The limits to how tall and thin towers can be has more to do with markets than engineers.
Within the next few years, the number of New York City skyscrapers that are 1,000 feet or taller is going to soar. Today, there are seven towers in the 1,000-feet-plus club. If construction proceeds as planned on projects now under way or scheduled to break ground this year, that figure will more than double.
In Manhattan, architects are building highest and fastest in Midtown. There, the supertalls aren't just tall—some of them are superskinny, too. A group of buildings along West 57th Street with residential units priced from $5 million to more than $100 million has transformed the Central Park perch into Billionaires Row, a signifier of America's new Gilded Age. In most cases, each unit is basically its own penthouse suite, occupying an entire floor of its building.
Taken on their own terms, the superskinnies represent a feat of architectural design. The new developments going up on West 57th Street may, in fact, be approaching the outer limits of the tall-to-thin aspect ratio for a structure. Just not for the reasons you might think.
"Structurally, there are a lot of very unique challenges, especially for a building that wants a high degree of special views," says Vishaan Chakrabarti, a partner at SHoP Architects and the director of the Center for Urban Real Estate at Columbia University. SHoP—the firm that designed the Barclays Center as well the forthcoming Domino Sugar Refinery development, both in Brooklyn— is responsible for what may be New York's, and the world's, skinniest supertall.
I use that term advisedly: "Supertall" is a category of building defined by the Council on Tall Buildings and Urban Habitat as between 984 feet and 1,968 feet in height. (Anything much taller than that is, technically speaking, "megatall," and not permitted in the U.S. by the Federal Aviation Administration.) The SHoP Architects project in Midtown—111 West 57th St.—is not only a giant at 1,350 feet. It's also strikingly narrow.
"The majority of the shear load is happening on the east and west façades," Chakrabarti explains, referring to one of the fundamental forces that architects and engineers have to account for in designing buildings. Those façades will boast another bravura element of the design: a system of terra-cotta blocks.
"[The design] gives the building a sense of stability," Chakrabarti says. "It’s a slender building, but a stable building. The main structural challenge is taking the core and using the east and west shear walls to stabilize the building."
With an aspect ratio of 1:23—that's the ratio of the longest dimension of the building to the narrowest—111 W. 57th St. is a true needle. Chakrabarti describes the building's feathered tiers, or setbacks, as "an essay on the setback." Which you will be able to detect even at street level.
But this building is no outlier in its structural components. The design features gaps in the building that allow wind to pass through, lessening the wind load on the whole. It also includes a tuned mass damper, a large object that acts as a kind of inertial counterweight. This is designed to balance a building, to keep it from swaying in the event of an earthquake and to ensure the comfort of occupants when the building is buffeted by strong winds.
(Here's the roughly 800-ton tuned mass damper of Taiwan's Taipei 101 skyscraper doing its job during the 2008 Sichuan earthquake.)
These are the kinds of structural advances that have utterly transformed skyscraper building in the last few decades. But they aren't particular to 111 West 57th St.
Down the road is 217 West 57th St., another supertall, superskinny Midtown tower. This isn't, strictly speaking, the project with the sharpest aspect ratio that architect Gordon Gill has ever designed. That honor belongs to the trident-shaped tower designed by architecture firm Adrian Smith + Gordon Gill for One Dubai in the United Arab Emirates, a $95 billion—billion—mega-development planned by firm Skidmore, Owings & Merrill (SOM). But 217 W. 57th St. will nevertheless be one of the skinniest towers in Manhattan and the nation.
"The complexity just increases when you get slender," Gill says. "The floorplates become smaller, but the views can become really amazing."
Adrian Smith + Gordon Gill is responsible for creating some of the best views in the world. The firm designed Kingdom Tower, an astonishing 3,280-foot tower under construction in Jeddah, Saudi Arabia. (Smith, while he was with SOM, designed Dubai's Burj Khalifa, currently the tallest building in the world.) No one boasts taller projects. And at 1,423 feet in height, 217 West 57th St. will be New York's second-tallest building, right behind 1 World Trade Center—and only then when you count the spire.
This new kind of skyscraper is popping up all over Midtown—but so far, only there. There are reasons that superskinnies haven't shown up in Chicago, Los Angeles, San Francisco, Dallas, or other cities. Nor are they likely to get much skinnier in Manhattan.
"From an engineering standpoint, there’s a ways to go," Gill says when I ask him how tall and thin the firm can build. "From an economic standpoint, we’re close to the limit."
"We cut slots, we punch holes, we create notches in the corners of the buildings" to mitigate the effects of wind, Gill says, on tall and thin buildings alike. But there are some places where superskinnies will just never go. No matter how pitched income inequality comes to be in San Francisco, these towers will never rise there. "For areas that are seismic, the slenderer buildings are not advisable," Gill says.
"It stops making sense in terms of elevatoring," Chakrabarti says. Vertical circulation—elevator shafts and related mechanicals—don't get skinnier as a tower does. For a project like 111 West 57th St., where each floor constitutes a single huge apartment, this means that elevators chew up floorplate as you rise. And since the entire tower yields only about 300,000 square feet of residential space, there isn't a lot of square footage to lose.
You could go taller with these skinny towers, but it would require something that no U.S. city has in spades: abundant land downtown available for development. The higher you go, Gill says, "the land area you require increases. Doubles. Triples." At a kilometer high, Kingdom Tower's footprint is still manageable. But at a mile high, the base that a building requires becomes much, much larger.
"You end up carving that stuff up [the base area]," he says. "There’s an economics to it that suggests that at this time, and this place, this is what we can do."
Even in Billionaires Row—where Midtown zoning allows skyscrapers to soar—the oxygen has mostly been used up. To build the towers that are rising now, in many cases, developers purchased air rights from adjacent shorter buildings. "At least in this corridor, most of the air rights have been used up," Chakrabarti says.
In other words, these superskinnies are unique—a "registration of the market," as Chakrabarti calls them. "It is a typology that’s happening, no question," he says, noting that SHoP has at least two more supertalls coming to New York. "But if I look at our overall portfolio, [superskinny, supertall] is not an enormous percentage in terms of square footage."
So fans and critics of these buildings shouldn't expect to see them copied everywhere. At least, not until design and engineering technology advances to the point that that the aspect ratio can be pushed to even leaner proportions in markets that could sustain these developments. Or, not until other markets generate the political climate that makes these developments possible.
"I just received a document a couple days ago from someone who's been working on a patent for building stabilization," Gill says. "I'm going to dig into that and check that out. The more we learn about how to tune that, the more interesting the structures can become."
*Correction: An earlier version of this post indicated that New York's Boston Valley Terra Cotta will provide terra cotta for the SHoP Architects projects. The workshop is one of the guilds being considered for the commission. And the Taipei 101 is properly located in Taiwan, not China.