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Boston Society of Architects Stair Project

Posted by Daniel Couturier on Mon, Oct 08, 2012

by Lian Chikako Chang / July 13, 2012

In September 2010, the BSA Atlantic Wharf Designer Selection Panel began meeting to determine what type of process should be used in selecting an architect for what would become the new BSA headquarters. Various ideas were debated, but, ultimately, the panel members agreed on a two-stage design competition. The first stage was an ideas competition. Entrants anonymously submitted one 24″ x 36″ board describing in plan, section, elevation, 3-D rendering and text a vision for the new headquarters. In the second stage, the panel made a shortlist of entrants, held in-depth interviews with each and awarded the number-one entrant the design contract. The notion of an ideas competition was central to the panel because, as the competition call for entries stated, "One important design challenge centers around making a connection between the first-floor concierge space and second-floor office space. The first-floor concierge space should engage passersby and have the ability to draw them up into the second floor office/arts space." Although all the entrants addressed the first-floor/second-floor design challenge in unique ways, it was ultimately the vision of Höweler + Yoon Architecture that carried the day. As the following article by Lian Chikako Chang points out, overall, the process the BSA engaged in—which culminated in a new headquarters that has as its centerpiece a 13-ton three-quarter-inch green steel plate stair—was one that encouraged both a free flow of ideas and a collaborative approach toward resolving the many design, bidding and construction challenges the team faced. After many years of providing architectural services to public-sector clients and working within that sector's associated constraints, it was refreshing to work with a client whose insistence throughout was one of total team collaboration. This project or, more specifically, this model of collaboration should be held out proudly as the standard for an example of how the design and construction process should work. –John Nunnari, BSA owners project manager; executive director, AIA Massachusetts.

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Couturier Iron Craft has recently completed this unique stairway project in Boston Massachusetts for the Boston Society of Architects Headquarters, known as BSA Atlantic Wharf. This staircase project was designed to give the appearance of an open air staircase hanging from the second floor ceiling using 13 tons of 3/4″ thick steel plate. Read the entire below as to the design, engineering & fabrication of this project.

BSA Space reveals itself in stages. The first thing to catch your eye from Congress Street will probably be a flash of green stretching up to the second floor. As you approach the entrance, the green comes into focus as a stair—bold yet immaterial, as it looked in the architect's original renderings submitted in the competition to design BSA Space. On ascending the stair, however, there is no mistaking the three-quarter-inch-thick folded and painted steel as anything but solid. Hard under your feet, the stair is shiny, slightly rough and massive as it folds up into a green wall that is continuous with the second-floor ceiling, also green. Finally, once you're immersed in the second-floor exhibits and meeting spaces, this green ceiling becomes an ambient marker—less graphic and more conceptual—anchoring BSA Space through this entrance stair.

How do you make a minimalist stair that weighs 13 tons, an emphatically material object that hides its details in plain sight, appear as a single surface? To find out, I spoke with architect Eric Höweler AIA from Höweler + Yoon Architecture, structural engineer Patrick McCafferty from Arup USA (Boston), project manager Jason Smith at Commodore Builders and fabricator Tom Couturier from Couturier Iron Craft.

Although the BSA stair has echoes of other single-surface stairs, including one by Carlo Scarpa in Castelvecchio, and another by Diller Scofidio + Renfro—where Höweler previously practiced—at the Juilliard School of Music, what differentiates this stair is the extension of the single surface across two stories. To invite visitors up to the second-floor BSA Space from the street, Höweler explains, "the soffit has to be the facade, and this facade also has to be the stair…. All three things are one gesture."

This one gesture is the result of several structural decisions. McCafferty, the project's engineer of record, explains that the stair is "hung from the second floor, bears lightly at the first floor…and cantilevers up to the soffit of the third floor." The connections at the second-floor plate are the heaviest but are painted white and recede from view. Because the connection at the soffit provides lateral restraint only, and the stair meets the ground with a grooved connection that allows for axial displacement, any "differential deflections between the third and second floor do not impose undue stress into the stair."

McCafferty reports that the biggest structural challenge was "satisfying the vibration and acceleration limits of the stair under footfall traffic," and that Arup used its own finite-element-analysis software to study the stair's dynamic response to loads. As a video of this analysis shows, stresses move through the stair's uniform three-quarter-inch steel in decidedly nonuniform ways. To address this, structure is hidden in elements that serve not double, but triple duty: the bolting surface between the upper wall and the stair turns up to support a handrail, with this extra material also serving as a beam. "Usually you'd try to get the engineer to minimize the depth of the stringer," Höweler jokes, "but in this case, we said, 'You can have two stories of stringer on one side.'"

