The first of the stadium's eight roof trusses was recently finished. Next week, it will be rolled off a temporary work platform to make room to erect the next truss.

Erecting the roof

The erection procedure for the roof trusses takes maximum) advantage of the inherent strength and stability of the roof structure. For example, the very stable, three-chord, long-span trusses allow a single truss to span the 655-foot clear distance without any temporary vertical or horizontal support.

This allows the trusses to be erected one at a time on a stationary platform, then rolled off to the side to make way for the next truss.

The configuration of the ballpark and the ability to retract the roof off the stadium and store it over the railroad tracks turned out to be real advantageous during construction.

The retractable roof panels can be erected away from the main bowl, which eliminates scheduling and staging conflicts with the rest of construction. This is much more efficient than the construction of two similar stadiums, the Toronto Skydome and the newly completed Bank One Ballpark in Phoenix, both of which required the retractable roof to be constructed over the stadium.

Sliding panels

The retractable roof consists of three independent panels. The two lower panels slide under the upper center panel, and then all three panels retract completely off the ball field and are stored to the east, over the Burlington Northern railroad.

Combined, the panels cover over 8.5 acres and weigh nearly 13,000 tons. There is enough steel in the roof to build a 60-story high-rise building.

The top of the tallest truss is 267 feet, which is equal to the height of a 25-story building. Special consideration was given to the location of the bottom of the lower trusses, which are only 167 feet above the playing field, so that a high-flying fair ball will not bit the bottom truss chords.

Making it move

The stadium roof is retracted by rolling the roof panels along a runway structure using a specially designed roof retraction mechanism. The retraction mechanism, designed and built by

Ederer Services Inc., a local crane manufacturer, consists of 128 36"inch-diameter steel wheels arranged in groups of eight, referred to as a "travel truck assembly."

The 16 travel truck assemblies run along elevated runway structures, which are' 100 feet tall on the south side and 50 feet tall along the north. NBBJ, the architects for the project, wanted the elevation of the north runway to be as low as possible, to minimize the visual impact of the runway structure from the seating bowl. This will give a more open feel to the ballpark when the roof is retracted.

The moving roof panels are driven by 96 10-horsepower DC motors connected through gears to the 36-inch wheels. The roof is designed to move at about 30 feet per minute (a very slow walking pace), allowing the roof to open or close in approximately 20 minutes under normal operating conditions. The roof will also be able to move against head winds of up to 30 miles per hour.

The retractable roof and supporting structures are designed to carry not only the wight of the roof, but also any extreme load conditions that may be inflicted by Mother Nature. This includes 70-mile-per-hour winds, lateral earthquake ground accelerations of up to 33 percent of gravity, and snow drifts up to six feet deep. In fact, the total weight of the design snow loads exceeds the weight of the 45,000 fans expected to attend opening day.

Giant shock absorbers

One of the unique features of the roof system is the eight "viscous dampers" mounted in the trusses at the north end of the roof panels. The viscous dampers are analogous to the shock absorbers in an automobile, with one major exception: the shock absorbers in a car are two inches in diameter and designed for forces in the range of 1,000 to 2,000 pounds.

The dampers in the retractable roof are 18 inches in diameter and designed to withstand ultimate forces of up to 1.6 million pounds. The dampers are used to absorb and dissipate the dynamic wind and earthquake forces in the roof structures. They will effectively cut in half the maximum seismic forces imposed on the roof, runway and travel truck system.

Construction of all three roof panels is scheduled to be completed by the end of November.

Kurt Nordquist is the stadium engineering team leaders for Skilling Ward Magnusson Barkshire.