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.