Some Buildings Have a Certain 'Vibe'

Feb 2, 2017

Sometimes, when you walk into a building, you feel a certain ‘vibe.’ Well, it may not be just the décor.  Many modern buildings with lighter materials and large open spaces will actually vibrate or even sway in the wind - and that can cause problems. But as Robbie Harris reports, researchers at Virginia Tech have come up with a new kind of solution for the shaking at their specially designed "Vibrations Testing Lab," the only facility quite like it in the country.   

Professor Mehdi Setareh, School of Architecture Design and his students have created a small device that can stabilize huge buildings. Graduate Student Sriram Sankaranarayanan, uses an iphone to measure the frequency of the floor’s vibration inside The Vibration Testing Lab at Virginia Tech, the only building like it in the country dedicated entirely to studying how buildings move

It’s a plain looking steel structure with a concrete roof, but its address gives it away.. Innovation Drive, Blacksburg Virginia.    

The whole thing is actually a measuring instrument - it's two stories high, and its walls and floors embedded with all kinds of sophisticated motion sensors. This is where Professor Mehdi Setareh and his students developed a way to reduce building vibration, much of which is actually caused by human beings just walking inside one.

“When you walk on a floor or you are in an auditorium dancing on a balcony, then the vibration might become too much and some people might get scared and leave the place."

Setareh teaches Architecture and design at Virginia Tech. He says vibration in today’s buildings have not led to catastrophe, but something that happened in the 19th century did.

“There were two footbridges that collapsed when the soldiers were marching in unison over them, one in England, one in France and more than 200 people died.”

The vibrations from the marching soldiers caused a positive feedback loop with the movement of the bridge itself and the resonance increased to the point of structural failure.  After that, marchers on bridges were told to make sure a few people remain OUT of step.

But even humans meandering around today in malls or other structures today can set off vibration problems. The Millennium Footbridge in London, which crosses the river Thames, had to be closed for a year and a half because of it.

To fix it architectural engineers use a behemoth of a device known as a tunable mass damper. Medhi says, “A damper is like a shock absorber in your car.”

The typical variety, weighing several tons or more is installed in buildings around the world. Now Medhi and his students have invented a much smaller, portable version that can do the same thing in a different way. 

Bryan Johnson is a graduate student in architecture. He says, “You can compare the vibration to sound cancelling head phones. They hear the sound coming in, like if you’re on an airplane, and they generate a frequency that is the opposite wavelength of the sound you’re hearing and that kind of neutralizes the sound.”

To demonstrate, Sriram Sankaranarayanan, a grad student in civil engineering exploits one of the coolest features of this Vibration testing lab.

“I’ll go up on my heel and I’ll drop down.”

All Sriram had do was stand on his toes and drop to his heels to make the whole floor vibrate - and sure enough, it is a creepy feeling.

Their new invention, called a Portable Tuned Mass damper is sitting on this floor and shaking right along with it. It’s about the size of a suitcase. He ‘tunes’ the metal plates and springs inside the box to move at the opposite frequency from the floor and when he does the heel drop again, there is no perceptible movement.

Sriram is one of the few that know about mass damping in his graduate engineering program.. They don’t even teach about these devices to structural engineering students.

But they may soon become better known because they make a huge difference, not only for comfort, but also for high tech instruments like MRIs, which have strict tolerances for accurate readings.  And while concept of damping vibration has been around since early last century, the jumbo version is anything but portable - and expensive.

“Our part here is just making it more accessible, make it cheaper, easier to put together and tune.”

Satareh has filed for a patent on his portable tuned mass damper.  The goal is to make the technology more accessible to architects as our buildings keep getting taller and our electronics more sensitive to even the tiniest vibrations.