When the New York Giants and New England Patriots square off in Super Bowl XLII this Sunday, the players will likely be too busy pounding each other into the turf to notice they're playing on top of a retractable grass field that qualifies as one of the world's biggest motion control systems. And that's just fine with the engineers at Uni-Systems, the kinetic architecture firm behind the field's design.
"The last thing we wanted was for the players to be aware that they're playing on anything other than a regular grass field on the ground," says Alan Wilcox, the project engineer responsible for the field. Yet in reality, this gridiron is really a grass-covered suspended floor created from 18-inch steel beams and supported by more than 500 wheels that ride on a system of steel guide rails.
Measuring 230 ft wide and just over 403 ft long and weighing almost 19 million lbs, the entire field rolls out of the stadium, spending most of its time outside where the grass can soak up the Arizona sun. Come game time, the field rolls right back inside, with the 750-ft move taking just over an hour.
Engineers who work on industrial motion control applications may wonder, "What's the big deal?" After all, the field moves on just one axis and does so slowly by industrial standards. It has simple stop-and-go controls, which an operator initiates from a portable control stand that plugs into either end of the field. The system has no braking capability. Yet what the field lacks in motion control complexity, it makes up for in scale. And working at such a large scale requires careful engineering attention to power-transmission, guidance and vibration issues.
Ground-Moving Mechanism
“One of the biggest challenges we faced,” says Wilcox, “was coming up with the field's mechanization system.” The handful of retractable fields built in Japan and Europe have used systems based on water bearings or even teflon-and-steel skid plates. Uni-Systems took a different design route: It went with wheels. “We had a lot of experience with driven wheels from the work we've done on retractable roofs and knew a wheel-based system would be robust and cost effective,” Wilcox says.
The entire field rides on an array of 542 15-inch forged steel wheels housed in wheel boxes that have been integrated into the field's structural steel framing at strategic locations. Seventy-six of these wheels are driven by 1-HP gearmotors from Nord Drivesystems. These units feature a 568:1 gear reduction, 2.9 rpm output on the shaft and a startup torque of roughly 3740 ft-lb, or 2.2 times the full load torque. Start-up torque for the entire field comes to about 285,000 ft-lb. “We don't need to move fast, but we obviously need lots of torque,” says Wilcox. It's worth noting, however, that the 11.5 ft/min speed of this retractable field is faster than those found in other stadiums.
The wheels travel over 13 guiderails that have been embedded into the concrete under the stadium floor and extend outside to the parking lot that serves as the field's second home. Most of the rails are flush to the concrete of the stadium floor — which creates a nice flat surface for non-football events like concerts.
The central rail, however, protrudes from the surface a bit because it plays an important role in guiding the field. Wilcox says wheel-based systems can experience off-axis steering forces from even a few misaligned wheels. In the case of such a big moving platform, these lateral forces can be significant. With loads on each idler wheel coming to about 40,000 lb, for example, the lateral forces for a single crooked wheel could reach as much 16,000 lb, Wilcox estimates, explaining that the coefficient of friction between wheel and guiderail is roughly equivalent to the relationship between their normal and lateral forces.
To overcome these off-axis forces and keep the field on track during its travels, Uni-Systems came up with an arrangement of guide rollers, 180 in all. Each one consists of a pair of spring-loaded cam rolllers that ride horizontally along either side of the central guide rail profile with the springs allowing load sharing between nearby rollers.
Tackling Vibration
Another important part of Uni-Systems' design involved minimizing the vibrations that players would feel on the field, not just when they hit the deck hard but when nearby players do. “What we did to take vibration out of this field went way beyond the usual vibration analysis work,” says Wilcox. Uni-Systems, for example, consulted with a floor vibration guru, Dr. Tom Murray, who teaches structural steel design at Virginia Tech. The company also worked with its structural engineering firm, Walter P. Moore, to fine-tune the stiffness of the structure as the field design came together.
One difficult aspect of fighting vibration again came down to the scale of this project. The field area measures over two acres, so vibration damping through structural means has obvious cost implications.
Then there's the challenge of knowing how much vibration damping is required in the first place. “You can do all the engineering calculations in the world but they won't tell you how much vibration is acceptable from the players' and coaches' point of view,” says Wilcox. To bring these subjective human-factors considerations in the mix, Uni-Systems built a series of sod-covered protoypes and asked the home team Arizona Cardinals' players, coaches and owners to test them.
What came out of all that prototyping and testing was a beefier structure that ended up weighing a whole lot more than the first-pass designs. Wilcox says the weight (dead load) rose from about 12 million lb to 18.9 million lb as the design matured, with much of that weight coming from a move to 18-inch steel beams from 12-inch beams and from other stiffening measures. “Vibration turned out to be a really big deal in the design,” he says.
Game Plans Change
The size of the steel beams wasn't the only thing Uni-Systems changed as the field went through the design and build process. As Wilcox explains, the long lead times associated with building a stadium meant much of the retractable field design took place after the construction had already begun on the rest of the stadium, forcing Uni-Systems engineers to adapt to some challenging new design constraints and change their game plan on the fly. Think of it as the engineering equivalent of calling an audible.
To take one example, the move toward beefier steel beams added weight, which in turn triggered design changes to the guidance system. Wilcox recalls that guidance system was initially to have used a double-flanged wheel instead of the guide rollers. Uni-Systems' prototyping and testing, which involved one rolling prototype that carried 100,000 pounds through a lifecycle test, inidcated that the flanged-wheel design wouldn't hold up. By that time, the guide rail had already been installed in the stadium floor, so Uni-Systems had to engineer a roller system that would function with the existing guide rail profile.
The deeper steel sections not only added weight but also created packaging challenges. “We were constrained on the height of the field,” Wilcox says. So room for larger beams had to come at the expense of ground clearance. The finished field design rides only about 3-inches off the ground. And that lack of clearance drove some of the decisions about motors. “One reason we went with Nord is that they had gearmotors we could customize to fit our available space,” Wilcox recalls.
Uni-Systems ended up finishing the retractable field in time for a pre-season game between the Steelers and Cardinals in the 2006 season. That was field's first game, but Sunday's game may prove to be its best.
For a closer look at the design of the retractable field, check out this complete slide-show — and check out images of the stadium and field below.
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