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June 7, 2001

A 'shimmering' canopy belies some steely calculation

  • The potential existed for three different structures to sway in three different directions at the same time, yet the common canopy connected to each of them had to remain standing.
  • By GARTH GLASCO
    Skilling Ward Magnusson Barkshire Inc.

    illustration
    Image courtesy of Skilling Ward Magnusson Barkshire
    The circular design of the arch allows the 8 inches of movement to occur in any direction, and a “keeper system” of overlapping steel plates around the rim of the baseplate prevents the arch from “uplifting” in heavy winds.

    When called upon to design a “shimmering block-long glass canopy” spanning Pike Street and uniting the development components of the convention center expansion, One Convention Place, and the original convention center, the task seemed simple enough.

    After all, as the structural engineers who designed the original convention center facility, which spans a 12-lane freeway and three city streets, it was likely that the right solution could be readily determined. But the design of the canopy proved more challenging and infinitely more interesting than originally anticipated.

    The challenge was presented in the fact that the 90-foot-wide canopy, known as the Pike Street cover, had to connect to and be supported off of three separate structures while addressing independent building movements caused by earthquake forces, wind forces and normal building variation. In other words, the potential existed for three different structures to sway in three different directions at the same time, yet the common canopy connected to each of them had to remain standing.

    Computer modeling was employed to predict how the three buildings move relative to each other when subjected to wind and seismic loads. As the structural designers for all of the development components, it was possible to analyze the computer models for the two new structures (the convention center expansion and One Convention Place) compared to the original convention center facility.

    The braced-frame design of the buildings provided increased building stiffness, which minimized movement. This analysis concluded that the new canopy had to be able to move 8 inches in all directions on a horizontal plane to avoid potential damage.

    Since the architectural characteristic of the canopy structure had already been established by LMN Architects, the project architect, the next step was to determine how that 8 inches of “play” could be designed into a safe canopy structure, while addressing all structural loads and providing the desired aesthetic appearance.

    It was also necessary to address wind uplift, as everyone who has ever walked along Pike Street on a windy day well knows. Through the development of a very complicated custom connection detail, a pre-tensioned tied-arch system evolved with a rigidly fixed connection on the north side of Pike Street and a “sliding” connection everywhere the arch touched on the south side of the street.

    Firmly attached to the new convention center expansion structure on the north, the nine steel built-up pre-tensioned arches, spaced 30 feet apart, span 90 feet to connect to a circular steel baseplate 4 inches to 6 inches in diameter and 1 1/2 inches thick. This baseplate rests on a Teflon and stainless steel sliding bearing plate assembly. Movement in the building on the north side of the street is mitigated by a sliding motion in the moving arch connections on the south. The circular design allows the 8 inches of movement to occur in any direction, and a “keeper system” of overlapping steel plates around the rim of the baseplate prevents the arch from “uplifting” in heavy winds (refer to diagram).

    The erection sequence for the arches presented its own set of challenges. The arches are composed of tension rods that converge beneath each arch at a “center medallion” (a 7/8-inch steel plate roughly four feet long and 2 1/2 feet tall).

    The arches had to be erected in a manner so that they had the capacity to resist wind and earthquake forces on the Pike Street cover, while the pre-tensioned rods had to stay in tension so that they would not become slack during assembly or after. All of this had to be accomplished on a canopy structure with one side that was allowed to move freely.

    This free movement also meant that the support rods for the arches had to be pre-tensioned in a manner that would allow the rods to lose some tension as the canopy moved, but not allow them to go into compression (or slack). Finally, the pre-tensioning could not distort the arches, as it was critical that LMN’s original geometry be maintained.

    Typically, rod tensioning is accomplished by pulling between two hard, fixed points. However, the sliding arch connection obviously prevented a typical tensioning approach. Intensive computer analysis was undertaken to experiment with a variety of pre-tensioning approaches: varying the sequencing of tensioning, applying different forces, etc.

    Some of approaches accomplished erection but allowed the center medallion to shift significantly and inconsistently from arch to arch. Others deformed the geometry of the arch beyond acceptable limits. Ultimately, it was determined that in order to obtain the needed amount of tension in the rods while maintaining the required geometry, the free end of the arch had to be pulled in by a distance of 16 inches, the rods installed, and the arch released. It was found that once released, constrained by the tensioned rods, the arches expanded a distance of just 1 inch.

    The resulting erection sequence required the 36-inch-deep built-up steel arches to be fabricated so that they were 15 inches wider than the desired end result, with a flatter top-of-arch geometry than the final position. These arches were brought to the construction site, where temporary construction cables pulled the ends of the arch toward each other (while flat on the ground) a distance of 16 inches.

    The rods were attached, then the temporary cable released, which allowed the “pre-tensioning” to occur as the arch tried to deflect back to its original geometry. This resulted in the desired uniformity from arch to arch once tensioned; in fact, the location of the center medallion varied by just two to three inches. The last step was for the contractor to erect the arches using temporary strongbacks.

    The arches are stabilized by a series of 18-inch-diameter pipes that run between each arch. These pipes support 12-inch-deep arched steel ribs that run parallel to the main arches and act as support for the glazing system. This design allowed the glazing to have to span only 15 feet, which eliminated the need for an additional structural mullion system. An additional series of 2-inch-diameter tensioned tie rods located in the plane of the glazing act as cross-bracing, preventing arch “rollover” and completing the arch support system.

    The pedestrian bridge under the Pike Street cover is enclosed by glass on either side, connecting to the overhead canopy using a specially designed structural mullion system. These glass “walls,” 60 feet high at their highest point, enclosed the pedestrian bridge so that it can be used for exhibit space.

    The next time you walk under the Pike Street cover, it’s likely you’ll pause for a minute to reflect on the process that went into developing this “shimmering” connection.


    Garth Glasco is a senior associate of Skilling Ward Magnusson Barkshire. He is structural project manager for the convention center expansion and Elliott Grand Hyatt hotel projects.


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