Moving Up in the World
Structural gymnastics help ultrathin, ultra-tall residential towers for the ultrarich make their mark on the Manhattan skyline.
In New York City these days, residential towers cannot be too slim or too tall. The improbably slender form of One57, now fully enclosed, is the furthest along of a new crop of super-thin, supertall, super-luxe residential towers. Designed by Atelier Christian de Portzamparc for Extell Development Company, with glass panels of blue, pewter, and silver, it rises 1,004 feet, hundreds higher than even its tallest neighbors.
Just blocks away, CIM Group and Macklowe Properties' 432 Park Avenue will top out at even greater height, 1,396 feet. The square floor plan, extruded without setbacks in concrete with punched windows, has reached about 45 stories. It is as proportionally pristine as a Sol LeWitt sculpture—an image of “pure strength,” says its architect, Rafael Viñoly. Nearby, SHoP Architects is still refining its design for the 1,350-foot-tall building planned for 111 West 57th Street. So far, it is the slimmest of about a dozen super-skinny, supertall towers planned or under construction in New York. The ultrathin buildings are intended to lure buyers willing to plunk down as much as $95 million for a home perched high in the sky.
“These towers are going up in neighborhoods that are distinctive and have tall-building zoning capacity yet have only small parcels available,” says Carol Willis, the founder of New York's Skyscraper Museum. “Evolving technology and unprecedented price points make these buildings work,” she says. Though super-slims are for now unique to just a few Manhattan neighborhoods, high-rise living worldwide will benefit from the innovations that make these super-slims possible.
Codes define a slender building as one that is more than seven times as high as the narrowest side at its base. In the Skyscraper Museum's recent show Sky High and the Logic of Luxury, Willis exhibited towers she calls super-slender—those with at least a 1:12 ratio. The slenderness ratio of 111 West 57th is a startling 1:24.
Zoning Sculpts Form
Regulatory requirements, especially zoning, substantially influence the design of the super-slims. Several will rise in a special Midtown Zoning district that permits high density and the transfer of “air rights”—subtracting unused development square footage from atop existing buildings and adding it to a new tower. All of the super-slims involve such transfers. And even though their assertive profiles on the skyline and the shadows they will cast have sparked protests, most are being built “as of right.” In other words, they are on sites that lack height limits and do not require additional zoning or design review.
Not all of the city's ultra-skinny towers are being built in Midtown Manhattan. An 830-foot-tall, 57-story tower with a slenderness ration of 1:11 is under construction at 56 Leonard Street in the TriBeCa neighborhood. The building, designed by Herzog & de Meuron, will have floors cantilevering as much as 25 feet, creating the impression that they are precariously stacked. It will be able to grow so much taller than its mostly low-rise neighbors because Alexico Group, the project's co-developer with Hines, acquired air rights from the entire block.
In most parts of the city, towers must not penetrate the “sky exposure plane,” which defines a buildable envelope that slopes away from the street, to permit sun access. For 53 West 53rd Street—a 1,050-foot-tall hotel and condo tower soon to begin construction next to the Museum of Modern Art (MoMA)—architect Jean Nouvel tapers the building's spire to fit within a pair of sky exposure planes converging from two streets. Similarly, 432 Park Avenue confines itself to the zoning envelope by rising from the center of the through-block site without setbacks.
At 111 West 57th Street, SHoP has opted to move its tower away from the street in order to stay within the permitted envelope while minimizing required setbacks. As part of an agreement with the city's Landmarks Preservation Commission, the tower will rise from the courtyard of Warren and Wetmore's 1925 Steinway office building, which will be turned into apartments as part of the project. The new tower's highest floors must still be set back, but SHoP makes the most of the requirement, creating an expressive stepped crown. Bronze window mullions turn into finials to visually dissolve the top of the building into the sky. “We wanted to make a New York City building, so we worked with the zoning to express the dictated setbacks yet take it further,” says SHoP partner Chris Sharples.
Structure and Stiffness
Addressing the unique structural aspects of ultra-slender towers without sacrificing too much sellable space to elevators, stairs, columns, and shear walls is the key challenge, explains Silvian Marcus, the director of building structures at WSP. The firm is the structural engineer for most of Manhattan's super-slims.
According to Marcus, high-strength concrete (around 14,000 psi) is essential in slender towers. Concrete mixes have improved a great deal in recent years, he says, with additives enhancing workability. Fly ash, a byproduct of coal combustion, is increasingly used to replace a large portion of Portland cement in these mixes, because it offers strength without the substantial heat normally generated during cement-curing, which can cause cracking. (It also reduces the landfilling of the ash, which can contaminate surroundings.)
