The Fantastically Engineered Roof Floating Over the U.S. Open
Jon Disbrow faced a perplexing problem. Architect for the Rossetti planning and design firm, his job was straightforward: to add a retractable roof to Arthur Ashe Stadium—the world's largest tennis venue—in Flushing Meadows, New York. But there was a catch. The stadium's location, a former ash dump site once home to the 1964 World's Fair, does not provide a particularly stable foundation, so adding any additional weight to the 23,500-seat venue simply wasn't an option.
"One of the things that is really unique about the roof opening for Ashe is it is just enormous," Disbrow, the principle architect on the project, told Popular Mechanics. He's not wrong. Today, the Ashe stadium not only boasts a new retractable roof, but the largest roof opening for a tennis venue. The size alone made the job a tough one. Soils that subside at a rate of a half-an-inch per year even without being loaded up with additional structures made it a tougher one.
But not an impossible one, and so Disbrow and the Rossetti team came up with their first plan for covering Ashe. The initial solution would be to remove concrete in the upper reaches of the stadium, replacing it with lighter aluminum, thereby giving the architects some much-needed room to breathe. The weight savings would have been significant enough to allow for a fairly standard, cable-supported retractable roof structure to be mounted on the stadium. But eight months into the design, the team found that wind loads rendered the whole thing unreliable in less-than-ideal weather conditions, so they were forced to scrap their first plan all together.
It was only in the wake of this failure that the team was able to come upon the final, functional, and unconventional solution: Placing an enormous retractable, metal umbrella around and over the entire stadium, without really touching it at all.
If you were to visit the Arthur Ashe Stadium today—or, say, for the start of the 2016 U.S. Open on Monday—you would find eight concrete pads around the stadium's perimeter, each with a set of steel beams that sprout out and spread as they stretch toward the roof's metal skeleton. Together they're easily able to support the 520-square-foot steel roof and its 250-square-foot opening.
But that's only thanks to the real magic, which happens under the concrete where steel piles run around 175 feet deep, to deal with the unstable soil. "They go through the terrible soil to bedrock and get filled with concrete," Disbrow says. "They are spanning vertically through all that old organic soil and ash and debris to get down to support the structure and make it stable."
A 15-inch gap runs the perimeter of the stadium, keeping it at a carefully calculated distance from the new roof, one small enough to make the two parts look integrated, but crucially wide enough to allow for seismic and differential movements.
The roof itself is composed of four primary roof trusses that cover the octagonal stadium and the two-piece roof—6,500 tons of steel, all together—wrapped in lightweight and durable Teflon-coated fiberglass membrane fabric (PTFE) which keeps out the wind and rain, and blocks some of the sun's harsh rays while not blocking out the light entirely.
When the roof panels close, they inch towards each other using a mechanism designed by a crane company, and can open or close in about seven minutes—quicker than the U.S. Tennis Association's upper limit of ten minutes—as a pair of winches mounted on either side reel in or reel out two-inch diameter stainless steel cable onto a winch drum. These winches, operated by five electric motors, can actually be monitored and adjusted in real time, so that if one side starts moving quicker than the other due to wind or some other variable, crews can correct the situation on the fly.
As the two pieces draw to a close, a 260-foot-long air-filled bladder inflates to seal the exposed joint. The ease of making that seal is the main reason the roof is only a two-parter in the first place, as opposed to something more sophisticated. "It gives us the fewest joints to keep it watertight," Disbrow says. "It is not like a couple of drops of water are no big deal. It is absolutely critical to keep all water off the court and off the spectators."
While the roof covers the top, that wasn't quite enough to finish the whole job. The roof also needed to close the rest of the outdoor venue, in order to turn it into a climate-controlled indoor stadium. So shutters roll down from the roof structure and sit on a sill of the existing structure, the only points where the two entities gently touch. The single point of physical connection between a nearly 20-year-old outdoor venue and the highly-engineered umbrella that is giving it new life.
From Popular Mechanics
Trenton H. Cotney
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