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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

Replacing the Black Box

By T.R. Witcher

Innovative structural ideas will help open up Seattle's new convention center.

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The planned addition to the Washington State Convention Center makes a virtue out of its urban site by arranging program vertically rather than horizontally. Image Courtesy of LMN Architects

January 5, 2016—Convention centers across the United States are in the midst of a slow transformation from places that encourage isolation from the life of their cities to venues that encourage dynamic interactions with the local community.

Perhaps no project captures this change better than a planned $1.4-billion addition to the Washington State Convention Center in Seattle. The project will not only occupy a full city block just to the northeast of the existing convention center, but will also include two adjacent parcels, which will be developed as an office tower and a residential tower. Because of the tight urban site, the convention center's underground loading dock will stretch across all of the developed parcels so the entrance ramp to the loading level will actually be built inside one of the towers.

The new convention center will offer roughly 1.23 million sq ft of space. In addition to back-of-house space, the structure will feature 150,000 sq ft of contiguous exhibit space spread across one floor; a lobby and three levels of parking above that; 100,000 sq ft of flexible-use space covering one floor above that; 120,000 sq ft of meeting rooms on two levels above that; and a 60,000 sq ft ballroom at the top of the structure—the largest on the West Coast.

The building's interior has been planned with adaptability in mind. The flexible exhibit hall will feature ceiling heights and column spans that are typical of such large-scale venues but the hall will have higher-quality wall, floor, and ceiling finishes than typical convention halls so that it can be used to host banquets and parties. The ballroom, meanwhile, will have the rigging necessary to host smaller exhibits. And the lobby will feature operable glass walls that will let conventioneers find each other—smartphones and tablets in hand—for temporary and impromptu meetings.

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The glassy facades of the planned Washington State Convention Center addition are meant to integrate the activity inside the building with the life of the city outside. Image Courtesy of LMN Architects

The convention center will feature a glassy facade that embraces the city rather than turn its back to it. "You're no longer inside a black box," says Rafael Viñoly-Menendez, AIA, LEED AP, a partner of the Seattle-based architecture firm LMN Architects. "You're really engaged with what's going on around the site."

Viñoly-Menendez says that the idea is for such spaces as the ballroom and flexible hall to be read from the exterior as "objects in space."

"Our basic approach is [that] the building, at the end of the day, is a fairly large facility, with primary spaces that you want to make easy for people to understand even before they enter building," he explains.

The interior functional spaces will be more opaque while interstitial spaces will be more transparent, thanks to large expanses of glass that will serve to connect conventioneers to the city, and residents to the activities at the center. "We're trying to make the building both capture some of the solid character of the pieces that need to be legible but [also] dematerialize the rest as much as we can," Viñoly-Menendez says.

This ambitious goal creates more than its share of challenges. For one, the flexible hall features 180 ft of column-free clear span, a difficult structural challenge because two floors of meeting spaces and a ballroom will be located above it. "It is unique to locate two levels of meeting rooms and a ballroom level above a 180-foot clear span," says Derek Beaman, P.E., S.E., the president of Seattle-based Magnusson Klemencic Associates and the principal in charge of the convention center project. "We've designed program stacks similar to that, but they've typically been supported on a more traditional 90 by 90 foot column grid. To maintain 180 foot clear below is very challenging."

So the engineers have devised an intriguing solution—what MKA calls "interstitial trusses." These extend from below the lower meeting room level up through the upper meeting room level to the ballroom, where their top chords are located. Spaced 60 ft apart on center, the trusses demand a great deal of architectural and structural collaboration so that they work in conjunction with the meeting room spaces and the occupants never know they are there.

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Customized trusses extend from below the lower meeting room level to the ballroom so that the exhibit spaces can feature large column-free spaces. Image Courtesy of LMN Architects

But the top floor creates an added challenge. As a ballroom, it will occasionally feature dancing. When dancing occurs on a long span, the natural frequency of the structure can match or be very close to the frequency of the dancers themselves, creating resonances within those trusses. While always a challenge with ballroom floor design, Beaman says, the problem is exacerbated by the fact that the two meeting room levels below are directly tied to the ballroom level via the interstitial trusses, and events could be held concurrently in these spaces.

MKA is still studying ways to limit the impact of vibrations coming from the ballroom, perhaps by making sure the structure is robust enough to minimize them or perhaps by using tuned mass dampers, as is sometimes done in stadiums to try to rein in the vibrations of a cheering crowd.

To meet the local seismic codes, the convention center addition will use buckling restrained braces—essentially a steel plate core with a concrete shell that is encased in another steel tube. Its advantage is that these braces can "take the same load in compression as they can in tension," says Thomas Meyer, S.E., a principal of MKA and the firm's project manager for the addition. "That makes them unique from a typical brace frame system that you would have a wide flange member or a tube with a different capacity in tension versus compression," he says.

That scheme creates a predictable and ductile system, Meyer adds. "We can use a larger response modification factor in the design, which essentially brings our earthquake loads down from what [they] would be if it was a shear wall system or a special concentric brace frame using conventional wide-flange or HSS [hollow structural section] shapes."

A conventional arrangement of bracing will support the more opaque spaces. At locations at which the facade is more transparent, the strategy will be to have the bracing seem more organic and less organized than in conventional system. "From one level to another these braces are dancing in their location on a given framing elevation," says Meyer. "It essentially activates the lateral system from an aesthetic standpoint. People looking in through transparent facade [will] experience these braces differently on different floors and from one end of the building to the other."

The greatest challenge, however, comes from the building's southeast corner, which will cantilever over both the express lanes of southbound Interstate and also slightly over the regular southbound lanes themselves. Putting a vertical column at the edge of that corner has proven too problematic: it would need to come straight down onto the median between the regular and express lanes, and due to a previous lowering of the express lanes, the median is, as Beaman puts it, "chock full of foundations already."

"To locate a column has proved very challenging, so a cantilevered solution will be implemented," Beaman says.

MKA has also considered a diagonal column, but currently the plan is to use another interstitial truss—this one 50 ft deep and located at the same level as the trusses supporting ballroom and meeting room levels. This truss would be anchored in the back by columns that can go down to the foundation.

When asked about the difference between the two plans, Meyer notes that if engineers employed a sloped column, they'd need something to keep it from falling over during construction, and in its permanent condition, it would put restraining forces on the lateral support system of the building. This in turn would place additional demands on the building's seismic systems. By contrast, Meyer says, "Vertically, the truss option is stable without engaging the lateral system."

The building should begin construction in spring 2017 and be complete in 2020. "We at MKA have been involved in a lot of convention center projects over the years," Beaman says. "This isn't the largest, but this is arguably the most complex and technically demanding that we have been associated with, which is tremendously exciting."

 

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