...cable," he explains. The system simplifies detailing because it eliminates complex diagonal member-to-chord connections, he says.

Super Support

Each Brunel is supported at its ends by a 171-ft-tall concrete supercolumn. The entire assembly resembles a giant table. TLCP Structural designed the supercolumns, complete with a lifting slot, to take all the loads of the jacking operation. The Brunels also support three fixed trusses, spanning the field at each end and secondary trusses. These now are under construction and span from the Brunels to the bowl’s perimeter concrete columns.

The transporter system is integral to the assembly. That meant Uni-Systems had to deliver the transporters much earlier than it has on its other two stadium projects. It also had to provide protection for the system during and after the lift.

Compared with Reliant Stadium in Houston, "we had an extra year of site presence at Phoenix and our design schedule compressed by one year," says Michael Becker, Uni-Systems project engineer. "This was a challenge for us because this is our most technically advanced roof to date," he says. "It also added to cost."

The 0-to-14-degree rail arc drove the transporter system design. The slope means drive forces vary significantly over the travel path, says Becker. That made it unsafe to use the system before it was fully functional. That, in turn, ruled out using the individual transporters as an erection aid for panel trusses, which had been done on other projects. For Uni-Systems, Schuff’s strategy to preassemble the roof into a unit on the field and then lift it was appealing, even with downsides.

The operable panels "kiss" at the 50 yd line, which is the peak of the arced rail. The configuration is well-suited for a cable drive system, which works somewhat like an elevator, says Uni-Systems.

The system rides on 32 steel crane wheels with a 36-in. tread diameter and is driven by 16 independent cable drums. Each has four, 7.5-hp electric-motor drive trains, for a total 480 hp. There are eight operable rail clamps.

The cable drive system is different from the cable drive system at Phoenix’s nearby Bank One Ballpark. There, a flat rail, closed-circuit system results in significant changes in the cable tension level as the drive system coordinates the movements of the three roof panels attached to each circuit, says Uni-Systems.

By contrast, the Cardinals stadium open-circuit system leverages the slope of the roof by using the weight of the roof panel itself to provide constant tension to the cables. "The advantage is the simplicity–the Cardinals stadium system is essentially a child’s yo-yo versus the finely tuned concert piano of Bank One Ballpark," says Mark Waggoner, the WPM associate on the job.

At the west end of each of the 16 panel trusses, a linear bearing is integrated into the connection between panel trusses and the transporters. These allow the west end of the panel trusses to move plus-or-minus 18 in. relative to the rail on which they are supported and over which they travel. The feature is critical to accommodate thermal expansion, deflections under load and normal construction tolerances without generating large thrust loads or "binding" in the system, says Uni-Systems, which has a patent pending on the linear bearing.

The rail was designed with a camber of some 20 in. to accommodate Brunel displacements and deformations during the lift. That meant the final rail position would be different from the installed position. To address this, WPM provided for vertical shimming every 13 ft at each rail girder support. Also, every 65 ft along the Brunel, the rail girder was fabricated and installed with a longitudinal gap of about 1 in. The rail was only bolted tight to the girder at the center two panel segments of the Brunel, with the remaining clamps allowed to slip. This prevented axial load transfer to the rail across the girder gaps, says the engineer.

As the Brunel truss deflected during the lift, the gaps between rail girders closed and the rail deflected into is proper radius. Connections will be tightened.

Stadium foundation work started September 2003. Hunt figured 60% of the concrete frame had to be done by the next August. That was when roof assembly had to begin to meet the lift date.

Stadium bowls usually are built in a "racetrack" fashion, level by level. "We couldn’t do that and meet our schedule," says Charlie Prewitt, Hunt’s construction manager. Instead, Kiewit divided bowl concrete work between expansion joints.

Workers cast the concrete vertically, "tower by tower," beginning in the northeast. Work included the slotted supercolumn, needed to receive the Brunel end bearing for the eventual lift. Supercolumns were poured in nine lifts, allowing the start of steel erection prior to the topping out of the supercolumns.

The concrete work then moved to the north end, the east side and the northwest corner, which included another supercolumn. After that, concrete work moved to the south, freeing up the north for steel work. The sequence was orchestrated like a fugue, starting with concrete and followed soon by steel, to keep concurrent operations from tripping over each other.

After installing sideline pipe shores, ironworkers began erecting the north half of the east Brunel. Following this, they moved to the north half of the west Brunel. They then picked the north-end fixed trusses. The retractable roof panels were picked last, in one piece. On the south half, the erection sequence was similar.

Over the length of the Brunels, the engineer allowed a sweep, or out-of-straightness, of 1 ft. After fabrication and prior to lifting, the sweep measured 11�4 in. "This is quite small when measured over 700 ft," says Griffis. "It represents very tight control over fabrication and erection tolerances."

Hunt puts the cost of the completed roof at $75.3 million. The cost of the lifted portion is $50.6 million, including $9 million for the mechanization.

Hunt has done four operable-roof stadiums. This one is on schedule, says Robert S. Aylesworth, Hunt’s executive vice president. He says operable-roof sports venues are "not getting any easier" to build because they are all so different. But he adds: "They are getting easier, generically, because we have familiarity with mechanization."

(Photos courtesy of Walter P. Moore)