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Constructing a Convertible

The latest retractable roof technology makes desert sun a home field advantage.

December 01, 2006 |

When the National Football League's Arizona Cardinals decided to build a new stadium in 2000, the desert climate was a major consideration of the design plan. The Cardinals had difficulty drawing fans to Sun Devil Stadium in Tempe, especially early in the season, because of the torturous heat that often exceeded 100 degrees F. The heat was even rougher on the players, not to mention the field's playing surface.

In the cooler months, however, the climate is perfect for outdoor events, and not just football. A new stadium could take advantage of the warm winters to hold open-air concerts and conventions.

The Cardinals found a way to shield fans and players from the heat early in the season and open the place up later, thanks to an innovative roof design. Completed in August, the $455 million University of Phoenix Stadium in Glendale, Ariz., is the NFL's second retractable roof facility.

The exterior design of the stadium resembles a barrel cactus as envisioned by design architect Peter Eisenman, along with facility architect HOK+Sport, Kansas City, Mo. The retractable roof is made from Teflon-coated fiberglass sheer-fill fabric from BirdAir that allows 12% light transmission into the stadium.

Operating on an incline

Two gigantic Brunel trusses (opposite) hold up the retractable roof at University of Phoenix Stadium in Glendale, Ariz. A Teflon-coated fiberglass fabric allows light to enter the stadium even when the roof is closed.

HOK+Sport designed the first retractable roof on Reliant Stadium in Houston; this is its second such project with structural engineer Walter P Moore. “These roofs have a long history of relying on principles of basic engineering, but the more we do them the more comfortable we get in changing what they look and operate like,” said Dennis Wellner, a founding principal of HOK+Sport and project principal for the University of Phoenix Stadium. “That was the fun part.”

Eisenman's slightly sloped roof design allowed the engineers and architects to take advantage of gravity to help the stadium's ABB ACS800 drives move two 185x285-foot sections of the roof. The two 550-ton panels are suspended between two parallel tracks along the east and west sides of the structure. The tracks are curved to follow the roof's profile, which slopes down from the 50-yard line toward the north and south ends of the building. Each roof panel rests on eight two-wheeled carriers (four along the west side of the roof panel, four along the east). Each panel moves down an incline varying from zero to 14 degrees as it goes from the closed to open position along the curved top chords of the Brunel trusses that hold up the roof.

“It's the most technically advanced retractable roof we've made to date,” said Cyril Silberman, CEO of Uni-Systems, the Minneapolis-based mechanization contractor on the project. Silberman said standard techniques such as the powered traction wheels that Uni-Systems used on previous stadium projects were not an option for the sloped roof. If the powered wheels failed on a slope, the huge roof sections could end up in the parking lot.

Uni-Systems previously worked on retractable roofs at Miller Park in Milwaukee and Minute Maid Park and Reliant Stadium in Houston. The company is working on retractable roof plans for proposed NFL stadiums in Dallas and St. Louis.

Raise high the roof beams

To construct the first inclined retractable roof in the U.S., the Building Team used new construction techniques. The two 700-foot-long, 87-foot-deep Brunel trusses that support the roof sit atop four 171-foot-tall, 17.5-foot-wide supercolumns. Because the lenticular span of the trusses is so long, the fabricator-erecter, Phoenix-based Schuff Steel, came up with the idea of building the trusses on the ground and then raising them to the tops of the four supercolumns once they were completed. This sped up their construction and minimized safety and design concerns.

“Most long spans like this are erected in place on shoring towers,” said Mark Waggoner, senior associate with Walter P Moore. “Had we constructed it that way, the design would have been completely different. We wanted to make sure our steel structure was harmonious with the idea of letting in lots of light, and the orthogonal structure we got from building it that way lets all that light and sun in.”

Waggoner said the spans aren't really even traditional trusses, but rather a combination of a compression arch and steel cable support, commonly used in bridge design.

The stadium's 90-foot trusses were constructed on the ground and lifted into place last February.

“Even though they're 90 feet deep, there really is little sense that the trusses are that deep or massive,” Wellner said. “The interior space is really defined by the underside of the roof, rather than being encumbered by these huge trusses that glide through it.”

Many members of the Building Team credit the modified design-build process that was used on the stadium for allowing them to work more directly with general contractor Hunt Construction Group, Scottsdale, and the project's sub-contractors. This allowed them to devise innovative concepts that have helped University of Phoenix Stadium draw rave reviews.

“It had a number of advantages,” Waggoner said. “Small things, like we didn't have so many volume RFIs to deal with, really opened things up to a more collaborative effort and really put eve-ryone on the same side.”

The Cardinals and the stadium's owner, Arizona Sports and Tourism Authority, have already scheduled late-season events (suitable for opening the roof), such as the BCS National Championship game January 8, 2007. The stadium has also been named one of the Top Ten Stadiums in the World by Business Week.

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