Surprisingly, wind loads are not as much of a design challenge as the oscillations those forces induce are. Building codes dictate stiffness to protect occupants and avoid distortions that crack windows or cause leaks, but codes do not require steps to counter the discomfort people feel when sway acceleration is too great (measured in fractions of g-force, or milli-gs). Such discomfort may sound trivial, but Marcus says minimizing acceleration is essential to habitability. WSP designs to approximately 18 milli-gs for winds that would be encountered over a 10-year period and as little as 5 milli-gs for more frequent low-velocity winds. That standard dictates a host of responses that affect building mass, structural configuration, and whether dampening devices are needed.
The simplest tactic to combat those forces is to add weight, especially at the top of the building, but Marcus says that's just a first step. The engineer must engage the building's entire structural system by strengthening the connection of the core to the perimeter columns and shear walls. To stiffen 111 West 57th Street, WSP thickened the shear walls that run the full length of the east and west exterior walls to as much as 3 feet, and linked them to the core with deep beams.
The building's 15½-foot floor-to-floor heights left plenty of space above the ceilings to conceal the stiffening beams. The dimension not only suited the engineer's purpose: the floor-to-floor height also coincides with the maximum code-permitted run of the exit stairs. That allows the floor space given over to the core to be minimized, especially as the two required exit routes are nested atop each other in a “scissor” configuration. The generosity of the resulting 12-foot-high finished ceiling has become a selling point.
Marcus varies core-and-perimeter tactics to suit the architectural intention of each building. The exterior of Viñoly's 432 Park Avenue is structural poured-in-place concrete that acts as a tube. (With white cement and careful attention to forming, the tube is also the finished architectural surface.) Even with monumental 10-foot-square windows, the tube is stiff enough to permit a 30-foot column-free zone between the exterior and the central core.
At 53 West 53rd Street, Nouvel aligns the elevators and stairs along the west wall, where the building abuts an office tower. Marcus says the structure “relies very little on the core” because of the extremely stiff concrete diagrid exterior that slopes to a pinnacle at 1,050 feet. The floor layout leaves a large unimpeded area for MoMA galleries at the base.
At 56 Leonard, WSP stiffens the structure by placing the core's perimeter wall outside the hallway that wraps the elevators and stairs. Enlarging the core increased the building's rigidity enough to minimize interior columns. As the building rises, the outer core wall vanishes. Shear walls wrap mechanical floors to further stiffen the building. They are set back 3 feet from the perimeter, behind a glass-louver facade.
Despite their prominence, the Leonard Street building's cantilevers neither contribute nor detract from the stiffness of the tower. Marcus calls them a “local problem” structurally, handled in a number of ways, with exposed, varying floor-slab thicknesses expressing the forces they resist. In full-floor cantilevers, short walls within the building envelope and columns at the far edge of the cantilever lock top and bottom slab together to attain rigidity without diagonal braces. They are essentially Vierendeel trusses, say the designers.
Confusing the Wind
Buildings with a slenderness ratio of 1:10 or more demand additional steps to reduce acceleration, such as dampers installed near their crowns. At One57 and 432 Park these take the form of tuned mass dampers, which consist of weights (typically between 500 and 1,200 tons) suspended from springs. Some buildings, including 56 Leonard, rely on a tuned liquid damper. It has a compartmentalized pool filled with water. In both types, the mass moves more slowly than the building sways, dampening the acceleration.
Marcus also slows accelerations by reducing wind forces on the buildings. The setbacks at 111 57th “confuse” the wind, reducing pushing forces on the windward side and the suction forces on the leeward side. As the design is refined, the architect will probably include gaps in the facade to let wind pass through the building, lowering the overall force.
Viñoly provides unenclosed floors at intervals up 432 Park Avenue's facades. The spaces between columns let the wind pass, and also offer Marcus the opportunity to conceal stiffening columns and beams that cannot be accommodated on occupied floors. A curved enclosure protects the core and mechanical equipment.
Commercial towers have, until now, led tall-building innovation. But as more skyscrapers house residential uses, including luxury hotels, the building type's appeal will push architects and engineers toward further collaborative refinement and innovation. “We are in a continuous search for improving materials and systems,” Marcus says. “To reach higher into the sky, we take small steps.”
James S. Russell, FAIA, is a New York–based architecture critic and journalist. He is author of the book The Agile City: Building Well-being and Wealth in an Era of Climate Change.