Few building structures rival the complexity of Minute Maid Park. Walter P. Moore says retractability made the analysis of the curving roof at least six times more complicated than a fixed roof of the same configuration.
According to Walter P. Moore, structural engineer at Minute Maid (formerly Enron) Field, the 42,000-seat major league park in Houston was a great deal for the Astros. The total cost of $248.5 million (under the $250 million budget) was 38 to 108 percent less than the cost of the three other U.S. retractable ballparks. It was built in less than 27 months and by the end of the first season, cumulative attendance exceeded three million fans, breaking all previous Texas sports season records . HOK’s designers used the historic train station as the front entrance. In addition to the roof, the project involved the world’s largest sliding glass wall (atop the left field wall) and a working locomotive as a nod to Houston’s railroad past.
The design called for a 6.5-acre roof that would dependably and safely move without compromising fan safety or playing the game.
With 29 pounds of steel per square foot, each of the three moving roof panels spans over 500 feet. It can withstand 110 mph (Category 5) hurricane winds when locked down. Designers and engineers also had to consider fast-moving thunderstorms that could strike when the roof was open.
The mechanization system is unique; computer-controlled 7.5-horsepower electric motors drive a total of 140 three-foot diameter steel wheels. It can be operated remotely by a single technician, and the system can alter roof movement in response to wind.
The transporter system uses a six-foot deep girder and a urethane spring suspension system to evenly distribute the weight over the wheels. This system works more like in-line skates than the “roller-skate” behavior of traditional wheel bogey systems, and thus eliminates large concentrated loads. The more forgiving rail tolerances allowed a more economical shallow foundation system that saved two months of construction time and millions in cost.
To improve the performance of the roof while reducing the steel tonnage, designers used passive dampers at the connection of the long span roof trusses to the vertical support trusses at the top of the glass wall beyond left field. These dampers only engage when high winds blow.
The tempered glass wall beyond the left field fence (more than 100 feet tall) opens with the roof. Reportedly the world’s largest, the wall will withstand airborne hurricane debris, and meets the most stringent hurricane codes.
The park demonstrated that a dependable retractable roof can be had with an austere budget and on an accelerated schedule. Prefabrication testing of the mechanized assemblies economically debugged the system, eliminating delays and cost swell. Instrumentation was installed throughout the roof to measure and report on deflections and movements in real time. This structural performance data is correlated with actual wind speeds, temperature, barometer, and humidity that are gathered and reported by a weather station built into the roof.
Few building structures rival the complexity of Minute Maid Park. Walter P. Moore says retractability made the analysis of the curving roof at least six times more complicated than a fixed roof of the same configuration. The roof, the giant sliding glass wall, the seating sections, and the foundations were structurally analyzed together to gauge the interaction between them, especially to understand the effect on the roof of foundation settlements. The building geometry is quite complex, particularly in three dimension. The roof is supported on parallel rail tracks that overlay a radial structural system for the seating sections in the lower superstructure. To achieve the aggressive schedule, the lower superstructure and the roof were built simultaneously but independently. Each primary steel truss was erected on a single fixed set of shoring towers located outside the seating bowl, then paired trusses were laced together and rolled into position as a unit.
Owner: Harris County – Houston Sports Authority
Associate Architect: Molina & Associates
Lead Structural, Civil and Traffic Engineer: Walter P. Moore
Mechanization Consultant: Uni-Systems, Inc. (Minneapolis)
Construction & Construction Management: A joint venture of Brown & Root Services (Houston), Barton Malow (Atlanta) and Empire Construction (Houston)
Minor league ballparks have evolved to incorporate similar amenities of a major league ballpark, but on a different scale. Historically, a minor league park would consist of grandstands, maybe a canopy, and a hot dog vendor.
Today, minor league ballparks have signature canopies and roofs, massive scoreboards, unique field configurations that create interest in the way the game is played, batting cages, elevated box seats—the list goes on and on. The addition of suites and club seats is a current popular trend. Team management offices and modern locker room facilities are no longer optional, but generally are program requirements. Today, minor league ballparks include inviting amenities such as team stores/souvenir shops and brewpubs. Patrons, team owners and stadium owners desire a design that fits into the community’s traditions—incorporating local aesthetics and culture into the ballpark design.
Baseball parks are also becoming even more family-oriented. Obviously, America’s favorite pasttime has always been about fun for the whole family, but some parks are actually now adding playgrounds. Today’s ballpark designs “knock it out of the park” when it comes to family appeal and overall community fun.
—Ken Conrad, P.E.,
and Timothy R. Santi, P.E.
Walter P. Moore
Engineers + Consultants
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