The Spiral: How to enjoy the great outdoors at 1,000 feet

It’s rare for occupants of super-tall buildings to be able to get outside into green space without a long journey down to the ground. It’s even rarer for them to have a completely uninterrupted view of the sky above – no matter what level they’re on.

But that’s exactly what office workers at The Spiral, 66 Hudson Boulevard will enjoy. Every floor has a flexible double-height atrium space, which opens out onto one of a series of landscaped terraces that spiral up the building as it sets back. Because of this unique design, by Danish architect BIG, no two terraces are in the same place, and none is overlooked by any other.

“The Spiral”, as this Manhattan skyscraper is known, is developer Tishman Speyer’s vision for the future of the workplace. As well as a human-centric design that prioritises health and wellbeing, it also offers fully customisable layouts and coworking studio space. It’s all the more unusual for being conceived in 2016, long before the Covid pandemic made fresh air and access to the great outdoors so important to workers, and flexibility so essential to employers.

What tenants won’t see is the structural ingenuity that made this possible.

Location

Manhattan, New York, USA

Client

Tishman Speyer

Architect

BIG

Project Status

Complete

Awards

2-star Fitwell
LEED Gold

Under the skin: The Spiral’s super-strong, super-complex skeleton

Making complexity simple

Tall buildings are usually an exercise in repetition: identical floorplates are stacked one on top of the other, sometimes growing smaller towards the top. Not at The Spiral. The spiralling terraces meant that the perimeter columns had to slope inward at each floor, so as not to obstruct the view. And as the terraces move into their new location at each level, the floor framing shifted to brace the leaning columns. The result is a complex yet economical structure where no two floors are the same.

“Sloping columns are straightforward to engineer, but the net effect of this across 66 levels is a high level of complexity, not just for the structure, but for the facade, the connections and the construction too,” says Jeffrey Smilow, executive vice president and managing director of building structures with WSP in the US. “To manage that, you need experience of making the complex straightforward.”

Number of Storeys

66

Height

1041 feet

Gross Floor Area

2.8m sq.ft

Incredible strength, but only where it’s needed

There’s no theoretical limit on the height of a building – the limiting factor is the width of the structure needed to support it, and how much lettable space this consumes. Structural engineers are constantly pushing back this frontier, using new materials and technologies.

At The Spiral, WSP has made full use of higher-strength steel: Manhattan skyscrapers are usually built using grade 50 steel; The Spiral uses grade 65. This is crucial to managing the complex stress patterns created by the cascading terraces, and it made possible the long, column-free spans that make the offices so open and flexible. It also means that the structure can be as slender as possible, maximising the available floor area. It reduces the cost and embodied carbon too: as the steel is so strong, less material is needed in the connections and the core.

There’s always a degree of repetitiveness with tall buildings, but The Spiral has no repetition at all. Our challenge as engineers was to deal with all that complexity and make it financially viable.

Jeffrey Smilow WSP

Jeffrey Smilow WSP

Enormous forces, tiny details

Super-tall towers are constantly buffeted by strong winds, so ensuring occupants are never aware of any movement is one of the greatest challenges of high-rise design. The spine of The Spiral is its reinforced concrete core, which is made from super-strong 12,000psi concrete. The outer structure is attached to this by huge steel trusses called “outriggers”. This dramatically increases the building’s stability and helps to manage the enormous, sometimes opposing horizontal forces generated by the sloping columns.

The steel connections holding all of this in place are just as geometrically complex as the structure itself. So rather than handing over to the contractor to produce the shop drawings that would be used to fabricate them, WSP’s engineers produced detailed computer models as part of the structural design. This integrated, collaborative process saved time on the project, and helped meet a challenging completion date required to secure the prime tenant.

Finding efficiencies in a wind tunnel

It takes several years to construct a super-tall tower – during which the exposed, partially completed structure must be able to endure all the same storms and seismic events that the completed building is designed to resist.

At The Spiral, the steel reinforcement within the core was assembled first, rising for 12 storeys before being encased in concrete. This temporary steel frame had to support the permanent steel below, as well as the weight of the construction deck and two tower cranes, all while being continually exposed to the highest wind loads.

Building codes focus on how completed structures will behave in the wind, and the effects on exposed structures are not well understood. To rectify this, WSP commissioned independent wind tunnel testing to find out how strong this temporary frame needed to be. In fact, testing revealed that the actual loads were significantly lower than standard assumptions – enabling a much more efficient structural design, and reducing the associated cost and carbon.