June 6, 2002

Unique design creates unique challenges

Photo courtesy of Skilling Ward Magnusson Barkshire The cone structure of the Museum of Glass, shown here under construction last year, was built as a series of columns around a compression ring, held in place by a temporary erection platform. The horizontal beams form the dramatic stepped ceiling of the museum's entry hall.

The dramatic design of Tacoma’s Museum of Glass epitomizes the city’s development goals of unification and rejuvenation. The facility provides a symbolic link to the past while serving as a physical link to the community, joining the downtown cultural district with the evolving Thea Foss Waterway.

Two prominent features of the museum -- the melding of the building into the landscape, and the stainless steel cone -- presented a unique set of structural challenges.

The roof

The rooftops of most structures are typically out of sight to the general public, with little or no attention paid to aesthetics. The roof was an integral piece of the museum’s composition, not only in form but in function.

The siting of the museum, located downhill from downtown Tacoma, meant that 90 percent of the viewing population would be looking at the museum from above. It was therefore essential that the "landscape" of the roof be as aesthetically appealing as any of the street views of the structure.

The rooftop also had to accommodate elevation transitions in a manner that was accessible, pedestrian friendly and inspiring -- incorporating reflecting ponds, a grand staircase, ramps, pavers and outdoor glass sculptures. In essence, the rooftop of the structure could not look like a roof at all!

Site constraints

Many other factors added to the complexity of the structural design. The roof elevation of the building was predetermined, set by the connection of the Bridge of Glass to downtown. The lowest elevation was also predetermined, with excavation depths limited by a remediation cap used to contain the site’s contaminated soils.

These two parameters delineated the narrow vertical confines of the building envelope. The rooftop also had to accommodate loads up to four times that of a typical roof to address the weight of the reflecting ponds, public assembly areas and pavers. Finally, since height limitations eliminated the possibility of separate mechanical, electrical and plumbing "zones," the structure had to totally integrate with those systems.

Grand gallery and staircase

The museum's long-span, high-ceiling gallery was placed directly below the 8-inch-deep reflecting pond -- structurally, a less-than-ideal location. While long spans often dictate deeper beam depths, particularly in heavily loaded locations, it was critical to minimize this depth. Closer beam spacing provided the needed strength in a shallower structure.

Another important consideration was uniformity of beam deflection, the degree to which beams sag under weight. Disparity in deflection across the structure, while acceptable by code, could result in leakage from the reflecting ponds into the gallery space below. To avoid this, the structure was carefully analyzed to "smooth out" deflections from one area to the next.

An interesting challenge was provided by the grand staircase, which gracefully wraps around the cone and ascends the site while sloping in two directions. This bidirectional slope meant that each beam framing out from the cone -- with spans of up to 60 feet -- had to be set at a different elevation as the structure responded to changes in grade.

A 3D model of the framing in this area was developed to assist the team with visualization, as well as determine top-of-steel elevations. Visitors can observe this complex geometry in the stepped ceiling of the grand hall, with its stunning and expressive volume.

The elaborate maze of mechanical, electrical and plumbing penetrations required close design team coordination. Structural integrity was a key consideration as the various systems were woven and threaded through a pathway of beam openings. In some areas, beam depths were increased not for strength, but to accommodate the large HVAC ductwork needed to move high volumes of gallery air without excessive noise.

Hot shop evolves

The museum’s prominent hot-shop cone, its design influenced by wood-burner structures observed throughout the Northwest timber industry in the early 1900s, evolved through several structural design concepts.

The cone was originally conceived as a glu-laminated timber shell with an internal steel cable-and-strut bracing system. That design was discarded in favor of a steel structure for various reasons, among them the feasibility of a wood structure surrounding an active hot shop.

A number of steel concepts were then studied, including a rolled-steel plate structure (similar to and fabricated like a silo or water tank); a shingled, rolled-steel plate shell with internal steel ribs; a system of columns with curved exterior members; and both tilted and nontilted cone configurations. The system that was selected most closely met the aesthetic, budget and constructability goals.

The 90-foot-tall, 90-foot-diameter cone is composed of vertical steel tubes, horizontal steel purlins and spiral pipe bracing built as a faceted structure, with a stainless steel cladding system forming the exterior curved surface. The thrust of the tilted vertical columns is resisted by a steel compression ring/wheel at the top of the cone and a concrete tension ring/wheel at the base. A 3D analytical model was created to predict the cone’s performance in an earthquake and to assist with detailing.

Erecting the cone

The cone structure was erected as a series of "trees." The trees, or full-height vertical columns with stubbed-out portions of the horizontal purlins, were leaned against a compression ring at the top that was held in place by a temporary erection platform.

Spacing of the cone’s horizontal and vertical structural members was dictated by the optimal span of the stainless steel cladding system. The diamond-patterned cladding pieces are bolted to the cone’s structural frame using light-gauge steel stud connectors. Applied in a swirling pattern on the tilted cone, each row of cladding shingles are by necessity a unique size and shape.

New Northwest jewel

As the grand opening draws closer, the final touches are falling into place. An amazing rooftop glass sculpture by Buster Simpson has just been installed, reflecting the promise of treasures inside.

Through a true group effort, Tacoma’s Museum of Glass embodies the excitement of art, the connection of community and the history of the Pacific Northwest.

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