IMAGINING A NEW URBAN typology—neither a building nor a sculpture, not quite infrastructure yet not purely art, as much empty space as metal—may have been the easy part. The real challenge with the Vessel, notes engineer Eli Gottlieb, managing principal at Thornton Tomasetti, was in the execution: orchestrating the symphony of forms and voids that makes this “public space that’s cascading” not only striking but stable and safe. From dimensional planning to construction strategy to material choices to vibration analysis, Gottlieb reports, the process required exceptional coordination amid technical and procedural innovation. “I don’t know that there are that many precedents for what this is,” he says. “How do you create a staircase that takes the public realm and lifts it up into the air around it?”
Asked by Related developer Stephen Ross to design a sculpture for the Hudson Yards development, and resistant to the convention of dropping an ungainly decorative object into a plaza, Thomas Heatherwick and colleagues decided to sidestep the implicit competition with works like Anish Kapoor’s Cloud Gate (the “Bean”) in Chicago’s Millennium Park. Heatherwick, in a recent televised discussion with Dezeen’s Marcus Fairs, asked, “How could we make something that brought people together, that didn’t block a space, so that you could look at each other as well as look out? Could we make a room, and could we let you use your body to navigate that?” The resulting structure, holding 8001,000 people at once on 16 levels with 80 landings, 154 flights of stairs, and nearly 2,500 steps, is a complex response to a simple desire: “We wanted to make a social device.”
The Vessel raises a subterranean model now well-known to be Heatherwick’s chief influence, the stepwells of Gujarat and Rajasthan, India, into an aboveground lattice of staircases and landings allowing improvised ascents, descents, and circulation through the space above the Hudson Yards platform. Since
it opened in March 2019, it has drawn torrents of tourists and a full range of rhetoric, from awestruck to scathing. Complaints about the Yards as a pseudo-urban theme park, its public subsidies for private interests, its initial photorights policy (quickly changed), or the perceived inutility of stairs to more stairs have not slowed the flow of Instagram-snapping climbers. Its silhouette serves as a glyph for the Yards as a whole. Love it or hate it—New Yorkers are already doing plenty of both—the Vessel is an instant icon.
It’s hardly a vessel in the sense of a container; it’s fully porous to the winds off the Hudson, though rigidly built, with a stressed-skin box structure, interior ribs, and orthotropic decks (the inner and outer staircases) cantilevered
off the central interlayer spine. Comparing its box elements to “airplane wings and fuselages or boat hulls” and its decks to boat and bridge construction, not buildings, Gottlieb outlines a sequence of challenges that the team had to overcome.
The first involved basic geometric details: “How many staircases? How narrow does it start? How wide does it get at the top? How tall is it?… Do you start with four staircases or five staircases at the bottom of it? When you get to the top, is it still the same number?… Is there some kind of different cascad-ing pattern that happens where the mesh becomes tighter?” The engineers and architects used parametric models in an iterative collaboration to determine dimensions and spacing, settling on an odd-rhythm pattern with a pentagonal base and seven expanding decagonal levels in plan, cup-shaped in section and pinecone-profiled in elevation. It rises, Gottlieb explains, from “a ring that’s about 45 feet in diameter at the base to a ring that’s 150 feet in diameter in the top. That’s a lot of stretch happening in this whole assembly.” As the structure of rising and falling staircases slopes outward, “you have to be watching out for headroom issues as you’re coming down a staircase, making sure that there’s enough rise, and the staircases you’re shifting enough so that you actually have space to be able to flow through it,” he adds.
“Then once you solve that, you start thinking about the structural challenge of actually building this,” Gottlieb continues. “As a team, we studied everything from being a full stressed skin, where the staircases are part of the whole stressed skin, to the final solution that we came up with, which is that just the central box, what we
call the interlayer, is a stressed skin, and the staircases on either side are separate orthotropic decks.” The staircases do not spiral, Guggenheim-ramp-style, or cascade all the way to the bottom, but compose individual rings, so that “all of their forces need to be transferred to the interlayer to get down…. It became more efficient for us to allow them to be orthotropic decks, which then allowed us to explore how the copper-colored finish material would actually be fabricated and installed.”
One decision that simplified fieldwork was to use a custom prefabricated module called a “dog bone”: a primary landing plus stair segments leading up and down on each side, with staircases spliced together at mid-run. Fabricated in Italy,
the dog bones were brought in by barge and trucked from the West Side Highway just a few blocks, minimizing clearances for transportation. “Erection actually ended up being controlled by flow of material to the job site, as opposed to the speed of erection on the job site,” Gottlieb reports; each dog bone went up in a few hours. The full assembly comprises 75 primary dog bones, 10 special dog bones at the base, five pedestal pieces, and six special pieces for the south-side angled spine and ADA-compliant elevator, the one way visitors with mobility challenges can ascend the structure. The entire Vessel uses 96 steel members.
