WHEN THE MUSEUM OF MODERN ART opened its expanded campus to the public last October, one part of the addition was exhibited to anyone who passed on the street, whether or not they planned a visit to the galleries. The Blade Stair, as the museum and architects Diller Scofidio + Renfro (DS+R) call it, hangs like a four-story sculpture within the new glazed atrium of MoMA’s west-end expansion, offering a tableau of museumgoers to all passersby. It is a design worthy of its surroundings; the entire 155,000-pound (or about 70-metric-ton) staircase is suspended from structural steel within the expansion’s sixth-floor ceiling via a 6-inch-wide vertical stainless-steel-clad wall that fully supports the stairs and landings, leaving the entire structure free of lateral bracing.
“In essence the stair is a facade element,” says Chris Andreacola, associate principal for DS+R, who worked in collaboration with Gensler on the museum’s expansion. “One of the motivations behind it and other parts of the facade, is the opening up to the street of MoMA, which they didn’t really have before.” Highlighting activity within the museum was a big motivating factor for the design, adds Andreacola, so the stair is arranged so that landings (and the people on them) are visible through the frameless glass facade. Inside, being able to take in daylight and the streetscape offers museum visitors a respite from their circuit through the galleries. The expansion added more than 40,000 square feet of gallery space that allows for increased flexibility in the type of exhibitions the museum can host (for example, a new double-height studio will host live programming, film, and performances). New streetlevel galleries on the expanded ground floor are free and open to the public, a gesture by the museum to bring art closer to the streets of Midtown.
The expansion to the west of the existing museum features a stack of vertically interlocking galleries of varying heights. This volume required a new vertical core, creating a functional need for the stair and two adjacent elevators. The decision to create the most minimal stair structure possible was in line with the way the rest of the facade was designed on the West 53rd Street elevation, where a similarly floating custom entry canopy welcomes visitors into a double-height space where they can see through to 54th Street. “The canopy and that facade at the studio and gallery spaces just to the west of the blade stair are all hung,” says Andreacola. “A lot of that was about really leveraging tension and being able to hone things down minimally so it’s efficient to hang these things.”
The desire to avoid having to connect and dead-load a staircase into every floor of the expansion drove the Blade Stair’s spare form. After DS+R developed the initial idea to hang the stair with Brian Falconer, a principal with the project’s structural engineer, Severud, they entered into a design-assist relationship with Dante Tisi, a custom metal fabrication firm with offices in the U.S. and Argentina, who in turn worked with engineers at Eckersley O’Callaghan to further develop the Blade Stair concept. By hanging the stair from the sixth-floor ceiling structure, the design team was able to create a single 6-inch-wide wall on which to hang stairs and landings. “Instead of having bending elements that take loads at every floor, you have a single tensile element that allows that minimalism, and that distilling down of the stair to its really essential parts,” says Andreacola. “The design and engineering project was to keep eliminating elements rather than trying to manipulate elements.”
Esteban Erlich, a project manager with Dante Tisi, agrees that the most challenging aspect of the design was that it was suspended from the 6th floor and free-standing on three sides, but adds “the structural analysis of stairs has changed a lot in the last few years—the analysis of vibrations and frequencies done by Eckersly O’Callaghan was very sophisticated.”
“With these landings that are basically cantilevered off the blade, the challenge was how to deal with the sway of the bridge as a pendulum, if you will,” says Andreacola. Slotted connections through the stair runs limit the lateral movement of each landing. The runs are essentially beams that provide stiffening for the landing element that is close to the facade, and far away from any connection. Landings were lightened as much as possible to limit acceleration, and ultimately the stair’s distillation to its most essential parts—treads, risers, and handrails—gives it the appearance of floating in mid-air. “How many joints can you get rid of? How minimal can you make it?” asks Andreacola, adding that the biggest limitation was the maximum size possible for transport and installation. The blade itself was composed of 4-foot-by-4-foot assemblies; these were installed a stair run at a time then field-clad with finished stainless-steel panels. Within these, the Blade wall is comprised of 12-inch-by-4-inch steel tubes hung vertically and spaced 12 inches apart. Dante Tisi fabricated the stair treads and risers in structural stainless steel, then bolted these to either the face or back of each tube, creating the run of the stair. “They are like beams cantilevered off the tubes,” says Andreacola, “then the tread is added to stiffen it and provide lateral support.”
For an installation where every component is exposed, finishing details were crucial. “The most challenging aspect of the installation was that all the components fit very tightly together, with very little tolerance, both in the joints in the wall panels as well as in the hairline joints of the solid stainless steel steps,” says Erlich. The team realized that welding the 3⁄4-inch treads and risers created so much heat that it distorted the stair’s finish, so Dante Tisi mechanically fastened the treads and risers and then plugged the fasteners to create a smooth surface with crisp edges.
The stair’s glass handrail, which consists of a single piece of glass for each stair run, went through a similar paring-down process. “Since there’s a natural triangulation at a tread riser we were able to take advantage of that and get rid of unnecessary bulk,” says Andreacola. The triangulation allowed the laminated, low-iron glass balustrade, which has a heatstrengthened inoplast interlayer, to be clamped with three stainless-steel compression pins at every tread riser. Andreacola says that modern glass technology, improved from even a few years ago, lets designers add significant stress to certain points, allowing the pins to work with reduced clamping contact area.
Ric Scofidio, who co-founded DS+R with Elizabeth Diller, also added, or subtracted, one final detail to the stair: a chamfered edge on the nose of the Blade closest to the stair runs, creating an edge profile that is about an inch wide rather than the 6-inch width of the rest of the hanging wall. “If you go down the stairs that chamfer takes you around the landing to the next run of stairs,” says Andreacola.
Rather than clamorous, the experience of using the metal stair is remarkably serene. Finishes like white oak cladding on the stair treads and risers and microperforated gray birdseye maple help to create a sort of absorptive vessel, preventing sound from migrating into nearby galleries from the stair’s atrium space. Much like in the galleries themselves, “You don’t feel overwhelmed,” says Andreacola. “There are things to look at along the way. It doesn’t burden you with the fact that you’re going up a stair, as you would feel in an enclosed stair.” On any given day, he adds, “I don’t see many people waiting for the elevator.”