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An in-progress look at the new transit hub’s massive skylight
After funding cuts and subsequent delays since construction started in 2005, the much-anticipated Fulton Street Transit Center is finally taking shape in Lower Manhattan. The $1.4 billion project will connect eleven subway lines with the PATH train, the World Trade Center, and ferries at the World Financial Center. In collaboration with artist James Carpenter, Grimshaw Architects designed the project’s hallmark—a 60-foot-tall glass oculus that will deliver daylight to the center’s concourse level. The hyperbolic parabaloid cable net skylight supports an inner skin of filigree metal panels that reflect light to the spaces below. AN took a look at the design’s progress with Radius Track, the curved and cold-formed steel framing experts who recently completed installation of the project’s custom steel panels:
Metal framing was an ideal choice for the skylight’s large structure, whose 90-foot diameter required a high strength-to-weight ratio that couldn’t have been achieved with a heavier material like concrete. Cold-formed steel (CFS) could also be manipulated into the complex shapes necessary to achieve the skylight’s irregular shape.
Though the project was originally designed as a stick-built structure, the design would have required workers to complete the construction of the complicated, sloping oculus walls while working five stories above ground. Proximity to the water raised concerns about severe storms that would have further compromised working conditions. The oculus also had to meet security standards surrounding the World Trade Center memorial sites, so the design team abandoned the stick-built approach and began to consult with Radius Track on an alternative construction method.
The structure’s total surface area is approximately 8,294 square feet, comprised of 44 panels arranged in two tiers. Panel width is a constant 8 feet, while length ranges from 19 to 33 ½ feet excluding two smaller end panels measuring 4 feet by 14 feet. The knife-edge element at the top of the parapet is 167 feet long, with a profile that changes continuously along the diameter.
Using BIM, Radius Track customized designs for the seven-layer panels that complete the walls of the oculus. The modeling software allowed the team to detect potential clashes within the panels and with other design elements early on, and also facilitated the rapid, offsite fabrication necessary for the project’s tight timeline.
The custom panels are designed not only for performance but also for geometric precision. The seven layers include framing (studs, track, blocking, and knife-edge panels where applicable), steel decking, DensGlass sheathing (a drywall material used in exterior applications), waterproof membrane, drainage mat, insulation and curved metal girts to which exterior cladding is attached, and Tyvek wrap. While the materials used in the project are traditional, the methods to connect the layers are not. Each layer has its own particular pattern, making attachment details between the layers critical. (For example, the CFS layer is a grid, the decking consists of linear ridges aligned with one panel edge, and metal girts span across the panel.) Each layer required its own design and subsequent coordination to ensure the finished installation was as precise as possible.
Several types of metal are used to create the oculus. The walls’ structural framing is 14 gauge (68-mil) cold-formed steel, a “beefier” design than Radius Track would typically employ because of high wind speeds and enhanced safety and security requirements that are now standard for government structures in New York City. Designers used 16-gauge CFS for the track that is wrapped horizontally around the oculus walls. Decking is VulCraft 3-inch steel decking and horizontal metal girts secure the insulation layers. At the parapet, Radius Track designed customized 16-gauge, laser-cut steel sheets to form the ever-changing slope that circles around the top of the structure.
Some sections are opening to the public ahead of the anticipated mid-2014 completion, and the complex is eventually expected to serve 300,000 passengers each day with 26,000 square feet of new space that will also include new retail stores and restaurants.