55 Hudson Yards, New York

Co-developers Related and Oxford Properties Group wanted column-free corners, a 10-foot clear ceiling and no penetration on the core apart from the openings to the elevators. “All of these things allowed for much better planning and much more efficient layouts for our tenants, but they compounded the challenges facing the engineers,” says Andrew Cantor at Related. “We wanted to have large openings for future stairs as well as setbacks for double-height terraces throughout.”

They also wanted the building to be constructed out of concrete – unusual for a commercial building in New York. “But that was the easy part,” says Jeffrey Smilow, executive vice president of building structures at WSP in New York. “The difficult part was what lies underground. Of all the projects we’ve done over the last 30 years, this has been the most complex in terms of integration with infrastructure and complexity below-grade.”

55 Hudson Yards is a 40-storey tower set back on a nine-storey podium, which incorporates a six- storey ventilation shaft building for the subway. One of the biggest challenges was how to weave the support structure for all of this through the substantial subway escalators. The tower is mainly supported on 10 caissons, 100ft deep with diameters of up to 6ft, which had to be driven very accurately.

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.”