From an article in Construction Dive by Jenn Goodman.
Adding to the constraints, the renovation of the 102nd floor of the Empire State Building in New York City had to be completed while a visitor terrace below was open to the public.
The culmination of a four-year reimagining of the building’s visitor experience, the $165 million observation deck project involved replacing the old windows — which only reached waist height — with 360-degree floor-to-ceiling glazing offering unobstructed, panoramic views of the city. In addition, developer Empire State Realty Trust required that an 86th floor visitor terrace remained open to the public throughout the construction.
“It was paramount to the client that their guest experience not be interrupted and that our work remained as ‘invisible’ as possible to those visiting the observatory,” said Kevin Brightly, project manager, Skanska USA Building.
Working at such great heights — at times more than 1,250 feet off the ground — presented unique challenges that required unique engineering solutions, Brightly told Construction Dive. An exterior crane or material hoist was out of the question given the need to keep the terraces open to the public from 8 a.m. to 2 a.m. every day and minimize the need for extensive overhead protection, he said.
To complete this complex renovation without disrupting normal building operations, Skanska and partners Plan B Engineering and Greg Beeche Logistics employed an innovative “cocoon” enclosure system to facilitate construction while protecting workers from the elements and visitors below.
This approach required extensive engineering and plan review before construction began, according to Brightly, and included analysis of the cocoon not only in its final configuration but also in partially constructed and dismantled scenarios.
The team relied on a variety of construction technology systems to design and implement the cocoon, including RISA-3D, Revit, Inventor and extensive laser scanning and BIM modeling.
Before it could be constructed, a structural “halo” with a hoisting carriage was first built around the 103rd Floor; this system was then used to assemble and hoist 16 trapezoidal modules from Level 90 to 102 where they were docked and connected to one another in succession. The exterior work was performed nightly between the hours of 2 a.m. to 7 a.m., Brightly said.
Upon completion, the 26-foot-tall, 30-ton cocoon provided 600 square feet of exterior work space across two deck levels. Its bottom deck was rated for 300 pounds per square foot and its sidewalls of Haki sheeting were designed to withstand 98-mph wind gusts. The walls were lined with wire mesh screens and debris netting so that no material could escape during demolition or construction, Brightly said.
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