Dr. Linda M. Hanagan, assistant professor of architectural engineering, says, "People don't think floors should move, and when they do, this motion can be perceived as annoying or even dangerous. This perception of danger can persist, even though no danger is present."

Stiffening or thickening the floors in an existing building to decrease excessive motion is costly and can take months to complete, disrupting the building's occupants, she adds. However, Hanagan's new approach can be more effective than other structural retrofits and often takes less than a week to fix the problem.

Hanagan's approach, for which Penn State recently filed a provisional patent application, uses active control systems installed on the floor or in the ceiling cavity below it to damp the vibrations.

For example, if people are doing the bunny hop, the hop, hop, hop motion can cause a long-span, steel floor to begin to vibrate in response to the up-and-down motion of the dancers. Each time the dancers hop, the regular "beat" can cause the floor's up and down motion to increase. Hanagan says, "This happens because a component of the ‘beat' is in resonance with the natural frequency of the floor."

The researcher's solution is to use strategically placed "smart" counterweights that sense the magnitude of the floor's motion and move to apply opposing forces to damp or diminish the vibrations. The active control system detects when the floor is moving up and down and when to react in the opposite direction to counter the movement.

The Penn State architectural engineer notes that her active floor vibration control system is similar in concept to the tuned mass damper (TMD) approach that is sometimes used to correct floor motion. Both Hanagan's approach and TMD rely on a moving mass to dissipate the energy in the floor system. The advantage of the active system, which uses floor velocity measurements to generate the amount of force needed to damp the floor motion, is that it requires less moving mass to get the same degree of control as a TMD system. One study showed that a TMD system would have to weigh 30 times more than the active control system.

"When TMDs have been ruled out as a repair option because the existing structure can't support the additional weight, the active system could be the only viable option," Hanagan adds.

The active system is cost effective too. "As recently as a few years ago, 20 pounds of force cost about $30,000 to generate. Today, thanks to the improved capabilities of commercially available linear motors, we can generate 500 pounds for about $50,000," she notes. The Penn State researcher is currently working on a non-exclusive basis with a commercial partner to apply her approach. She also measures and assesses floors on a consulting basis. In addition, she conducts research to provide design engineers with better tools to predict vibration behavior and prevent vibration problems before a building is constructed.

Hanagan's early work on active control of floor vibration was supported in part by the National Science Foundation. The active control approach has been described in publications including Experimental Implementation of Active Control to Reduce Annoying Floor Vibrations in the Engineering Journal, fourth quarter issue, 1998. Other publications are noted on Hanagan's website at http://www.engr.psu.edu/www/dept/arc/server/faculty/hanagan.htm

Note: If no author is given, the source is cited instead.

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