When engineers made plans to reinforce and upgrade the carrying capacity of the Empire State Building’s mast and tower by adding 39 tons of steel, they had to find a way to protect pedestrians from falling rivets, tools and materials. Roofing the observatory and building bridges over the sidewalks 1,250 ft below were lousy options. The top of the iconic New York City building has an open-air observatory at the 86th floor and premium viewing spaces at the 102nd and 103rd levels. Annually, these spaces host about 4.3 million visitors and generate about $85 million in revenue. Soaring above the busy streets, a 200-ft-tall steel broadcast tower bristles with antennas that generate about $20 million more. Together, the observatory, mast and tower are the crowning jewel of the 86-year-old icon, which is owned by the Empire State Realty Trust Inc.
They came up with a design for a sheltering “cocoon,” which sits on a 560-sq-ft aluminum elevated work platform, or “dance floor.” The platform is braced from below by steel brackets through the conical ice shield, which is there to shatter ice falling from the tower.The search for an alternative to scaffolding dates to February 2014, when the ESRT’s building engineer, engineer-of-record Thornton Tomasetti, site safety engineer Plan B Engineering and contractor Skanska USA Building Inc. began to consult with New York City Dept. of Buildings officials to devise a plan that would not only protect the public and workers but also allow for the strengthening of the mast and tower without having to resort to sidewalk bridges.
Further, the outer edges of the dance floor are guyed by cables leading up to the roof of the cocoon and the tower above. Encircling the exposed base of the tower just above the ESB’s roof, the floor is enclosed by walls created by aluminum truss towers arranged in an octagon and bridged by panels of ballistic cloth, which slide in edge tracks from bottom to top. To reduce exposure to dangerous winds, the panels can be drawn down in a few minutes by rollers.
The assembly is designed to meet city codes for a 300-psf live load and a three-second wind gust of 98 mph. “It was wind-tunnel-tested to failure at 140 miles per hour, and it was the test-sample support structure that failed. It wasn’t the envelope,” says Peter Sjolund, the ESRT’s senior vice president of construction. “You are on the top of the Empire State Building, the most famous office building in the world. You don’t want to be on ‘film at 11.’ ” “The planning of the project took two years, including six months to design the cocoon and [perform the wind] testing in Florida,” says Tom Durels, ESRT’s executive vice president and director of leasing and operations. “That gives an idea of the amount of thought and engineering that went into it.”
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