Second Skin

The facade of the recently opened Liverpool Department Store in the Mexico City suburb of Interlomas gives away very little of the functional building inside. Beneath its glowing metal cocoon lies a conventional three-floor concrete structure resting on a three-story parking deck. The edgy exterior is a departure for the Liverpool chain, with its slew of 75 stores in the area. Established in the mid-19th century, the store originally specialized in European fashion shipped through the port of Liverpool. The conservative retailer was ready to “break the mold” and establish a new direction for its store designs according to Gerardo Salinas, partner at Mexico City–based Rojkind Arquitectos.

The clients not only wanted a fresh face for the 325,000-square-foot store, they needed it quickly—it had to be designed and built in only nine months. Adding to the challenge, the footprint of the building and the form of the parking garage on which the store would rest were preordained. The semicircular site is hemmed in on three sides by a lacework of freeways. “There is no way to reach the site by foot, so the facade will be viewed mostly by car,” Salinas says. And since the client’s program precluded window openings, the architects needed a bold gesture to give the exterior a kinetic energy.

A pair of folded hands with interlacing fingers inspired the form of the facade. The idea is realized in bands of undulating horizontal ridges and valleys, creating a swirling skirt of stainless steel. The form is at once easy to comprehend and complex in its articulation. The visual impact of the building is in the subtle play of light and shadow across the folds, an effect enhanced by the surface treatments of the finish material.

Early in the design phase, the architects decided they wanted a metal skin, but they needed a broader understanding of the potential surface treatments and the required details. So the facade fabricator, A. Zahner, of Kansas City, Missouri, built a full-scale mockup of an approximately 20-foot square section of the wall. The team looked at several options, such as aluminum and zinc, but eventually settled on stainless steel. The 316 stainless steel alloy contains molybdenum, which makes it resistant to corrosion and discoloration from acid rain and air pollution. “The client was willing to pay a bit more for a material that’s almost maintenance-free,” says Salinas.

The steel panel system sits above three levels of parking that are surrounded by black-anodized expanded aluminum mesh. While the stainless wrapper acts as a rainscreen, a concrete-block wall assembly behind it performs as the thermal envelope, freeing the team to play up the expressive qualities of the skin. The architects selected two surface treatments to create a subtle variation: a brushed finish with a delicate random brushstroke pattern and a bead-blast finish which they arranged in separate bands to increase the sense of depth of the facade. A third treatment—small raised ovals—swims across the facade like a school of fish.

These treatments were selected, in part, to help mitigate glare. “We decided in the beginning we were not going to use a mirror finish,” says Salinas. He points to a much-publicized glitch with another stainless-steel-clad building—Frank Gehry’s 2003 Walt Disney Concert Hall in Los Angeles—where drivers and nearby residents complained about visually distracting conditions and heat produced by reflective concave surfaces. Fortunately, the mirrored panels only made up about two percent of the skin on Gehry’s building and the issue was addressed by resurfacing the finish with electric sanders to reduce their reflectance. But the hoopla over the incident alerted design professionals to the issue.

For the Liverpool project, the team was also wary of the condition known as “oil canning”—a waviness or surface deformation sometimes visible in sheet metal cladding. Generally, surface distortion in sheet materials becomes more pronounced as they get thinner and more flexible. “All metal panels will oil can to some extent,” says Zahner’s director of engineering Paul Martin. “The width of panel, geometry and thickness are the main contributors, he says. To control distortion in the department store’s cladding, Zahner used 16-gauge (.063”) stainless steel backed up by aluminum sheathing and limited panel width to two feet. The panels also have inverted seams, which turn inward at the edges and serve stiffeners.

Since the Liverpool store was an international project, the construction documentation and construction phases had a different trajectory than is typical in the United States. There were few construction drawings and no conventional shop drawings. The fabricator generated installation drawings but performed almost all of its shop work digitally, which helped the team meet the project’s tight time line, according to Martin. “We were doing everything in 3-D, including unfolding the form and generating the patterning and surface finishes,” he says.

The eccentric form of the skin, with its curving plan and continuously changing section, meant that none of the 7,500 panels were identical. To process this complexity, the fabricators started with their own digital modeling of the skin’s geometry. “We took the form from Rojkind’s model, sliced it up, and generated all the different parts,” says Martin, adding that each panel had as many as 50 components. Zahner fabricated the elements with CNC (computer numerical control) machines which rely on computers to direct cutting, bending, and surfacing.

Zahner usually installs the facades it fabricates. In this case, the client, acting as general contractor, hired its own Mexico City–based crew. To insure that installation went smoothly, the crew’s supervisor rehearsed the assembly process several times on a mock-up at Zahner’s Kansas City plant. A Zahner employee was also on site in Mexico City during much of the construction process.

