Forest City touts new technology to keep B2 from swaying, previous plan (heavier, more expensive) unmentioned
Here’s how it works. Six water-filled pipes on the roof of the 32-story building — making up about 0.5 percent of the building’s total mass — will stymie the tower’s vibrations. The NASA-designed disruptor will control the water’s movement and change how the liquid and building would usually react when wind or other vibrations occur.
...The polyvinyl chloride pipes, each three feet in diameter, will be installed once the building is completed, which is slated to be early next year, Malsch said. Robert Sanna, director of construction and design at Forest City, said B2’s lightweight modular material necessitated the use of a disruptor.That's not quite the full story. Actually, the modular tower, now the tallest in the United States, was previously supposed to use a different technology common on larger buildings, a tuned mass damper.
Most skyscrapers use a different technology, called a tuned mass damper, which uses a steel or concrete weight to resist movement and giant tanks of water to weigh down the building, said Steve DeSimone, president of DeSimone Consulting Engineers.
...But buildings shorter than 800 feet typically don’t require them, DeSimone said, adding that he “wouldn’t put a damper on a 32-story building.”
Dampers also typically require some movement to kick them into action — a system often compared to a pendulum— which [NASA's Rob] Berry sees as a pitfall, especially when something like an earthquake requires an immediate reaction.There are not many earthquakes in New York, so surely there were other reasons, and the Real Deal supplies them:
Thornton Tomasetti, which has the exclusive right to apply the fluid harmonic disruptor to tall buildings in the U.S. and is bringing it to B2, is billing the technology as a cheaper and lighter alternative to traditional dampers.Those attributes--"cheaper and lighter"--likely drive the change at B2, aka 461 Dean Street. After all, the delayed tower is already way over budget. And, as described below, it may have been a challenge to lift the previously planned tuned mass dampers to the roof of B2.
The previous plan
The previous plan was described in a 7/11/14 article in the in-house magazine of the engineering firm Arup, a designer on the B2 tower. In Engineering the factory-built tower, Arup's David Farnsworth wrote:
The lack of concrete in the modules makes the structure as a whole very light compared to typical construction. This, combined with the building’s orientation and massing, meant that the structure would sway more in high wind than conventional buildings of similar height. We therefore incorporated two 100-ton tuned mass dampers (not typically found in buildings of under 40 stories) into the design to reduce wind-induced motions to acceptable levels.Given that B2's heaviest single module (the largest piece of the largest apartment) is only 24 tons, according to Farnsworth's article, that means each mass damper would have been more than four times as large.