FRIDAY, Nov. 2, 2001 (HealthDayNews) -- Doing aerobics in a multistory building could be hazardous to your sense of equilibrium. So could simply walking across a crowded room.
That's because the floor may start bouncing up and down, a phenomenon that usually isn't dangerous but still makes people nervous.
Though fixes for this disconcerting quirk of architecture and physics can be difficult, a researcher at Penn State University has developed a device she says can keep the bouncing at bay.
"You can think of it like the shock absorber in your car," says Linda M. Hanagan, an engineering professor who recently applied for a patent for the device.
Hanagan says it's not very common for floors to bounce, but people often are stunned when they do. The cause lies in the inability of a floor to adjust properly to the vibrations that people create as they move on top of it, she says.
Most of the time, the floor vibrations are too small for people to notice, but she says sensitive equipment, like medical MRI machines, can detect movement.
Hanagan says people usually sense vibrations as a "fast jiggle" that can be felt several floors away from where the action is.
"When you start to talk about aerobics classes, people are generating really large forces. The vibration gets into the columns and can bounce the whole building. You feel it, and it's disturbing. Sometimes it's annoying, and sometimes people feel it might be unsafe," she says.
Ordinary buildings can be susceptible, Hanagan says. "A lot of our phone calls come from plain, old offices with steel-framed floor that have lightweight slabs in them. When someone walks around on that office floor, it generates this kind of steady up-and-down motion. If you're trying to read a computer screen, it's very difficult to function with that type of vibration going on," she says.
"It is almost never a safety issue. In fact, the kinds of amplitudes you would have to get for it to be a safety issue are almost unimaginable," she says.
The collapse of an upper-story dance floor in Israel earlier this year, which received extensive television coverage in the United States, apparently had nothing to do with vibrations, Hanagan says.
To stop bouncy floors, building owners can turn to time-consuming retrofit, stabilizing programs with new columns underneath, she says. Or, engineers can build "damper" machines that use weight that adjust to vibration.
Hanagan says her new device takes damping one step further by using a motor and a sensing mechanism to detect when a floor begins to vibrate too much.
"It's a pretty sophisticated shock absorber," she says. But it's not as cheap as the kind under your car. The 500-pound device, about the size of a large trash can, will cost about $50,000, she says.
Kent A. Harries, an engineering professor at the University of South Carolina, says the science behind Hanagan's device is sound, but it may be easier for some building owners to simply not allow activity like aerobics that cause floor vibrations.
But he says the device might be useful in situations that present unusual challenges, such as a hypothetical 1920s movie house that's been converted to a concert venue. If the balconies were to vibrate when music is played, he says a damper would be just the thing to keep them steady.
What To Do
For information on reducing floor vibrations in your home, read this article from the Journal of Light Construction.
To read more about the collapse of the dance floor in Israel, visit CNN.com, which also includes links to articles on other similar incidents.
