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New Atlanta Falcons Stadium Design More Than a Box With a Lid

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Image courtesy of 360 Architecture
Pinwheel-shaped kinetic roof of the next home for football's Atlanta Falcons is just one of many features that will redefine convertible stadium architecture. Cantilevered petals ride on tracks supported by 70-ft-deep box trusses and backspan trusses.
Video and animation courtesy of 360 Architecture
New Atlanta Stadium's Bill Johnson talks about convertible-stadium architecture.
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At last year's high-stakes interview for a lead architect to shape a $1.2-billion retractable-roof stadium for the National Football League's Atlanta Falcons, 360 Architecture's Bill Johnson staged quite a show. Figuring correctly that his firm's comparative inexperience and lack of local connections put it at the bottom of the short list for the 1.8-million-sq-ft New Atlanta Stadium, he decided to do something "over the top" to win the work.

"We were the dark horse. Our best chance was to challenge the status quo," says Johnson, a senior principal with the Kansas City, Mo.-based 360, formed in a 2004 merger of CDFM and Heinlein Schrock Stearns.

Johnson threw caution to the wind for the winner-take-all interview. During a 90-minute pitch on April 16, 2013, he and his team presented a "convertible" stadium with a radical new design approach; they also broke with tradition by brazenly engaging the selection panel in their presentation. The audience participation and stagecraft were somewhat out of character for Johnson, a rather understated Midwesterner.

First, 360 handed out props to the audience, which included Arthur Blank, Atlanta Falcons owner, and Rich McKay, Falcons president and CEO. There were decks of "what if" cards and small, transformable toys, each with a what-if label: "What if you could create a game-day experience that pulled fans away from the comfort of home? What if the scoreboards provided views of the game seen only at the stadium? What if there was a camera embedded in the tips of a football? What if select seats shook during a hard hit?"

Johnson

"All of us have some form of attention-deficit disorder, and when we got bored, we started reading the cards," says McKay. People laughed, he adds, when they came upon a wild card tucked into the 20-question deck. It asked, "What if you just hired us right now?"

That almost happened. By the end of the evening, 360 had jumped from fifth to first place, beating out EwingCole, HKS, Populous and TVS Design. Johnson got the good news the next day.

McKay explains, "The ball gimmick and the cards were all tied into [a bigger message] that we have to look at this stadium differently. That struck home."

The Falcons were especially impressed with 360's first interview slide: "Permission to Challenge Everything, Act Responsibly, Experts Outside the Industry, Embrace Change, Re-imagine the Game Day Experience, Reject Current Thinking."

"We've kept that as our go-back-to slide every time we have to make a decision on the project," says McKay. It even inspired the development's marketing slogans: "Re-imagine the Stadium Experience and Reject Current Thinking."

Blank had made it clear he wanted an architectural icon for Atlanta. Johnson, tired of a seating bowl in a box with a lid—one of which he had designed in the mid-1990s, while at Ellerbe Becket, for the Arizona Diamondbacks—jumped at the chance to introduce something fresh. "We are building the idea we brought to the interview," he says.

Johnson generated his description-defying design, which includes folded profiles and edgy shapes, by starting with the roof. At the interview, the 360 team, which also includes structural engineer Buro Happold Consulting Engineers PC, kinetic architecture consultant Hoberman Associates and mechanical-electrical-plumbing engineer WSP, unveiled a scale model with a motorized roof. "To get it done in time, we had five different 3D printers all over the country going all at once," says Erleen Hatfield, a BH principal.

Breaking interview convention, Johnson invited the audience to gather around the model. Chuck Hoberman, also the inventor of the transformable-toy handouts, then invited the Falcons' Blank to press the start button. The Falcons' McKay says the 360 team's jitters were palpable until the roof opened successfully.

Laying Foundations

Crews are just starting foundations at a state-owned site that abuts the 22-year-old Georgia Dome stadium, which will be razed after the new building opens. The new multipurpose venue, with 71,000 permanent seats, is set for completion by mid-2017.

The state-owned development is a public-private partnership, which includes $200 million from city bonds to be repaid by an existing hotel-motel tax. The Falcons Stadium Co., which holds design and construction contracts, will pay rent to the state.

The Falcons are getting an open-air stadium that can be climatized. When the roof is open, "we can turn off the air-conditioning," thanks to more than 30,000 sq ft of louvered panels and operable windows working with the 105,000-sq-ft roof opening, says Adam T. Kyle, a WSP senior associate. The roof likely will be open when the temperature is 45°F to 75°F. During cool nights, the opening will be used to precool the building. On hot days, the roof will stay closed to minimize heat gain. If rain threatens, the roof can stay open because the artificial-turf field has a drainage system.

