One such force device would use hydraulic pistons moving counter-weights.  Another method involves placing adjustable tendons along the sidesof structures.  In fractions of a second, sensors in the systems can read thestructural vibration patterns caused by earthquakes or high winds and adjustthe tension on the appropriate tendons to reduce the excessive forces ormotions of the building," Whorton said.

All the actively controlled buildings in operation today are in Japanwith the exception of one in Taiwan.  A TV tower in Nanjing, China, also is tobe retrofitted with active vibration control.  Other new construction willincorporate the technology, particularly in seismically active regions; and itmay 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 theglobe," Whorton said.  "The active control technology for vibration isolationis mature and is fully capable of doing the job.  This is a technology readynow for commercial applications."