Fulton Center Sky-Reflector Net Lower Manhattan’s device; and the engineer wanted bustling transit hub to make sure, quantitatively, that is crowned with an daylight could penetrate Fulton Center’s lower levels.” ambitious tensile Kostura adds, “In a normal proj- structure that required ect, you can almost get away with rigorous collaboration dividing up the systems that support between designer, the interests of different stakehold- fabricator, and installer. ers. Here, achieving our goals required intensive collaboration.” Sky Reflector-Net was NAVIGATING AMONG NINE SUBWAY conceived by James Carpenter lines and the streets of Lower Design Associates (JCDA) in Manhattan, 300,000 people response to Arup and Grimshaw converge daily to walk through Architects’ initial 2003 design of Fulton Center, which opened to the Fulton Center pavilion. While the public on November 10, 2014. that scheme would go through Just as stunning as the number multiple iterations for value engi- of users is their initial reaction neering, the three-story structure to the new transit hub’s atrium consistently featured a dome pavilion. Spotting the artwork Sky whose south-facing, 53-foot- Reflector-Net that crowns the diameter oculus would top out eight-story hall, they turn from at 108 feet tall. JCDA namesake hardened commuters into tour- James Carpenter imagined ists: Travelers pause under, pho- suspending a skewed hyperbolic tograph, and orient themselves paraboloid—or hypar—armature by the skylit cable-net structure from the dome’s sloped oculus, cradling 952 Alanod-coated and tracing its double curvature in perforated anodized-aluminum panels that brighten and, evoking panels, each measuring 0.145 the New York subway system’s inches thick. historical reliance on sunshine, The varied responses bear reflect natural light into subterra- out the installation’s multiple nean circulation zones. functions. As Arup’s Zak Kostura, Carpenter’s vision was com- structural engineer for the project, pelling enough to win the public- explains, “The artist came to the art commission for Sky Reflector- table wanting to create an experi- Net. Developing it beyond ential effect; the architect wanted concept required interdisciplinary people to use it as a wayfinding support. “That hypar shape didn’t 2 Metals in Construction Spring 2015 Fulton Center Sky-Reflector Net 3 Right Tightly choreographed fieldwork required two boom lifts and a swing stage operating simultaneously. Below center A form-found structure, the net adopted its final geometry once 56 tie rods at its base were tensioned on-site. Below Cruciforms connect panels. Rod segments have serial numbers to ensure proper placement. assume the weight of the panels, the connections, or the posi- tion of the boundary form, which would all dictate the final design,” Kostura explains of the myriad factors that would have to be calculated to realize the complex geometry. JCDA subsequently worked with Grimshaw as well as Arup, which was tapped as prime consultant to the MTA in 2003, to bring the idea to life. While Carpenter did not origi- nally picture it as such, budgeting dictated approaching the hypar as a purely tensile structure. Arup reverse-engineered tensile forces and node positions according to Carpenter’s preferred geom- etry by executing more than 113 iterations of a computational model. Next, modeling yielded the unstressed length of each cable segment, along with 1,056 x, y, and z coordinates that would ultimately position the 952 infill panels into 17 rows. All but the top and bottom rows comprise rhomboid panels, and Kostura notes that a tradi- tional hypar-shaped net would Above Three teams of contractors per- Below A skylight at the top of Fulton have included horizontal banding formed the panel installation, connecting Center sends light into the atrium to panelize it into stiffer triangles. each panel to nearby cruciform at its below, much like the subway-illuminating “Jamie [Carpenter] and Grimshaw four corners. sidewalk skylights that used to punctuate New York City streets. made an adamant assertion that the panels should be diamond- shaped and not triangulated. That posed an immense structural challenge of creating a soft net from rigid, delicate panels, which would waft in the intense amounts of sun and airflow in the atrium.” The panels are bolted to one another via paddle connec- tors, held between 112 quarter- inch-diameter steel cables that crisscross to form the 17 rows. The integrated team had to treat a seemingly minute detail of Sky Reflector-Net—the left, right, and bottom holes where each panel would receive the paddle connec- tor—with the same scrutiny as its cable lengths. Indeed, if attached with less tolerance than the maxi- mum billowing of the net, then extreme movement could rip the This page top and opening spread: Zak Kostura; this page bottom, facing top and center and following page: MTA/Patrick Cashin; facing page bottom: Jennifer Krichels; concluding page: James Ewing Zak Kostura; this page bottom, facing top and center following page: MTA/Patrick This page top and opening spread: aluminum piece from its source. 