The topping out of Milwaukee's Miller Park retractable-roof stadium last November was a milestone in many ways. Just a 
month earlier, lucky fans of major league baseball's Brewers franchise autographed the 18-ton steel box chord that was lifted into place, as workers and others involved with the project paused for a moment of silence in memory of three construction workers killed in a July 1999 accident that delayed the park's opening a year.
With Miller Park's debut in April, Brewers fans no longer have to cope with cold temperatures in the spring or fall, or have their outings delayed or canceled because of rain. The stadium's unique, seven-section roof will not be the last retractable covering to be constructed over a stadium. Owners of multiple use stadiums want to be able to schedule events well in advance, without worrying about cancellations resulting from inclement weather.
Retractable roofs allow the use of natural turf, and combine the best features of open-air and domed stadiums. Professional teams prefer natural turf over artificial surfaces for sporting events. And the cost of a retractable roof is a relatively small percentage of total project costs. For instance, the retractable roof on Enron Field in Houston cost about $50 million, or 20 percent of the project's $248 million cost. The cost of providing mechanization for an operable roof - including motors, transporters and electrical controls - was $7.7 million, or less than 3 percent of the total project cost.
The ability to control the climate and to use natural turf are the two main reasons for the popularity of retractable roofs, says Craig Stockwell, vice president of Dallas-based A/E HKS Inc., which designed Miller Park in association with Columbus, Ohio-based architect NBBJ and Milwaukee-based Eppstein Uhen Architects. 'The Brewers were interested in a retractable roof from the get-go - you really have to have one in Milwaukee. Watching baseball in 30-degree weather just isn't appealing.'
Stockwell credits the roof proposal for constituting '90 percent' of the reason why the project was awarded to his team. The roof opens and closes in a fan-shape motion with 37 roof pieces assembled into seven roof panels. The panel pivot point is above and behind home plate, and the panels' outboard ends ride on rails constructed on top of the outfield wall. The highest point of the roof is 330 feet above the field, while the lowest point, above second base, is nearly 260 feet high. Two of the seven roof panels are fixed - one along the right-field line and the other along the left-field line. In the open position, three panels stack on the left-field side and two on the right-field side. Designed to open or close in about 10 minutes, the roof allows fans to enjoy temperatures 30 degrees warmer than outside when the roof is closed. While not air-conditioned, Miller Park is ventilated on rainy summer days when the roof is closed.
Stockwell says a major challenge was how to economically seal the roof. 'It's not air-tight or hermetically sealed when it closes,' he says. 'It has a lot of gaps. Our intent was not to make it weatherproof, but weather-resistant.'
Because the roof has a radial configuration and its seven sections aren't attached to the same pivot point, each section tracks in a slightly different manner. The concrete track beam has three rails to accommodate sections with different radii. As the sections move across above each other, the distance between them varies. 'The real feat was to prevent the various parts from hitting each other, but still allow the roof to close tightly enough to form a seal.'
HKS worked with Japan-based Mitsubishi Heavy Industries to develop the roof's operation, using its proprietary technology. Mitsubishi had done operable roof projects in Japan on a much smaller scale. The technology originated with Mitsubishi's crane projects for shipyards. 'We adapted it to move these big roof panels,' Stockwell adds. Outfield wall also moves
Existing technology also was used to create a movable outfield wall for Miller Park, which has been overshadowed by the more conspicuous operable roof. The outfield-wall system, designed by Uni-Systems of Minneapolis, can be opened to the outside. It is based on the company's experience with large movable wall structures, including aircraft hangar doors and 210-ft-high doors for a NASA rocket facility. For Miller Park the outfield panels are 72 feet high and 700 feet long. Two of the steel-framed units are operable and three are stationary. During inclement weather translucent panels on one door admit daylight.
Reliant to ride on retractable roof
In Houston, professional football action was formerly housed beneath the permanent dome of the Astrodome. The new Reliant Stadium -the home venue for the new Houston Texans NFL franchise - is designed with a retractable roof and will open in 2002. Reliant Stadium was designed by Kansas City-based HOK Sport in association with Houston-based architectural firms Hermes Reed Architects and Lockwood, Andrews and Newnam.
'After studying multiple roof schemes, a uniquely simple solution was pursued for cost and maintenance reasons,' says David Manica of HOK Sport, the project's lead designer. Two retractable-roof panels are envisaged that open at the 50-yard line. Five parallel trusses, 30 feet deep at midspan and spanning 385 feet, support each panel. These 10 trusses move along rails that are anchored to the top of tapered long-span 'supertrusses' along each sideline. The supertrusses span 675 feet and range in depth from 50 feet at midspan to 72 feet at their ends. They are supported by 'supercolumns' at each corner of the field. The supertrusses cantilever 125 feet beyond the supercolumns to support the movable roof panels in their retracted position. Two fixed trusses, located at a level just below the retractable panels, support roofs above the north and south end zones.
In order to reduce the roof's weight - and its cost - the operable portions are covered with fabric. This reduced the amount of structural steel required.
Uni-Systems also designed mechanisms for retractable roofs atop Houston's Enron Field and a new Arizona Cardinals stadium in Phoenix scheduled for completion in 2004.
Uni-Systems' focus has been primarily on the aircraft and space markets, according to company president Cyril Silberman. 'We've seen a lot of people struggle with retractable roofs because they haven't done the kind of analysis and engineering of the mechanisms that a more sophisticated customer such as Boeing or NASA would.'
Wind weighs heavy on roof design
Silberman says that developing a mechanism for an application such as Enron Field's 9,000-ton movable roof is relatively straightforward, as experience can be drawn from applications in the shipyard and other industries. The more challenging aspect of the engineering is to satisfy safety standards and local building requirements. In Houston, for example, design parameters include uplift and factors associated with hurricane-force winds.
When wind blows as a roof is being opened or closed, the design must keep it from over-running, which is more critical than achieving its programmed speed, Silberman notes. For example, a roof under a wind load of 50 m.p.h. has half a million pounds of pressure pushing it down the tracks.
At the Arizona Cardinals facility, the operable roof is designed to climb up a barrel-shaped dome and descend down the other side. When the roof is in the descending mode, with the wind behind it, the roof could reach a speed of 29 m.p.h., causing it to fail. 'Making sure that this type of failure could never happen is the real challenge of building mechanized roofs,' Silberman says.
He emphasizes that retractable roofs require sophisticated controls - for example, to keep track of the wind, to make sure all motors are operating at the proper speed and to confirm that roof sections are moving evenly down the tracks without binding. High-tech features of these systems include variable-frequency drives that count the electrical pulses going into the motor. The positions of individual panels are tracked for the operator's observation. Dynamic braking is used to magnetically slow the roof prior to the application of brakes.
Silberman believes this type of technology epitomizes the direction in which movable roof systems are heading. 'The aerospace industry is using it, and stadium construction needs to follow in its footsteps if retractable roofs are to be built to operate safely and reliably,' he says.