Patrick McCafferty: "Arup's in-house finite element software was used to analyze the dynamic response of the stair in order to fine-tune the structural design and detailing requirements." This video, courtesy Arup Boston, shows the stress contours as load is applied to the stair.

The minimalist design was also challenged in terms of meeting the code that requires the space between treads to reject a four-inch-diameter ball. Deflections meant that glass risers could not span from tread to tread but instead had to cantilever from a single edge. An alternate solution would have been to use perforated-metal sheet risers, which Höweler comments "everyone" had suggested. But the glass dematerialized more, so they worked out that detail to accommodate the "tremendous amount of moment" imparted on the joint when the risers are kicked. For Höweler, "If the stair seems sort of fetishized, I think it is…. We were hyperconscious of the fact that this is the BSA, and architects are much more critical than the general public…so there's a didactic quality to it."

For me, this didactic quality lies in the fact that the stair's clean expression is achieved by distracting attention from necessary secondary elements—having structural elements serve double duty as bolting surfaces and triple duty as handrail supports, by painting connections white, by insisting on the most transparent treatment for risers—instead of hiding them with additional materials. The stair may read as a single surface, but for those who examine it, its complexity is in full view.

To be in full view, everything had to be built according to exacting standards. As project manager Jason Smith recounts, "Everyone knew the stairs were going to be the crown jewel of the space, and the biggest challenge was finding the right contractor…. We went to just about every steel subcontractor in New England that we thought was capable. Being able to do it is one thing, but the issue was doing it on time and on budget." In the end, the team ended up talking with Couturier Iron Craft in Comstock Park, Michigan.

Tom Couturier told me that, as the fabricator, his main concern was to keep everything straight. "In a curved stairway, there are a lot of things that won't show. A straight stairway is easier to fabricate, but if you have any issues, it shows. We had to move around the piece when we were welding to minimize the concentration of heat. If we started to weld all in one spot, there could be a big bubble." This kind of deliberation from Couturier is what convinced the rest of the team that his firm could do the job within the six weeks that were allotted to go from shop drawings to mock-ups, fabrication, transportation and installation.

The most difficult stage of fabrication was figuring out how to make the bends between the side stringers and the stair treads; Couturier says that it was a compound pitch, and the radius had to be precise every time. "We ordered special dies to make that bend." Couturier's team also started with the flattest steel they could find by visiting their supplier to handpick material. The stair was welded on its side so it could be best braced, and, once it was welded, it was fully assembled at Couturier Iron Craft and then disassembled for shipping.

The final hurdle was getting the pieces into Atlantic Wharf and in place. Smith had to budget for "taking apart the side of the building to bring in the stair and putting it all back together again," and he commends Couturier for doing "an amazing job of planning the rigging" for the crane. Everyone was holding their breath as the first and largest piece was brought in. "We felt confident—there was a lot of planning—but there's always a 'What if?' The tolerances were very tight," says Smith. "If it was an inch longer, I don't think we could have done it," adds Couturier. But the installation actually "happened quicker than expected," according to Smith, and the project finished on time and on budget.

I asked Smith and Couturier what makes a project go so smoothly. Smith says that the most important thing is to encourage "a free flow of ideas" so that innovations could come from any team member. An early example of this occurred when Smith pushed to have the stair welded offsite and installed with bolts. Onsite welding would have been far costlier, with work limited to between 1:00 and 6:00 am, and less precise. As Höweler recalls, "My instinct was to insist" on welding onsite to reduce the number of bolts: "I was initially going to stamp my feet." But he allowed himself to be convinced. Like Smith, Couturier praised the rest of the team for fostering a collaborative process. "We all had input…to the point where we felt that this was our own project. The architect and the contractor created a sense of ownership, [so we could] take this on as if this were something going into our own business or our own home."

For an entrance stair for the Boston Society of Architects, whose new space is meant to be a home for exchange and collaboration between design and construction, and the profession and the public, this sounded just right.

Lian Chikako Chang has a PhD in history and theory of architecture from McGill University and is currently a MArch I student at Harvard University's Graduate School of Design. She loves tennis, gardening and food.

Tags: Project Highlights