If there is one characteristic that marks the design of a “super-slender” tower, as perhaps best exemplified by the luxury residential tower at 432 Park Avenue, it is probably its conspicuous simplicity.
The simple design of 432 Park Avenue, a 1,396-foot-tall concrete and glass supertall residential tower in Manhattan, owes its inspiration to a trash can designed by Austrian architect Josef Hoffman. It is tall, it is skinny, and it costs a king’s ransom to live in one of the building’s 104 apartments. The price-per-square-foot for an apartment nudges the $10,000 mark, a considerable price to pay for the building’s enviable views of Central Park and five-star amenities. While the design of the building itself has been called, among other things, “a genuine clunker,” the building’s 96 floors make it the tallest residential building in the city.
Very tall residential towers are rare worldwide because they are expensive to build and, in most cities, prohibited by height caps or other zoning regulations.
The ingenuity of 432 Park Avenue and other slender towers like One57 and Nordstrom Tower is not so much in their aesthetic as it is in the novel way these towers have used Manhattan’s zoning laws to create a new generation of skyscrapers.
“This is a completely different skyscraper form what has been created by the vernacular of New York City’s zoning laws and its high land values,” says Carol Willis, founder and director of the Skyscraper Museum.
And 432 Park Avenue is certainly slender. The tower, which stands 1,396 feet tall and has a base of 59 feet, has a “slenderness ratio” (the relationship of the width of the base to the building’s height) of 1:15. This is slender, although not nearly as slender as the 82-story Steinway Tower at 111 West 57th Street, which boasts a slenderness ratio of 1:23.
Very tall residential towers are rare worldwide because they are expensive to build and, in most cities, prohibited by height caps or other zoning regulations. This chart from the Skyscraper Museum shows the seven places in 2017 where tall and slender apartment towers have stretched beyond 305 meters (1,000 feet). Within each tower, apartments range from many small rentals to large luxury condos, and from fewer than 100 units to nearly 800.
Using FAR to Build High
New York City owes the recent construction of super-slender towers to two aspects of New York’s zoning laws: FAR and air rights, per Willis’ 2015 research paper for the Council on Tall Buildings and Urban Habitat, The Logic of Luxury 2.0.
FAR (floor area ratio) is the principal bulk regulation controlling the size of buildings. It determines how much floor area a developer can have in a building that is relative to the size of the lot, per the New York Department of City Planning.
The buildings that these developers bought the air rights from will remain low forever.
A revision to the city zoning laws in 1961 introduced two new provisions that Willis says “re-set the approach to real estate development.”
Says Willis: “First, it established the principle of ‘as-of-right,’ which allows property owners to design and build whatever they wish without a public review process, so long as they follow zoning rules and do not exceed the maximum FAR allowed for that lot. Second, it created the concept of air rights, which said that if an existing building has not used all the FAR allowed in that lot, the unused air rights could be sold to the owner of an adjacent lot and used there.”
Per the 1961 zoning laws, any properties that share a minimum of 10 feet of their lot line are permitted to buy the air rights of those neighboring properties.
Most super-slender tower developments have employed this method by purchasing unused air rights for transfer to their site to “consolidate and concentrate their collective FAR into one tower,” Willis says.
“The buildings that these developers bought the air rights from will remain low forever,” Willis adds.
Gary Barnett of Extell Development Corporation spent over 15 years buying up the unused air rights from the smaller neighboring properties near One57, Extell’s 90-story residential tower on 57th Street that’s been dubbed “the Billionaire Building” for its high-net-worth tenants and $100 million-plus penthouse apartment.
“Developers sometimes demolish the underbuilt structures to create a larger site for their project, even if the tower will cover only a portion,” says Willis.
Building Slender to be Sturdy
Standing tall and slender requires sound structural engineering. The center of 432 Park Avenue has a 30-foot by 30-foot core bounded by concrete walls that acts similarly to “the backbone of a body,” says Silvian Marcus, the engineer behind 432 Park Avenue, in a 2014 interview.
WSP, the structural engineers behind One57, used “the most advanced high-performance concrete” for the shear walls and columns that supported the tower’s unique shape, while a liquid damping system was used to mitigate lateral movement.
The emergence of these super-slender towers around Central Park has sparked concerns from protesters who say these new towers have already started to cast shadows that extend three-quarters of a mile into the park. The Municipal Art Society of New York launched its Accidental Skyline Initiative in 2013 to chart out the impact super-slenders and other developments have on Central Park and NYC neighborhoods.
Willis argues that New York City’s zoning code’s allowable FAR is a cap-and-trade system, so “long, thin shadows are a trade-off for more openness on the street below.”
“We may see more and more of these buildings, but there is a finite amount of FAR that can be built out on the island of Manhattan,” Willis says.
She adds: “It may look to people like this is a trend that will multiply until we blot out the sun. Those are not the conditions of New York.”
13 September 2016 by Daniel Rosen
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