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.”

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.