Streamlining the counterweight systems that limit a building’s wobbles
Take note, high-rise builders and daring designers. You know those giant counterweights, called tuned mass dampers, that keep spindly skyscrapers and bridges from swaying in the wind? Well, now there’s a pint-sized portable tuned mass damper.
Wait a second. How does a regular tuned mass damper even work? A little background: especially at great heights, a structure can be strong without being stiff, and when it lacks stiffness relative to the forces acting on it—wind, footfalls—it can swing perceptibly. The building won’t topple, but the movement can cause occupants alarm, discomfort, even nausea. A notorious example, Boston’s John Hancock Tower, swayed as much as three feet off its base before dampers were added.
Increasingly common in Manhattan, the typical tuned mass damper (TMD) is essentially an enormous counterweight on springs or a pendulum, built into the top portion of a tower. When high winds push on the building, the weight swings in the opposite direction, staying the tower’s shift. But in order to make it work, engineers have to “tune” the damper to match the building’s natural frequency, ensuring that the weight swings just enough to counteract the wind. (Frequency being the speed of a vibration.) Practical Engineering explains it all with a fun video here.
The drawbacks are size and cost. For example, the 728-ton steel ball that stabilizes the 1,667-foot-tall Tapei 101 tower in Taiwan cost $4 million—and it takes up six stories of prime real estate.
A lighter package
By contrast, a portable tuned mass damper, under development at Virginia Tech, weighs about 275 pounds. And a building owner with no technical training can adjust the damper’s settings with a five-dollar iPhone app.
Of course, the damper itself will cost more than $5, but it will be affordable, says its inventor, Mehdi Setareh, an architecture professor and the head of the Vibration Testing Lab at Virginia Tech. “What I’m trying to do is reduce the cost so it can become more common,” Setareh told us. “It’s like what Henry Ford did with cars.”
Setareh’s invention is best suited to those modern buildings constructed of lightweight materials and given swooping, eye-catching shapes with computer-aided design. Setareh is himself an award-winning structural engineer who helped design the dramatically cantilevered headquarters of MFP Automation Engineering (right).
Such bold designs have become more common—and along with them, so have complaints about noticeable vibrations, according to Setareh. His lab has studied and helped modify structures such as a theater balcony in Detroit and the monumental stair in the Zaha Hadid–designed Broad Art Museum in East Lansing, Michigan (left). “The floors are designed for strength,” Setareh hastened to point out. “When people walk on the floor, it holds the load, no problem.” But a slight bounce gives users the unwelcome impression of shakiness. “It can be scary, or at best annoying.”
Rather than sacrifice aesthetics or boost costs by building with heavier beams, said Setareh, architects (or, after the fact, building managers) can apply a simple fix. Acting like a shock absorber, the portable tuned mass damper (PTMD) consists of a weight on springs, tuned to have a natural frequency close to that of the floor. When the floor is rattled by foot traffic, the springs are activated and the movement of the weight pushes back against the movement of the floor, neutralizing it. “When the floor goes down, the damper mass goes up,” Setareh said, “and when the floor goes up, the mass goes down.”
The device sits two feet high and weighs a fraction of the floor, but it compensates by moving ten to twenty times faster than the floor vibrates. “So even though the mass of the [PTMD] is, say, 200 pounds, it’s like 4,000 pounds of force moving against the floor,” Setareh said. “That’s how it reduces the vibration substantially,” up to 60 percent in the Virginia Tech lab’s testing facility (see below).
A result of rocketry
That’s a fine solution for a variety of unique buildings—hospitals, theaters, corporate headquarters. But what about a straightforward residential high-rise? It’s hard to imagine a small, convenient version of the titanic TMD at Tapei 101.
And yet, a team at NASA has developed a damper the size of a coffee can (though it sits in a long pipe filled with water) that can stop a tall building’s shakes in a heartbeat. Spun off from rocket technology, a variation of this “disruptive tuned mass” has been installed on the new 32-story 461 Dean in Brooklyn, the world’s tallest modular building. It turns liquid into a secondary mass that absorbs and dissipates the energy of a building’s vibrations.
Engineering firm Thornton Tomasetti licensed the technology and adapted it to 461 Dean (right), which is built of lightweight steel modules. Thornton Tomasetti’s rooftop system, which they call a fluid harmonic absorber, consists of four U-shaped PVC pipes, each 50 feet long and three feet in diameter. Air springs are fitted to the outside of the pipes and tuned to a certain “sloshing frequency,” explained Michael Wesolowsky, a Thornton Tomasetti engineer. The tubes are pressurized so that during a wind or seismic event, air pushes on the water, and then vice versa.
“The liquid is tuned to slosh back and forth at exactly the same frequency as the building’s movement, but out of phase,” said Wesolowky, “pulling the building back in the opposite direction,” thereby keeping it steady.
“It’s like the air spring in your car,” added Elisabeth Malsch, another Thornton Tomasetti engineer, “if you have a newer, fancier car.”
Already, the system has performed well on high-wind days, Malsch said. And it has advantages over the gargantuan conventional TMDs. The absorber at 461 Dean (the first of its kind in a commercial application) is “cheaper, lighter,” said Malsch. “It required very little reinforcement” to the building below. It’s also easier to tune, not to mention maintain. After all, “we know how to patch leaks in pipes,” Malsch said.
Like the PTMD out of Virginia Tech, the fluid harmonic absorber can be installed after initial construction is complete, if a structure turns out to behave differently than predicted in high winds. The Smithsonian speculated that historic landmarks built before earthquake codes might be retrofitted with the NASA-bred system, another benefit of which is its fast-acting nature. (Skyscrapers can still sustain damage during the few seconds it takes for a quake to activate a conventional, pendulum-style damper.)
Malsch can attest that older buildings are good candidates for the addition of such a cutting-edge streamlined damping system. Her office is in Manhattan’s 40 Wall Street, at 71 stories the world’s tallest tower when it was completed in 1930 (before being surpassed by the Chrysler Building a few months later). On windy days, the structure’s age is apparent—or audible.
“Every six seconds,” said Malsch, “you can hear it creak.”
This post was written by Suffolk Construction’s Content Writer Patrick L. Kennedy. If you have questions, Patrick can be reached at PKennedy@suffolk.com. You can connect with him on LinkedIn here or follow him on Twitter at @PK_Build_Smart.