Council on Tall Buildings and Urban Habitat
56 Leonard, New York City
Featured September 2018

56 Leonard was recognized as an "Award of Excellence" winner in the "Tall Building Construction" category at the 2018 Best Tall Building Awards

Other Featured Tall Buildings

“56 Leonard is at the very edge of what is structurally possible. After all there are many buildings with cantilevers – but not many buildings with hundreds of cantilevers.”

Keyvan Kazemi, Project Manager, Alexico Group

Completion Date:
2016
Building height:
250 m (821 ft)
Primary Function:
Residential
Owner/Developer:
Architect:
Herzog & de Meuron Architekten (design); Costas Kondylis Design (architect of record)
Structural Engineers:
MEP Engineer:
Other CTBUH Member Consultants:
Cosentini Associates (security); Enclos Corp. (façade); Langan Engineering (civil, environmental, geotechnical)
Other CTBUH Member Suppliers:
Schindler (elevator)

56 Leonard has been called the “Jenga” building due to its irregularly projecting floors, as well as the irregular spacing and location of the balconies throughout its 57 stories. Above the fifth floor, there is a 6-meter cantilever to the west and a 4.6-meter cantilever to the south, both of which were constructed over occupied New York Law School buildings.

To build the superstructure safely, emphasis was placed on designing a custom, stackable perimeter screen system that provided full, passive fall protection. From the 45th floor up, all concrete-shaping forms had to remain in place to provide the necessary structural support to form and cast the floors above. The system was engineered so that once the concrete on the roof was cured, removal of the formwork could proceed from the top down until it concluded on the 45th floor platform.

The irregular floor plates as seen in this structural plan required careful detailing. A view up at the finished project highlights the intricacy of the perimeter balconies.

The project was designed with architecturally exposed concrete on the edges of the floor slabs and the underside slabs of all balconies, in addition to the columns and walls of the interiors. The façade is a full-height window wall system that consists of 3.4-to-6.7-meter-tall panels, which were installed from each floor using a robotic arm. The amount of exposed concrete on the project required extensive mockups of the columns and slab edges, as well as for the patching, cleaning and grinding of the concrete. The project team also consulted with experts in the concrete field from around the United States and the world to discuss how to get the concrete mix as flawless as possible because the design intent was for the concrete to be untouched after being stripped.

The prevalence of exposed concrete on the interior as a high-end finish required extensive mockups of the columns and slab edges. View of the perimeter screen system at work.

The interpolation of the balconies along each façade presented an interesting potential. Normally, a perimeter protection system to guard against worker falls would have to be fitted loosely around both balcony projections and the superstructure. But in this case the 260 balconies from the 3rd floor to the 43rd floor do not stack over each other, allowing the team to install them separately in a second wave. This allowed a separate perimeter protection system to be installed, as tight to the slab edge as possible — thus adding an extra layer of safety — which progressed up this portion of the building for superstructure construction, before returning to install the cantilevered balconies below.

Related Links
CTBUH Skyscraper Center Profile:
56 Leonard