The building was only able to touch the rock below in two thin strips that do not even span the whole site. Otherwise, it lands on the New York’s Metropolitan Transport Authority facility in predefined locations with predefined loads. The problem was that those predefined points were based on a completely different design. The previous plans for the site, for which WSP was also the structural engineer, envisaged a steel-framed tower with a diagrid structure. Now they had to work out how to land a concrete building with a completely different form in exactly the same way.
“Generally speaking, if you compare a typical concrete office structure to a steel-framed building you’re dealing with roughly twice the load,” explains Smilow. “But we had to match the steel building’s loads, and in the right locations.”
Engineering Tricks to Reduce the Weight of the Concrete Structure
WSP used several tricks to reduce the weight of the concrete, creating a structure that is light enough to meet strict load requirements while also being very strong. They specified a 12,000psi high- strength mix, using lightweight aggregate, and the floor slabs were post-tensioned – a common technique in other markets but believed to be the first major use in New York.
At the very top of the building, a system of outriggers links the core to perimeter columns spaced 30ft apart. All of this allows the floor slabs to be very thin – just 9 inches – even with spans of 40ft and 35ft off the core. The post-tensioned system was carefully planned to leave free areas so that interconnecting stairs could be added anywhere throughout the floor plate. As Smilow explains: “Every engineering decision that we made had to keep in mind flexibility for tenants.”