Material research for the skin ranged from “bonded bimetallic materials to individual cupronickel alloys,” finally settling on a cladding of polished stainless steel, deposition-coated to resemble copper and polished to mirror-like reflectivity. The hard-coat deposition is durable enough to withstand salty air blowing in off the Hudson without corroding or forming a patina. “It should maintain its color and its reflectivity over time,” Gottlieb says, with normal cleaning. Scratchiti or other forms of vandalism are unlikely, since most of the cladding is out of reach; 12-foot level-to-level heights, plus vigilant security around the concrete base plinth (the only area where visitors come close to the polished surface), ensure that malcontents will not leave their marks.
Prefabrication was extensive enough to allow not only the distinctive cladding but preinstallation of details such as shoes for handrails and mechanical systems (pipes, drainage, lights, speakers, wiring) concealed inside the structure. All bolts are interior as well; “you actually have to climb inside [the dog bones] to make up the connections between them,” Gottlieb says. “That allowed us to get a very refined geometry fitup on them, and in fact the whole structure went together within a few millimeters, [needing] almost no adjustments in the field to the shims that were actually designed into it.” The overall look is seamless, as sleek as a cinematic spaceship.
Almost nothing in the Vessel is vertical except for the edges of stairs, the seams of the safety glass panels, and the cab (though not the movement path) of the elevator. Its entire aesthetic appears to defy verticality, an anomaly in its hyper-vertical city. With this non-orthogonal geometry transferring loads at angles, and a relatively small base bearing its weight, the design team took exceptional steps to stabilize it, accommodating visitors’ fear of heights or sensitivity to vibration under seismic or wind forces. Vibration tolerance being partially a matter of perception and expectations in context, Gottlieb notes, it was essential to keep this potentially lively structure as stable as possible.
“You can imagine it works a little bit like a vertical spring, because of the natural cascading through it,” he notes. “That’s been analyzed and designed to be able to take all those vertical stress loads down, and it does act as a cantilever from the bottom.” The narrow pinch point at the base rests on a 12-foot-tall steel box element, the pedestal or plinth, below the walking surface (organized around a central blue light used for eerie photographic effects). The elevator pit and a mechanical room lie beneath the pedestal, which “bolts down to a steel grillage of three-foot-deep fabricated plate girders that form a continuous mat, effectively, underneath the entire width of the Vessel. Those then in turn sit on a set of major primary steel elements that span to columns and caissons that go down to grade underneath. So there is a complicated foundation for this base, but the base of this is a very stiff assembly; it’s all bolted down and tied together under tension, [with] high-strength bolts that hold it all down…. It is designed to be able to resist all those wind loads and all those seismic loads as a cantilever off of that base that has very solid foundations all the way to rock underneath it.”
To ensure that crowd behavior cannot compromise the structure’s strength or stiffness, the engineers analyzed various extreme unbalanced-load scenarios, such as a concert or other event on the plaza that would cause
all occupants to stand on one side or the other, large-scale races to the top level, or rhythmic movements that could create a resonance phenomenon, like the mass lockstep that gave London’s Millennium Bridge its “Wobbly” nickname. “We actually went to a testing facility that RWDI [Rowan Williams Davies and Irwin] has up in St. John’s, Newfoundland, and recreated what the visuals would be like on the Vessel, looking off of it into the surrounding neighborhood, and recreated what the movements… or the acceleration and vibrations would be like, and actually ran the entire design team and the ownership team through what those different experiences might be,” says Gottlieb.
One technology commonly used in much taller buildings adds to the Vessel’s stability: ten tuned mass dampers (TMDs), averaging 12,000 kg each, designed and fabricated by the German firm GERB and installed by Cimolai. “We tuned them in the field,” Gottlieb reports, measuring how the structure moved when excited by people jumping, then adjusting the TMDs to cancel motion. “I’ve been up there with one of my coworkers who doesn’t love heights that much,” he says; this colleague pronounced the Vessel “actually pretty comfortable in the end.” Integrated project delivery is sometimes an organizational buzzword; in the case of the Vessel, it was a practical response to the conditions of the project, where prefabrication solved problems of a tight site and a novel design. “We weren’t just imagining the end structure, but we were imagining, ‘How do you actually assemble this?’ and having the contractors at the table with us from the very beginning,” Gottlieb says. “Everything was designed to be totally integrated to be as efficient as possible, which allowed us to minimize the materials, optimize construction speed, and deliver on the tightest tolerances.”
The perspectives the Vessel allows are intriguingly unfamiliar, verticalizing pedestrian life, looking both outward and inward in ways no other Manhattan sites allow. (They may also be evanescent, since the view of the Hudson and New Jersey is likely to shrink in several years when the Western Yard adds seven more buildings.). A purposefully collaborative process, advancing the arts of offsite construction and quality control, seems appropriate for a “social device” that scales up and condenses a common action—passing other people on stairs—to generate a multidimensional human and urbanistic spectacle.