Monday, October 26, 2015

Forest City touts new technology to keep B2 from swaying, previous plan (heavier, more expensive) unmentioned

So here's how you announce a switch in technology for an innovative building with a troubled history: you don't.

A 10/23/15 Real Deal article, From NASA to Brooklyn: Here’s why skyscraper living doesn’t make you queasy, is subtitled "Forest City will test-drive the fluid harmonic disruptor at B2," explains that Forest City Ratner, working with structural engineer Thornton Tomasetti, will be the first building to use the above-mentioned NASA-developed technology to keep the building from swaying:
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.

The previous technology

Indeed, the Real Deal describes that technology, but doesn't mention that it was the original plan for B2:
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.

On the other hand, any pioneer use of technology, as with the modular plan itself, can be vulnerable to glitches.

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. 

And given that "B2’s tower crane can lift a maximum of 26.5 tons," as Farnsworth wrote, that could have posed a challenge for assembling the mass dampers, at least for the weights at the center of them. (The weight of the heaviest component  of the mass damper was not stated.)

And while Farnsworth didn't say explicitly that the mass dampers would be on the roof, available evidence is that such installations are on the roof or very high floors. 

So "cheaper and lighter" might make a big difference for B2.

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