Hurricanes and the sail effect dominated the thinking of the design team for the nation’s first retractable-roof baseball park engineered to withstand 146-mph winds. To keep the lid on the 36,000-seat new Miami Marlins Ballpark during severe storms, without adding too much extra weight, the roof’s structural engineer called for parking the roof panels 10 ft apart, in an almost-closed position. The gap-mode strategy, which reduces the sail effect, lightened up the roof by some 1,000 tons of steel, the engineer says.
“Putting a big sail on top of this stadium in a hurricane-prone region was the [project's] biggest challenge,” says Aaron White, a principal in the Tampa office of structural engineer Walter P Moore (WPM).
The $515-million facility is Florida’s first open-air major-league park built exclusively for baseball. Its contemporary look—dominated by its white color and its spaceship-like form—is intended to support Miami’s modern architecture. The ballpark is also designed to contrast with other recent major-league parks in the U.S., which have red brick exteriors that offer a traditional or “retro” look, says Greg Sherlock, a principal with architect Populous, Kansas City, Mo.
Rails on Beams
The facility’s gently sloped roof, covered in corrugated metal, contains three movable panels skirted by fixed panels. Bliss & Nyitray Inc., Coral Gables, Fla., engineered the fixed roof and the rest of the structure.
A 578-ft x 260-ft upper movable panel is flanked to the east and west by lower movable panels, each 548 ft x 145 ft. The panels are composed of steel trusses on trussed legs, spaced about 40 ft on center. The trusses span over the field, their ends bearing on concrete beams on opposite sides of the ballpark. Each beam contains a pair of parallel transporter rails.
Each truss leg sits atop a two-wheeled transporter, like an in-line skate, that supports, constrains and moves the panels relative to the building structure, says Alan Wilcox, project manager for the travel mechanism supplier, Uni-Systems Inc., Minneapolis. To date, WPM and Uni-Systems have collaborated on six movable-roof sports venues that have been built, including the Marlins’ ballpark.
The forged-steel drive wheels on the transporters roll on a forged-steel crane rail anchored to the concrete track beam. The transporters support more than 1 million lb of load.
Very Efficient
The panels of the 7,800-ton roof move using a traction drive system, powered by ten 76-hp electric-drive motors. “A steel wheel on a steel rail is very efficient as far as friction goes,” Wilcox says.
Thirty-eight variable-frequency drives with regenerative capability control the motors that power the retractable panels. “At higher winds with a panel this size, there is actually enough area [for the wind] to push the panels down the track,” Wilcox says. “The motors aren’t having to generate any power. Instead, they are becoming generators and that power can get dumped back into the electrical grid.”
The roof is designed to operate in winds up to 40 mph and to withstand winds up to 146 mph, as required by the Miami-Dade building code. The majority of the country uses a design wind speed of 90 mph, but WPM’s first retractable-roof ballpark in hurricane-prone Houston is designed to resist 110-mph winds. The “code pressures” on a Miami structure are 263% higher than most of the rest of the country, White says.
Consulting with wind engineer RWDI of Guelph, Ontario, WPM came up with a plan to set the roof into an almost-closed position when a hurricane approaches. Each panel moves to the west, enough to create 10-ft gaps between all movable and fixed panels.
The openings allow the wind to move through the roof structure, which reduces design pressures considerably, White says.
The movable roof has several wind-resisting mechanisms to prevent unwanted movement. Electrically actuated motor brakes and rail clamps automatically engage when the roof is stopped to prevent motion in the direction of the rail. A series of manual tiedowns connect the transporters directly to the track beams. Also, a series of 4-in.-dia uplift clips, permanently bolted to the bottoms of each transporter, engage the bottom of the railheads and prevent uplift.
Parallel Rails
Each rail beam contains parallel rails—an outer one for the upper panel and an inner rail for the lower panels. When the panels are retracting, the east panel moves under the upper panel and the entire assembly moves to the west—parking over an entrance plaza outside the ballpark’s footprint. The “storage space” over the plaza drove the three-panel design, White says.
To start roof construction, crews from Baker Concrete Construction, Coral Gables, Fla., cast the supercolumns in their final positions while they cast the rail beams in sections on the ground. Workers then jacked the beam sections onto the supercolumns.
Seating-area concrete work followed. “It was like two projects in one,” says Patrick S. Delano, senior vice president for Hunt/Moss, the general contractor joint venture of Hunt Construction Group, Indianapolis, and Moss & Associates, Fort Lauderdale, Fla.
To erect the retractable-panel trusses, crews from steel erector LPR Construction Co., Loveland, Colo., built a row of five falsework towers between opposite rail beams at the west end, where retracted panels would park.
Each truss system was then erected onto its transporter. The strategy, similar to the one used for the retractable roof at Safeco Field in Seattle, was to use only one set of falsework towers for all the movable trusses. “We saw a cost savings not having to reset the shores” for each truss, says Peter Radice, project manager for LPR, a subcontractor to fabricator Structal Heavy Steel Construction, a division of Canam Group, Boucherville, Quebec.
The approach also kept the seating bowl and field clear. “We could work on the bowl area and build the trusses outside,” says Brett Atkinson, Moss’ project executive.
Roof assembly began in April 2010, beginning with the eastern-most truss. The west lower-panel trusses followed the east panel trusses. The higher, middle panel trusses came next. Crews erected and bolted each truss in five sections. They then rolled the completed truss down a ramp, using the transporter system, which LPR also erected. The job was challenging because of heavy lifts and the truss rolls, says Radice.
The roof will remain open most of the time, letting in the sun or the rain to hydrate the natural turf. A single operator working from a personal computer can open or close the roof, which moves at 39 ft per minute, in 10 to 15 minutes, says Uni-System’s Wilcox. The operator also can move a single panel to shade a part of the field.
The roof isn’t the ballpark’s only architectural element that moves. A six-panel glass curtain wall in the left-field stands, 60-ft tall and 240-ft wide, biparts like sliding doors, thanks to an overhead rail system. The opening offers an unobstructed view of the city skyline and lets in ocean breezes. “Miami is so much about the sun and coast location,” says Populous’ Sherlock.
Miami is also about hurricanes. The roof protects against rainouts, but games are canceled if a hurricane threatens. That said, Hunt/Moss anticipates the ballpark will be ready for opening day in April—no matter the forecast.
Courtesy of: www.southeast.construction.com
