In New York City, when a developer hires an architect to do a residential project, they all want the same thing: to maximize floor area. Wringing the most out of a Manhattan lot, however, has been complicated ever since the sepia-toned year of 1911, when zoning laws were passed that imposed setback requirements and restricted floor area ratios in an attempt to keep at least some daylight and air moving down to the city’s streets and sidewalks. No longer could developers multiply the dimensions of their sites ad infinitum into the wild blue yonder, or that’s what legislators thought.
The ever-wily Big Apple real estate players quickly found loopholes to the law, buying up so-called air rights of adjacent properties. By those means, while an entire city block couldn’t become a monolithic bulwark, at least someone in the vicinity could transform their lot into the definition of density. The apotheosis of that trend is One Madison, a new spindly glass condo tower on 23rd street at the base of Madison Avenue. Designed by CetraRuddy with structural engineering courtesy of WSP Cantor Seinuk, the building rises as a virtually unbroken column from its modest 59-by-58-foot site to an ultimate height of 621 feet. With 3,300 square feet per floor, and encompassing a total of 181,400 square feet, the tower features a daunting 12 to 1 aspect ratio. It stands, in other words, at the very upper limit of slenderness.
Designing a building to fit these measurements presented the team with a unique set of challenges and opportunities. The typical conditions for a mid-block site such as One Madison’s would mandate party walls on the lot-lines. These solid, windowless faces would become the obvious place for structural shear walls, leaving the rest of the interior, with the exception of the core framing, open for programmable space. Views would then only be possible through the front wall to the street, and through the rear wall to the back of the lot. Here, however, above the 5th floor, the necessity for party walls was negated by the fact that the developers had purchased all surrounding air rights. One Madison, in perpetuity, would stand alone, presenting the possibility of taking advantage of impressive 360-degree views, including Madison Square Park.
With this goal in mind, Cantor Seinuk got busy working out a structural plan that would pull the building’s bracing in from the exterior. What they came back with was a cruciform system of shear walls that slices through the central axes of the plan, clearing up the corners of the tower. With that in place, only five additional columns were needed to manage the loads—four of them exposed and one buried within a wall—creating a vast amount of open space and allowing the perimeter to be dedicated to views.
Courtesy David Sundberg/ESTO and Cetra/Ruddy [click to enlarge.]
In addition to the building’s 360-degree views, CetraRuddy designed five-floor volumes that step out from the northeast corner of the central column of the tower, creating wraparound terraces for certain units—an added perk for the condo buyer. These volumes created 9-foot cantilevers off of the columns and shear walls. Canto Seinuk’s structural design manages these without load transfers, meaning that there is a singular load path throughout the entire height of the tower and identical structural profiles on each floor, allowing the team to optimize this profile for the most ideal interior layouts.
The structure itself is cast-in-place reinforced concrete. Concrete’s inherent fire-resistant properties mean that profiles can be kept thin, as extra fire proofing isn’t required, thus maximizing usable floor space.
It is also extremely rigid, a quality valued in residential high-rise construction where sway is a major concern for tenants who don’t want to see water sloshing about in the bath or toilet. In spite of the material’s inflexibility, however, One Madison’s extreme height and slenderness meant that a significant amount of sway would be a factor that had to be mitigated. Cantor Seinuk’s solution was as elegantly simple as it was unusual for residential construction: they designed a liquid tuned mass damper for the top of the structure.
The liquid tuned mass damper works much in the same way as other types of mass dampers, such as the famous giant metal ball atop Taipei 101 that moves on hydraulic pistons against the direction the building is being pushed by wind or global motion as a counter balance that centers the building’s gravity. One Madison’s damper is simply three large concrete tanks of water sitting on risers. As with the metal ball, the water sloshes about in directions opposite to those in which other forces push the building, keeping the tower upright and everything nice and steady inside.