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Dallas-area light rail system requires Texas-sized formwork

inlandpipeThe Dallas Area Rapid Transit Authority (DART) operates a 45-mile light rail system in and around the Dallas area.

The system is recognized as one of the most successful urban light rail systems in the country, connecting points north, east and south of the city with the downtown area and continually increasing ridership and attracting new development as it has expanded. Thanks in part to funds from the American Recovery and Reinvestment Act (ARRA), DART launched a major expansion to its Orange Line that will include rail service to and from the Dallas/Fort Worth International Airport. When the $1.3 billion expansion is completed in 2013, it will become a 91-mile system with more than 60 stops (up from 55).

Precast/prestressed products for one crucial element of the project are being supplied by Bexar Concrete Works I of San Antonio. Jorge Hinojosa. P.E., M.S. of Bexar, chose Hamilton Form to design and fabricate the formwork because of the company's experience in supplying complex forms for major infrastructure projects. The expansion is being developed through a process in which a single entity—Irving, Texas-based Kiewit, Stacy and Witbeck, Reyes, Parsons—is responsible for both design and construction. “On a complex design/build project of this magnitude, the forming system plays a critical role in the project’s success," Hinojosa said.

One section of the rail system will travel over a 550-ft. precast/prestressed concrete bridge that spans the Trinity River levee, located just northwest of downtown Dallas, near the old Texas Stadium site. The main span of the bridge consists of a 260-ft. clear span made up of two 97-ft.-long x 10-ft.10-in. haunch girders at the piers and a 160-ft. middle “drop-in” beam section. The end and center spans of the bridge utilize typical TX82 girders. In total, Bexar supplied 12 haunch girders for the bridge in a five-week window. (The two 145-ft. spans on either side of the center span were not supplied by Bexar.)

Hamilton Form fabricated 82-in. sideforms for the typical girders with Vibrotrack installed low on one side and high on the other to provide uniform concrete consolidation. To improve productivity, Bexar was supplied with enough soffit to allow three beams to be set-up at once so that the sideforms could leap-frog down the soffit for additional pours.

The soffit for the beams was unique due to the atypical bottom width of 33 inches. Hamilton Form designed the understructure of the soffit so it could be modified to a more common 32-in. width after this project is completed. For the haunch girders used at the piers, Hamilton Form designed a variable-height soffit configured to the profile of the bottom of the beam. The configured soffit allowed the 11-ft. 2-in.-tall sideforms to be built as a rectangular profile, providing better stability and ease of handling.

Sealing of the form against the soffit was accomplished by a thru-tie system located 2 inches below the bottom casting surface. For shipping and handling considerations, the haunch girder was post-tensioned rather than prestressed so that the form required end blocks. Two pour openings were provided to prevent the concrete from dropping the full height of the form during the pour.

Due to the height of the beams, transporting the girders from the San Antonio plant to the jobsite in Dallas, nearly 300 miles away, was challenging since the product would not fit under the I-35 overpasses, forcing the transport trucks to get on and off the highway about two dozen times. This made the journey take the better part of an entire day to arrive at the bridge site, although Bexar was able to transport two pieces on certain loads to save time.

According to Hinojosa, a great deal of temporary shoring prep time was required before the girders could be lowered into place. An added challenge in terms of placing the large girders was their weight, he added, especially the 200,000-lb.-plus center pieces. "Ever since Hurricane Katrina, the U.S. Army Corps of Engineers will not let any heavy equipment or product on top of a levee," he explains. "So the middle beam had to be set with a single crane pick, extending about 200 feet over the levee as well as a cluster of power lines that serve as a main electrical supply to Dallas."

According to Hinojosa, the forms were delivered on time, performed as expected and helped the project stay ahead of schedule. “We never have any doubts in haunch girder formwork with header," he said.