Engineering Sky Villas: 56 Leonard Street, New York

56 Leonard is one of the most distinctive landmarks on the Lower Manhattan skyline. The building is a staggered series of irregular boxes, with cantilevers ranging from 10ft to 25ft, designed by Swiss architects Herzog & de Meuron – the practice behind the ‘Bird’s Nest’ Olympic stadium in Beijing. When complete, it will provide 145 luxury apartments in the TriBeCa district, conceived by the architect as “houses stacked in the sky”.

Location

New York, USA

Client

Alexico Group

Architect

Herzog & de Meuron, Costas Kondylis and Partners

Project Status

Completed in 2016

Design Challenges

The design started out as a relatively regular rectilinear shape, but it evolved on a weekly basis until no two floorplates are alike. “Just by looking at the structure, you can see it presents many structural challenges,” says Hezi Mena. “It’s like a large sculpture. There was no simple solution for the whole building. Every floorplate has to be able to support itself, so we had to treat each one in an innovative, imaginative way.”

Every floorplate has to be able to support itself, so we had to treat each one in an innovative, imaginative way

Hezi Mena, WSP

Structural Solutions

The engineering team’s solution is to use a very strong concrete structure, concealed to allow a completely glazed exterior with views from almost every angle. The central core is linked to the external columns by outriggers at the mechanical levels 32 and 46. At the top is the ‘swimming pool’ – a slosh damper filled with 130,000 litres of water to temper the building’s movement in the wind.

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Supporting the cantilevers was one of the biggest conundrums for Mena’s team. For the smaller ones, the thickness of the concrete floor slabs provides sufficient support. For the larger ones above 15ft, there are additional beams, and for the very largest, a Vierendeel truss – a perpendicular column that engages two floors, without obstructing layouts or views. Throughout the structure, there are many ‘walking columns’, where loads are transferred from one location to another as they progress down the building. There are no dividing shear walls in the apartments at all, to allow residents to lay out their living spaces as they wish, or to combine apartments horizontally or vertically.

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Luxury Villas with Views of Manhattan

The greatest cantilevers are at the top of the building. The uppermost 10 floors each comprise just one apartment, known as ‘sky villas’. “It won’t feel as if each occupant is living in a high-rise building but in a house in the country,” says Mena. “They will have views all around, and they won’t really be aware of who’s above or below them. It will be a very different experience to your average New York City apartment.”