Engineering a solution

Achieving the tower’s double-cantilevered design was a literal balancing act: at the level of each sky garden, to open up expansive views, half of the perimeter columns are removed. The building essentially has the same challenges standing up as a tree with two gigantic notches chopped out of it,” says Dmitri Jajich, the structural engineering director for the project.

The team first proposed a system of trusses, all in heavy steel, to transfer the load from the perimeter to the building core. This would have been the strongest and lightest structural solution, but complex and time-consuming to build. In London, the most important financial consideration is not the cost of materials, but the time it takes to get a project built. “Our real triumph was not the engineering solution per se,” Jajich says, “but rather developing something that could be built without undue complexity and allowed the schedule to keep moving.”

Ultimately the team found a hybrid solution that balances strength, efficiency, and constructability. A structural steel truss rings the perimeter of the level above each garden level, while a system of post-tensioned concrete connects the truss back to the concrete building core. By using concrete for both the core and transfer system, it could be built by the same construction crew, using the same material — saving time and allowing the project to be completed on budget.

The post-tensioning strands used to reinforce the concrete transfer system are at the size and scale that might be used on urban infrastructure — much larger than anything typically used in residential buildings. “We’re not bridge designers, but we often have to design buildings that work more like bridges to get them built,” says Jajich. “When this challenge came up, we weren’t shy about tackling it.”

Another feat was optimizing the space around the robust structural framework on the two transfer floors. Usually, a building’s mechanical systems would be housed within these levels. Instead, the design team turned them into some of the building’s most distinctive living spaces.

“The steel members are exposed, and the concrete floors and walls were intentionally exposed to better explain the structure of the building to its occupants,” Design Partner Kent Jackson says. “It was an important way to bring out the character of the building and allow the residents to live within the structure.”