4 Metals in Construction Spring 2015 Fulton Center Sky-Reflector Net 5 Using computational fluid dy- the artwork in summer 2010. He The enlarged design team namics to model the cable net un- continues, “On many projects, relished further opportunities der 815 different conditions, the it is not until after contracts are to ensure accuracy. Enclos and team arrived at peak width and awarded that we’re able to ex- Schopfheim, Germany–based height values for the holes of 0.23 change model information.” panel manufacturer Durlum inches and 0.08 inches, respec- Enclos and Arup used the conducted parallel engineering to tively. Hole dimensions were set same software platforms in validate production of the 8,524 with construction tolerances. A modeling the cable net and its square feet of anodized alumi- similar parametric design process panels. They did not necessar- num. Similar quality control was also generated the cruciform- ily incorporate the same criteria. performed during the mockup shaped paddle (see diagram), Namely, “We set up our tools to of 13 panels at the Westford, which swages the crisscrossing anticipate field conditions that are Massachusetts, workshop of con- cables with a through-bolt; the not absolutely precise, but are tractor TriPyramid, with one en- paddle’s horizontal and vertical within construction tolerances of gineering team manually deriving pieces measure 0.25 inches thick adjacent trades,” Vaglio says. the panel geometry and a second at their ends, and taper to 0.125 Strength of panel-to-paddle team automatically generating it This spread Work on the surround- deflections we needed to be able the redundancy. And while that FULTON CENTER SKY-REFLECTOR NET inches at that bolted midpoint. bolts was one inconsistency from the parametric model. ing pavilion building continued to lock each kingpost to the rods diligence clearly paid off—the throughout cable net installation. All elements freely rotate around that arose from the different Finally, Enclos deferred once tensioning was finished,” Sky Reflector-Net cable net was Location: Fulton at Broadway and Cortland Street, New York, NY Work was performed carefully to the bolt axis to account for panel perspectives. In turn, Arup and fabrication of tensioned AESS ensure the relatively delicate cable Kostura explains. “So we came up installed in one piece during a Owner: MTA Arts for Transit and Urban Design and the MTA Capital Construction movement. Bolts at the tips of Enclos discussed the movement stainless-steel tie-back rods until net system was not compromised by with a novel detail involving cylin- single morning for minimal disrup- Company (MTACC), New York, NY the horizontal and vertical pieces tolerances associated with a site surveys confirmed as-built construction activities nearby. drical nuts that could be torqued tion to adjacent work, for ex- Sky-Reflector Net Artist: James Carpenter Design Associates (JCDA), New York, NY connect paddle to panel. more robust bolt, forecast the data: Each 0.375-inch-diameter against external threads on the ample—Vaglio adds, “We learned Architect: Grimshaw Architects, New York, NY The meticulous engineering of network-wide ramifications of rod addresses construction coupler until they bear on a flange that no matter how precise our Structural and Mechanical Engineer, Facade Consultant: Arup, New York, NY Sky Reflector–Net might suggest individual adjustments, and then tolerances via adjustability. At the within the pocket of the kingpost. modeling, there’s always a point Construction Manager: Lend Lease, part of a joint venture with PB Americas, a direct file-to-fabrication ap- identified the exact locations to oculus, for example, rod segments Those nuts were left loose on the when the reality—the construction New York, NY proach to execution. But this proj- increase bolt size. All of the cable were connected by couplers set rod assembly until tensioning was site—decouples from the model Structural Steel Erector: Imperial Iron Works, Bronx, NY ect’s delivery required overlapping net’s bolts are stainless steel within a hole at the base of each complete. The result was a stable and craftsmanship is paramount.” Architectural Metal Erector: Jordan Panel Systems Corp. (exterior/interior metal digital efforts. “We engage in ASTM Grade F593 316, and nuts kingpost—the vertical member system with compact, almost Double- and triple-checked in panels), East Northport, NY modeling before we even sell a are stainless steel ASTM Grade that connects the tension rods to invisible connections between the the abstract and perfected in the Ornamental Metal Erector: Capco Steel Erection Co., Providence, RI project,” says Jeff Vaglio, a Los F594 316. Eighth- and sixteenth- the oculus’s upper lattice of tube kingpost and rod.” field,Sky Reflector-Net spins its Curtain Wall Fabricator and Erector: Enclos, New York, NY Angeles–based associate director inch-thick Neoprene and Teflon- sections.