The facility, as designed, has three main roof elevations. The lowest, like a skirt, consists of sloped, downward-pointing triangular panels that bend, like a rigid drape, to become facade panels. Near the base, the panels alternate with the operable-glass curtain wall.

The fixed roof steps up to a flat plane—in plan, a broad, eight-pointed star with a 385-ft x 310-ft center hole. The uppermost roof elevation is the operable portion. In the closed position, eight triangulated and bent "petals" form a segmented oval, like slices of a pie, that cap the opening. In plan, when the petals retract over the fixed roof, they resemble a giant pinwheel.

All the panels are clad in ethylene tetrafluoroethylene (ETFE), a lightweight, fluorine-based plastic that has high corrosion resistance and strength over a wide temperature range. ETFE can be transparent, like a window, or translucent.

After deciding to go for the job, Johnson approached Hatfield. "I said, 'This may sound crazy, we might not make the short list, but would you be willing to throw your hat in the ring?'" he recalls.

Hatfield

Her answer was yes. Hatfield, a 23-year veteran of structural engineering, had long dreamed about designing a retractable roof. Hoberman soon joined to advise about roof kinetics. "Chuck became a muse for this project," says Johnson. Hatfield adds, "He helped us get off the original idea of rotational movement, which is tough to execute on such a large scale."

BH principal Craig Schwitter, who, in 1999, opened the North America office of the London-based multidisciplinary engineer, soon suggested that an oval opening would better reflect the rectangular field.

Hatfield calls the circle-to-oval switch "Craig's complete disruption" because it further complicated the structure by reducing repetition and forcing a nonorthogonal grid. Variations in petal sizes meant unequal forces on the supporting trusses, which increased analysis time by 20% to 30%, says Hatfield.

To develop the geometry, Johnson credits as instrumental the use of a graphical algorithm software editor, called Grasshopper, in conjunction with 3D modeler Rhinoceros. Both digital tools were developed by Robert McNeel & Associates.

The roof structure grew from the architecture. The flat, fixed-roof structure, which will contain 10,100 tons of primary steel, is a two-way system with a criss-cross pattern, in plan, of primary, secondary, backspan and gutter box trusses. The four primaries, each 70 ft deep with a 12-ft-deep top chord, look, in plan, like a racked tick-tack-toe board (see drawing).

The primaries are planned as the main support for the fixed and movable roofs. Each spans 715 ft between reinforced-concrete, 179-ft-tall megacolumns, sited just outside the seating bowl. Megacolumn plan dimensions vary from 21 ft x 19 ft to 29 ft x 14 ft.

The primaries will span diagonally across the corners of the field to frame the corners of the opening. They support the inner travel rails of four of the eight cantilevered petals near each petal's back section, or heel. They also support the 58-ft-tall video halo.

Four 70-ft-deep secondary box trusses, also with a 12-ft-deep top chord, will complete the framing of the opening and provide support for the other four petals' inner rails. Each secondary truss will span from a primary truss to its own megacolumn (see drawing).

Each petal is framed by three main trusses, which taper from 30 ft deep at the heel to 4 ft at the tip, to shape the roof slope. The petals are from 196 ft to 232 ft long and 128 ft to 160 ft wide. They cantilever 156 ft to 192 ft from the primaries, which they overlap by 40 ft. The cantilever puts a downward force on the inner travel rail and an upward force on the outer rail, which is supported by a backspan truss. Each backspan truss spans from a primary truss to yet another megacolumn.

The petals appear to open like a camera shutter. By moving together on the diagonal, they create an illusion of circular motion. The travel mechanism, thanks to the synchronization controls, is the most complicated and most expensive—by 20% or 25%—of his six completed operable roofs, says Bart Riberich, president and owner of mechanization consultant Uni-Systems Engineering Inc. (see sidebar here). A major technical challenge is coordinating the deflections of the fixed and movable roofs, thanks to the long and broad spans, adds Mark Silvera, Uni-Systems' vice president.

Track lengths vary with petal width from 225 ft to 375 ft. Each petal will bear on eight, two-wheel bogies along the inner rail (see drawing, p. 45). The uplift on the outer rail will be resisted by six roller assemblies.

 

The level track, a modest wind profile and an abundance of downward load on inner-rail wheels allow for the most simple and inexpensive petal drive option: twelve 7.5-hp traction drive wheels, says Uni-Systems.

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