Tom Sawyer - 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.
Thee 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.
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.
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 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.
"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.’ ”
Stealthy Work
Scott Seydor was project manager for the cocoon construction. An architect, he joined Skanska USA in 2011 as a design manager for mission-critical, design-build data-center work, which brought him to the attention of Skanska executives looking for someone with a strong design background to manage mission-critical work in the Empire State Building, he says. One one such project, Seydor managed the replacement of the Art Deco mast’s 480 pieces of glass between the 90th floor and 101st floor.
When he heard of the mast-strengthening project, Seydor says he jumped at the chance to manage it, too. “I said, ‘We can do it right. We can do it safe. Let me do it. The project needs to be done.’ ” He adds, “I live in Philly. I get up at 3:30 in the morning because I get to do this. … I am an architect, and this is the most iconic building in the world.”
Few New Yorkers—not to mention the hundreds of thousands of annual visitors who meander around the observation decks—are aware of the work being performed above them by a small Skanska USA crew and the handful of steelworkers from Skanska Civil NE, a sister business unit. “It’s such a small space, there’s only so many guys you can throw at it,” Seydor observes.
Because the materials-handling path makes use of the passenger elevators and public observation areas, crews had to construct and, starting this week, will have to deconstruct the cocoon between the hours of 2 a.m. and 7 a.m., when the public spaces are closed.
From the loading dock, materials are navigated through security scanners, down to a lower concourse and up a service elevator to the 79th floor, where they are carted to a passenger-elevator bank for a lift to the 85th floor and then transferred to a manually operated 1930s “mast elevator” for a ride to the 102nd floor. After that, materials are hoisted up ladders and through hatches by hand and with chain falls out through a single “submarine hatch” in the flat-topped conical roof that crowns the building.
Due to handling constraints, including the size of the legacy elevator in the mast, the largest piece taken up was a 6-ft, 4-in.-long W12 steel beam, says ESRT’s Sjolund. A support for the platform framing, it weighs 2,833 lb.
To maneuver beams out of the submarine hatch and into place, the assembly sequence first required the installation of chain hoists on lifting booms projecting from the tower. The steel beams were placed first, building up the ice-shield roof just beyond the perimeter of the planned dance floor. Then, crews erected a hard-sided wall around the first work area to prepare for assembly of the dance floor, just above.
Continue reading on enr.com.