In Manhattan, Asia, and the Middle East, a new super-tall, super-skinny skyscraper form is gaining popularity. Though it’s unlikely they’ll ever be the dominant form, it seems that they fill a niche in crowded cities.
The structures’ smaller footprint enables them to squeeze on to smaller lots and make use of the vertical dimension. A project in Manhattan designed by Eran Chen of ODA New York - 303-305 44th Street - will contain 41 floors in its 600-foot height, though the building will be only 47 feet wide.
The design employs 16-foot gaps between floors to make room for gardens, enhance views, and reduce wind loads. The total gross floor area is 116,731 square feet, and floor plate area is 2,700 square feet.
This form of structure has its own appellation: the slender skyscraper.
“The designation of slender is actually based upon a ratio between the width of the building and the height, which is different than a 'skinny' building, which is based only upon width,” Chen said.
What’s driving this new trend? According to Chen, the skyscrapers of yesterday, such as the Empire State Building, Chrysler Building, the Bank of America building, and Rockefeller Center were created as symbols of corporate power. Today’s “slender” skyscrapers, however, are residential projects.
“Private wealth domination keeps growing in the modern world, and perhaps this typology can be seen as the contemporary version of palaces belonging to royalty during the renaissance,” he said. “One can look at New York City’s skyline as a depiction of how we function as a society; its buildings mirror the economic situation. As each slender tower slowly rises up, so too do private owners slowly substitute corporations.”
New York City’s zoning regulations also facilitate slender skyscrapers, as air rights can be transferred to adjacent sites, so that “even a small footprint site can accumulate enough air rights to reach groundbreaking heights," according to Chen.
The slender form creates additional design and materials challenges, but can yield stunning spaces. With their smaller footprint and maximized height, slender towers find it more challenging to absorb increased wind loads. That challenge has resulted in perhaps the tower’s singular design element: its gaps.
“By ‘stretching’ the building vertically beyond its original program, we were able to create gaps, 16 feet in height, between every two floors,” Chen said.
The gaps contain full-floor sculptural gardens equal to the footprint of the building, suffusing sunlight throughout the space. The central core and perimeter design will dramatically reduce the wind load without sacrificing the 360-degree view.
Due to their smaller footprints, slender structures are often designed as single-floor apartments.
“In that way, they allow for the improvement of the quality of interior spaces in addition to offering those apartments 360 degree views and a four-side orientation to vistas of the city, similar to those from an airplane,” Chen noted.
He added that structural integrity can be achieved with a strong core.
“The floor plates of a slender tower require a strong core that is well integrated with the rest of the space,” he said. “Often times with slender towers we utilize the façade as support to the core by bringing back the structure to the façade."
However, this approach does require some interior space.
“The building core becomes a part of the apartment, and because of the extensive structural elements required, the space efficiency of the floorplates is low," Chen said.
Other techniques can help fine-tune the performance of the tower.
“Adding a counterbalance, such as water tanks or concrete slabs, on top of slender structures increases stability and acts as pendulum centering the slender tower,” Chen said.
Another example of the slender form, also in Manhattan, is Rafael Viñoly's 432 Park Avenue. At 1,396 feet tall, the tower will have a short reign as the tallest residential structure in the Western hemisphere when it’s completed in 2017.
Coming down the pike, a project from SHoP Architects will feature even more dramatic form. At 1,428 feet tall, the 58-foot-wide tower at 111 W. 57th Street is even more slender in proportion than the ODA and Vinoly projects.
As expected, structures such as these come with a hefty price tag. 432 Park has units for sale from $17 million to $81 million per unit, while some units at 111 W. 57th Street are reportedly priced at $100 million each.
Is there a limit to the cost of these projects?
“Cost of construction,” Chen said. “If there are enough people out there who are willing to pay north of $10,000 per square foot for a single floor apartment in the sky, then this limitation evaporates and the slender tower can move forward and be built.”