As the panels rolled out of the fabrication plant, sections were shipped to Mexico in sequence for assembly and erection. The installers put together every other bay of the 1.7-ton truss assemblies, including skin, sheathing, and aluminum trusses, at an off-site facility. The skin assemblies were attached to a tubular steel “translation structure” which spanned the gap between the concrete frame and the skin. Once the fabricated bays were put in place on the building, the components for the missing bays were filled in on-site.

According to Salinas, the attention to detail and the advance planning paid off. Only 14 panels needed to be manufactured a second time due to fabrication or installation problems. Instead of being discarded, the rejected components were installed inside the building.

Like the Liverpool Department Store’s skin, the new facade of the Edith Green/Wendell Wyatt Federal Building in Portland, Oregon, encases a conventional structure in an embellished exterior. But instead of the solid armor of the Mexican store, the Portland project has a metallic veil. The facade, now almost completely installed, is part of a $133 million renovation, slated for completion in 2014. The overhaul is expected to earn the 18-story, 1976 precast-concrete office tower a LEED Platinum certification and help it comply with current security standards.

The revamp, designed by a pair of Pacific Northwest firms—Portland-based SERA Architects and Bainbridge Island, Washington–based Cutler Anderson Architects—encases the structure in a high-performance double-glazed glass curtain wall that is both energy-efficient and blast-resistant. The scheme also includes a shading screen for the building’s west elevation. By adding the screen, the project team was able to significantly reduce solar gain and maintain occupant comfort with radiant heating and cooling. Without the shading device, the building would have required more cooling capacity and a much less efficient VAV (variable air volume) mechanical system, according to SERA associate principal Lisa Petterson.

The client, the General Services Administration (GSA), rejected an early scheme for a living wall that included climbing vines due to concerns about maintenance and the two-year time frame required for the plants to grow to full shading capacity. However, James Cutler, Cutler Anderson principal, wanted to maintain the screen’s organic look. In collaboration with cladding fabricator Benson Industries, he developed a system of six bowed arrays made up of extruded aluminum reedlike members. The reeds vary in size and are combined in a manner that allows the screens to be fabricated readily but lends them a slightly random quality.

The team relied on BIM (building information modeling) to get all the details worked out quickly. “It was critical to be able to see the design in 3-D and in context with the other conditions,” says SERA’s Gauri Rajbaidya. The modeling software allowed both the architects and the fabricators to articulate the design and refine it swiftly.

The west elevation’s screen responds to the low-angle sun and is devised to shade 50 percent of the glazing at peak solar exposure, according to Petterson. “It needed to be a vertical scenario, but the south and east facades—which are actually oriented southeast and southwest—required a combination of horizontal- and vertical-shading systems.” The result is that each facade is articulated in response to its specific orientation. For the south and east elevations, the reeds were adapted to create vertical finlike screens that bracket either side of horizontal light shelves. The 2-foot-deep light shelves shade the lower portion of the windows and reflect daylight into the building through upper windows.

SERA worked with the University of Oregon’s Energy Studies in Buildings Laboratory to analyze the shading and daylighting system. The architects tested several facade configurations in an artificial sky—a chamber that simulates overcast skies—to assess daylighting levels. The team also placed a partial-facade mock-up on a rotating table called a heliodon, which replicates sun angles at a given time of year. The data allowed the designers to fine-tune the screen and the light shelf systems.

The tubular reeds would have been 280 feet tall if they were continuous. But because aluminum has a relatively high coefficient of thermal expansion (a metric that describes how materials respond to temperature changes), designers needed to develop an assembly that would allow the reeds to expand and contract. They divided the tubes into roughly 30-foot-long sections that span between horizontal supports spaced every two floors or 25 feet. Each reed cantilevers several feet above and below the supports, creating a rhythmic pattern. “We spent so much time on these joints because you see them when you look out the window,” explains Cutler. “They’re right in your face.”

The reeds are trapezoidal in plan, splayed so that the narrower face is on the interior. Besides meeting the shading needs, the splay is meant to reduce the feeling of enclosure. The tubes, which vary in depth from 3 to 5 inches, also have a slightly “pillowed” front face and rounded corners. Sharp corners would have created sharp shadows and emphasized the tubes’ barlike nature, explains Petterson.

As with the Liverpool Department Store, it’s the chorus of thoughtful details that make the facade sing. Both projects illustrate how the technology of modeling programs, BIM, and computer driven fabrication have broadened possibilities for the architect. They also demonstrate the value of collaboration between architects and fabricators in realizing complex design concepts.

Michael Cockram is a freelance writer, educator, and sustainable design consultant living in Fayetteville, Arkansas.

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