Four-sided compatibility: Cutainwalls

August 11, 2010

Designers of an 11-story Seattle office building received an apparently contradictory message when they first discussed the project's requirements with owner Vulcan Inc. The client wanted a "progressive, innovative, leading-edge type of building" — but one that nevertheless would be a good neighbor to other buildings in the Chinatown International District in which it would be located, and to the renovated Victorian-style buildings of adjacent Pioneer Square.

"That had a lot to do with how the [building] form and the skin materials evolved," says John Savo, principal in charge with locally based project architect NBBJ. The reconciliation of Vulcan's goals resulted in a design that incorporates curtain wall elements in an unusual combination.

"The form is quite unique to the site," Savo says. Each side of the building is designed to be compatible with the neighborhood it faces. "While it is a contemporary building, it's something you wouldn't build anywhere else."

The designers narrowed the possible schemes to three. Two were "pretty conservative," says Joey Myers, NBBJ's lead project designer; the third was anything but. That's the one Vulcan chose.

Instead of employing a single exterior design vocabulary, the designers used three wall systems — glass, metal, and precast concrete. "Because of the aggressive approach to the form, we have walls that slope out, that slope in, and a curving curtain wall," says Savo. A full-height glazed north wall slopes inward at a 15-degree angle, and the west wall angles outward at four degrees.

The 15,500-sq.-ft. "waterfall wall" on the north elevation faces downtown Seattle. At the top, it transitions into a skylight. Glass in the four-sided silicone system is glazed to internal supports. The 11-story tall, uninterrupted glass wall is on a direct sight line from Second Avenue, a major route leading south from downtown. It also is a contemporary rendition of the lunette-shaped Great Window of Union Station, which it faces.

The west wall, glazed with vision and spandrel panels, is a parallelogram that slopes outward as it rises, extending to the property line at the top. This wall complements the scale and character of the large structures it faces, including Seahawks Stadium. Its spandrel areas have a solid frit coating on the No. 4 surface and a pattern of 1/8-in strips on the No. 2 surface, in order to create a shadow box effect. Its dynamic parallelogram shape was inspired by the streamlined locomotives seen once seen on the site, a former rail yard.

The east side of 505 Union Station faces older, mainly masonry buildings. Its precast concrete cladding reflects the neighborhood's pedestrian-oriented scale and massing. The wall's rectilinear expression is scaled to the proportions of neighboring historic buildings. Aggregates were selected to match the masonry colors of neighboring Union Station. Precast concrete also clads the south wall, the building's least visible elevation.

Corner conundrum

The intersection of the inwardly sloping north wall and the outwardly sloping west wall was the most complex detail of the curtain wall. To test the performance of this section, NBBJ created a model of a mock-up using a process normally reserved for making prototypes of manufactured articles. An outside supplier ran NBBJ's 3-D computer files through its stereolithography device, which directs laser beams at a pool of liquid resin to transform the resin into a solid model.

The building is glazed with green-tinted, low-e vision glass with a 60% light transmittance, and dark green fritted spandrel panels. The bottom two floors, including ground floor retail space, have Viracon-brand clear glass with a low-e coating. The laminated safety glass of the skylight (from Metco Systems) has a dot-matrix frit pattern with 8-33% coverage to provide sun shading.

Achieving the U values prescribed by Seattle's energy code tested 505 Union Station's designers, says Scott Johnson, lead technical designer with NBBJ. The north wall was a key element in determining the building's overall energy performance. Its four-sided silicone glazing system has no exposed aluminum members that would transmit heat into the building, and it is more energy-efficient than a thermally broken curtain wall. "Because of that, we were able to average out energy loss through glass on the remainder of the building," he says. The Seattle code has since been tightened, and 505 Union Station probably would not be able to meet its more stringent U values.

Floor sizes run an average 30,000 sq. ft., since the impact of inward and outward sloping walls tends to offset each other, according to Peter Pran, design principal with NBBJ.

Pre-construction meetings with Dallas-based Curtain Wall Design and Consulting Inc. and other consultants focused on resolving or simplifying the angles, both in plan and section, to make connections between systems and the complicated corners easier to engineer, Myers says.

The design of the curtain wall focused on the inwardly sloping north wall, the outwardly sloping west wall, and the corner where the two meet. These are features that attract views to and from the building and establish the dynamics between it and the urban fabric. Locations that would provide the best exterior views for building occupants also influenced the placement of large glass areas.

Terry Lundeen, a principal with structural engineer Coughlin Porter Lundeen, Seattle, says a major objective was to integrate the building structure and the curtain wall system to maximize the transparency of the north façade. Because this wall's slope continues down to the base of the building, a severe earthquake that damaged braced frames could cause the structure to move laterally. A moment frame within the building core is designed to counteract such a post-earthquake condition. Because 505 Union Station does not have a typical vertical/horizontal structural grid, gravity thrust forces must be taken out through the floors. "We needed special connections to make that happen on the north and west sides, which have sloped structural elements," Lundeen says.

The building rises from a two-level parking structure, constructed earlier, that 505 Union Station shares with three other buildings. Garage columns were placed on a grid that anticipated a uniform configuration for the buildings that would follow. But because 505 Union Station departs from that pattern, gravity loads on its west elevation were transferred 12 ft. inward to accommodate columns supporting its west wall.

Texas Wall Systems of Dallas engineered and fabricated all exterior elements except the four-sided system. It was "one of the top two or three" most challenging jobs the company has handled, says president Larry Long, and was focused on "making everything come together aesthetically, both outside and inside."

The unusual exterior wall design presented significant constructability challenges, according to Martin O'Leary, on-site project manager with Seattle-based Baugh Construction, now part of Skanska USA. The building's original construction schedule, which envisioned a typical rectilinear building, was about 12 months. Incorporation of the more complex wall systems added about three months to the pre-construction phase of the schedule. The shell and core were completed within the 15-month schedule.

O'Leary says the bulk of pre-design, design coordination, and shop drawing centered on the northwest corner of the building. Because of the curtain wall's numerous compound angles, 3-D modeling proved invaluable in enabling Building Team members to understand the complexity of the curtain wall, he says. It was used throughout the project — as a curtain wall bid document, for development of shop drawings for the offsite mockup, and for quality control.

Vulcan, which occupies the building's top five floors, oversees the business interests of Microsoft co-founder Paul Allen, who also developed the city's Frank Gehry-designed Experience Music Project and Seahawks Stadium.