Sep. 11, 1998 Earthquakes, windstorms, traffic and explosives cause motion that can be catastrophic to buildings or bridges. National Science Foundation (NSF)-funded engineers Billie Spencer Jr. and Michael Sain at the University of Notre Dame are designing systems that counteract damaging structural responses to such events. These "smart buildings" adjust to changing conditions without requiring massive amounts of energy to do so.
"This type of research is important because it pioneers a novel concept for the optimal performance and safety design of buildings and other civil infrastructures, particularly those under the threat of earthquakes and other natural hazards," said Chi Liu, National Science Foundation program manager.
Acts of nature, terrorism or even traffic create ever-changing forces on most structures. Buildings rely primarily on strong materials and a structure that dissipates energy to resist damage. Increasingly, though, mechanical means are being explored.
Traditionally, buildings are built to sustain damage in order to survive during severe earthquakes, according to Spencer. "You wouldn't want that in your car -- for it to break every time you go over a pothole."
To prevent such damage, manufacturers put shock absorbers in the suspension of automobiles to dampen the effect of thumps and bumps. Engineers are using the same concept, to design shock absorbers for buildings. However, the best systems must adapt quickly to change.
"When controlling buildings during non-critical times, you want to have the dampers soft so there are no jerky movements, which helps protect the contents. But during an earthquake you want increased damping," Spencer said. In other words, during stable periods, building designers seek the soft, cushy, boat-like ride of a luxury car, but during a catastrophic event, they seek the tight-suspension control of a sports car.
The shock absorber Spencer and Sain are developing for use in buildings relies on the same premise as the shock absorber most used in cars with a piston in an air- or fluid-filled cavity. Unlike your car, however, the associated damping forces can be automatically adjusted.
Spencer and Sain's shock absorber uses an oil suspension of tiny iron particles. The viscosity of the fluid -- and the magnitude of the damping effect -- can be modulated by creating a magnetic field.
"The fluid is like water or a light oil, but when it is in the presence of a magnetic field it becomes thick like pudding," Spencer said. Sensors in the building can determine -- in real time -- the way the building is moving and modulate the damping forces on a series of the smart shock absorbers.
Another important feature of Spencer and Sain's system is that it requires very little power; each shock absorber requires only about 50 watts. The system could easily run on batteries, especially important during earthquakes where power is frequently interrupted.
In tests, a three-story structure exposed to the same forces as the 1940 El Centro earthquake showed that the magnetically adjusted shock absorber was much more effective than, for example, a shock absorber without any on-line provision for adjustment. The magnetically controlled damper reduced the peak effect of horizontal displacement and acceleration on the third floor by almost 75 percent and 50 percent, respectively. The displacement relates directly to the health of the building - if it is too large, the building may not return to its normal shape. The acceleration, on the other hand, relates to the protection and comfort of building occupants - forces which are felt by persons and which can be tolerated by expensive equipment.
Spencer and Sain are working with Lord Corporation to develop the details of the technology. They presented their work at several international conferences this past summer.
Editors: For more information, see: http://www.nd.edu/~quake/
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