The Cardinals Stadium roof will be the first roof in the U.S. to operate on an incline. Each panel must traverse an incline varying from zero to 14 degrees as it moves from the closed to open position along the curved top chords of the Brunel trusses. Because the impact of gravity becomes greater on the system's cables as the roof nears the completely open position, the speed of the roof slows from a rate of 6 inches per second to a rate of 3 inches per second. "This is so we will never have inertial problems and trouble stopping [the roof in case of emergency]," explains Cyril Silberman, C.E.O. of Uni-Systems, the project's mechanization consultant.

Thermal movement is also a source of concern for roof operation. To avoid binding or locking of the system as the 16 trusses supporting each panel expand in the Arizona sun, the system has a linear bearing integrated into its western end. The bearing works much like a sliding collar, allowing movement of 18 inches in either direction when trusses lengthen or contract.

The Cardinals Stadium is only the second NFL facility topped with an operable roof. The first was Reliant Stadium, home to the Houston Texans, designed by HOK Sport and completed in 2002. Two more NFL operable roof facilities are under way-one for the Indianapolis Colts, to open in 2008, and another for the Dallas Cowboys, to open in 2009. HKS is designing both facilities.

The Colts' facility, known as Lucas Oil Stadium, will have the first operable roof supported on more than two parallel rails, according to Tarek Ayoubi, project manger for Walter P Moore. Rails for each of the two 596-foot-by-163-foot panels will run atop five peaked box trusses that span between two 752-foot-long superframes running along the field sidelines, 300 feet apart. The multiple supports "allow a larger opening at lower cost," says Brian Trubey, AIA, HKS design principal.

The challenge is to keep each panel's five sets of motors working in unison to avoid racking and overstressing. A computerized control system, devised by Uni-Systems, will keep the wheels in sync. The roof will also have a linear bearing much like that installed at the Cardinals Stadium. "Thermal expansion issues become a little more important with multiple rails," says Silberman.