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We spoke to the engineers behind the 6,500-ton steel roof of the world’s largest tennis stadium.
During the annual two-week U.S. Open tennis event, engineer Mark Sharamitaro often watches some of the world’s best tennis in the Arthur Ashe Stadium master control room. There, he can relax ... until he can’t.
The Billie Jean King National Tennis Center in Flushing, New York, hosts the U.S. Open annually. Thanks to upgrades in 2016 and 2018, the site now boasts retractable roofs over both Arthur Ashe Stadium and the Louis Armstrong Stadium completed in 2018. Sharamitaro was instrumental in the design of both systems and has been the lead operator of those roofs ever since they came online.
When all goes well, he can simply press a button in his master control room and watch each of the steel-railed roofs featuring polytetrafluoroethylene (PTFE) fabric move at 25 feet per minute. But when things don’t go well, he’s still the one in charge.
“I’m worried about unexpected storms because of the wind,” he tells Popular Mechanics. “It is not because of the wind and the structure, but because the wind pushes water places where I won’t want water. That is pretty much my overall concern, that something abnormal would get into the system,” says Sharamitaro, president of Clayton, North Carolina-based Sharamitaro Industrial Solutions.
Ashe, celebrating 25 years during the August 29 to September 11 event, required careful engineering just to allow a retractable roof over its top when it was added in 2016. Not only did the roof at Ashe need to avoid loads on the existing structure, it also needed to be fully watertight.
“We looked at baseball and football, and if a drop of water hits a football field, nobody cares,” Sande Frisen, a partner at Detroit, Michigan-based Rossetti Architects, tells Popular Mechanics. “That is absolutely unacceptable at the [United States Tennis Association]. The seam is right down the middle of the court, [water] is a complete dealbreaker. Our allowance of a leak is zero.” Rossetti is the company that served as architect of the stadiums. New York City-based Hardesty & Hanover designed the mechanization system for Ashe.
The Ashe and Armstrong roofs have plenty of similarities. That’s partially to ease maintenance, Frisen says. The most obvious difference, though, is that Ashe has a sloped roof, about 60 feet higher in the middle than at the edges, and Armstrong is flat.
As the largest tennis stadium in the world, Ashe seats 23,500 and must manage a much more generous opening. The 250-foot-by-250-foot roof opening requires four primary roof trusses to span the octagonal stadium. The roof is 6,500 total tons of steel wrapped in a lightweight and durable Teflon-coated fiberglass membrane fabric (PTFE). To minimize seams, the roof comes in two panels. Each movable panel weighs about one million pounds.
“It is like pulling a wagon up a hill with two ropes, one on each side, and then trying to keep them center, with no skew.”
Armstrong is also a two-piece roof, but as it seats only 14,000, the opening is smaller, and its .03-inch-thick PTFE and steel system weighs 284,000 pounds per panel.
Then come the winches. While Sharamitaro says the winch functionality is essentially the same between the two roofs, Ashe has four winches and Armstrong has only one. At Ashe, each panel has a north winch and a south winch, driven by ten electro-mechanically powered motors. All in all, there are five 40-horsepower motors per winch, with two winches per side. Total horsepower: 800.
“It is like pulling a wagon up a hill with two ropes, one on each side, and then trying to keep them center, with no skew,” Sharamitaro says. “You don’t want one side to crawl up the hill faster than the other.” Each winch has independent controls so the system can coordinate the panels to meet in the middle at the exact same time, creating a smooth visual effect.
Alternately, in Armstrong, four 40-horsepower motors (160 total horsepower) for the one winch operates four ropes, each connecting to one end of each panel. The steel rope doesn’t allow stretching, and keeps each of the roofs in the exact same configuration for opening and closing.
“In Armstrong we monitor the skew, but I have no control over the skew, it is all done when we tension the cables,” Sharamitaro says. A cable adjustment system was incorporated into Armstrong, he adds.
Both roofs slide on railroad-style rails requiring rail clamps to hold the roof in place. The railroad system isn’t unique to the tennis center, but using winches isn’t common. Many football venues incorporate motors driving wheels, which produce torque. By having the motors power the winch, there is one big gear on the drum and small pinion gears around the winch. All the torque goes into the drum and to the two-inch-diameter rope.
“You are not worried about the wheel slipping,” Sharamitaro says. Removing the threat of a wheel slip was paramount at the tennis center because the roof will need to close at a moment’s notice and sometimes even as inclement weather approaches. At the larger stadiums, decisions on roof closures are often made days in advance of events.
“At the USTA, the philosophy is, it is an open-air event as long as they can make that happen,” Sharamitaro says. “We will be positioned in the control room waiting for a storm to come in. We wait until they see the rain is going to hit and say ‘close the roof now.’ And we close it just in time and when the rain passes, we open it back up.”
Of course, everyone hopes the roughly six minutes it takes to close the Ashe roof and roughly seven minutes, 30 seconds to close Armstrong is enough time. (Ashe travels at a max speed of 25 feet per minute and Armstrong moves at 17 feet per minute.)
With sensors on the roofs monitoring everything from skew to winch power, they must also be careful to not operate the roof in high winds, simply because the only time the rail clamps aren’t set is when the roof is moving. If a wind gust pushes past 50 miles per hour, they stop and set the rail clamps. If it is above 45 miles per hour for three consecutive seconds, they stop. “The roof could theoretically be repositioned by the wind due to the fact it is kind of like a wing of a plane,” Sharamitaro says. “We have never had to stop because of wind during the event, but have had to during commissioning and maintenance times.”
Operation differs between Ashe and Armstrong, because the roofs slide on an arc that has a 780-foot radius. At Ashe, the winches pull the panels up the incline, but then can use gravity in a controlled descent to draw them back down, Frisen says. At Armstrong, the winch must do all the work of opening and closing.
Ashe takes more horsepower to get that heavy panel up the incline than the lighter panel on the flat Armstrong roof, but Armstrong requires a more complicated system. “There is a ton of cabling,” Frisen says. The one winch feeds four lines, some spooled one way and some the other. “The rope diagram for Armstrong is significantly more complicated because you are pulling in two directions at once, Frisen says.
Once closed, the seal proves paramount. “That is definitely a bit of an art and science together, for sure,” Frisen says. “It is a very complicated piece.”
While conceptually quite simple, Frisen says the reality of making a watertight roof work was more complex, especially since the retractable roof world doesn’t typically need a 100-percent watertight solution. So, they got creative. At both stadiums, one panel overlaps the other. Sets of bristle brushes multiple feet in length cover the overlap to help stop wind and rain. Blowers then inflate an air bladder to help seal the gap—this can only be done once the roof has stopped moving. On the back side of the bladder, a gutter catches any escaping water and sends it away from the court.
“It is a belt and suspenders and a little bit of duct tape to make it work,” Frisen says. “The system works very well, the reality of executing that takes a lot of work.”
At Ashe, shutters can roll down from the side of the roof structure and sit on a sill of the existing stadium, the only point where the two entities gently touch, allowing this highly engineered umbrella to turn Ashe into a climate-controlled indoor venue. Armstrong uses 13.5 miles of terracotta louvers to help naturally ventilate the venue.
With the roofs opening and closing throughout the U.S. Open—and at least every night of the event—Sharamitaro and his team have plenty to monitor. Sometimes, though, all it takes is a push of the button and watching a winch do its thing.