The Arizona Cardinals kicked off the NFL season in style last month, hosting division rival San Francisco 49ers in the franchise's state-of-the-art, 64,000-seat multipurpose stadium in Glendale.
The $355 million venue, designed by New York-based Peter Eisenman with HOK Sport, Kansas City, Mo., features a bevy of high-tech amenities, including a retractable roof, 88 luxury box suites, and an exterior skin composed of 10,000 insulated metal panels that are rounded to collectively form the shape of a barrel cactus.
But the stadium's most unique high-tech feature won't be visible to the average fan (or player) on Sundays. Beneath the lush-green all-grass gridiron surface is a complex, custom-engineered platform that allows the entire, 2.2-acre playing field to be wheeled right out the door—in less than an hour.
The retractable field is a first for the NFL and one of only four such systems in the world. (The other three are soccer fields: Ajax Arena in Amsterdam; AufSchalke Arena in Gelsenkirchen, Germany; and Sapporo Dome in Sapporo, Japan.)
The field will be wheeled indoors on gamedays and during occasional NCAA football contests. For the remaining 350 days of the year, it will bask under the desert sun on the south side of the stadium, where conditions for maintaining the natural grass are optimal. This approach ensures that the Cardinals will be playing on a lush, healthy playing surface on Sundays. It also permits the Arizona Sports and Tourism Authority, owner and operator of the facility, to host a wide array of events in the 1.7 million-sf venue, including conventions, concerts, monster truck rallies, and NCAA regional and Final Four basketball tournaments.
Designed by Minneapolis-based Uni-Systems under a contract from construction manager Hunt Construction, Indianapolis, the 234x403-foot, 9,450-ton retractable field rides on 542 15-inch-diameter wheels along 13 rail lines embedded in the concrete sub-floor. The system is powered by 76 one-horsepower motors that collectively output an average speed of 11.5 feet per minute.
The field structure includes all the components required for the natural grass field to thrive—irrigation, drainage, and an optimized turf system installed by CMX Sports Engineers, Phoenix.
To ensure that the field travels straight along the 741-foot-long path, Uni-Systems designed 360 three-inch-diameter guide rollers that hug the center rail, according to Alan Wilcox, PE, project mechanical engineer with Uni-Systems.
“These guide rollers are mounted in pairs on spring-loaded mechanisms that deflect away from the rail as the loads increase,” says Wilcox. “This deflection allows the field to absorb short-duration, potentially damaging loads produced by local deviations in the rail. They also distribute long-duration loads across multiple sets of rollers to effectively 'steer' the field back into place.” The guide rollers limit lateral movement to slightly more than 1/8 inch.
Wilcox says the design team took extensive measures to ensure that vibration within the structure was minimized. The team built four mock-ups that were tested by the Cardinal staff members. Testing led to several key adjustments to the system, including beefing up structural steel members and the composite concrete deck.
“It was a balancing act between increasing stiffness and the cost impact to do so, which was significant considering the size of the field,” says Wilcox.
As for the result, Wilcox says the Cardinals have already received the thumbs up from the players.
“After the opening pre-season game, Ben Roethlisberger and some of the Cardinals players said the field is one of the best playing surfaces they've been on,” says Wilcox. “They said it played fast, and no one mentioned vibration.”
For a detailed slideshow on the design and construction of the retractable field, visit: www.bdcnetwork.com/contents/pdfs/cards.pdf.