Highways/Bridges: Best Project
Sundt Construction Inc. completed the $25.9-million West 7th Street Bridge, connecting downtown Fort Worth to the burgeoning cultural district, a month ahead of schedule in November 2013.
The original span was built in 1913 and expanded in 1953. When the Texas Dept. of Transportation and the city of Fort Worth decided to replace the 99-year-old bridge, which was in serious need of repair, they had two main priorities: construct a signature bridge the community would be proud of and minimize disruption to the busy thoroughfare.
The old bridge was replaced with a new 980-ft-long structure, designed by TxDOT, which calls it the world's first precast, network-arch bridge. It features four vehicle travel lanes, a center-raised median and two 10-ft-wide pedestrian lanes.
The team found several ways to speed up construction and minimize traffic closures during the 23-month project. Sundt began building the 12 precast, post-tensioned concrete arches off site a year before work started on the bridge itself. Each arch is 163 ft long, rises more than 20 ft above the roadway surface at its highest point and weighs about 640,000 lb.
Once the arches were complete, Sundt transported and set them in place in pairs along either side of the old structure, primarily at night. When all of the arches were in place, the team closed and demolished the old bridge and constructed the new one in its footprint in an aggressive 120 days.
The arches were cast lying flat on their sides, then post-tensioned and installed with stainless steel rods that run from the top of the arch to the tie. After they cured to a concrete strength of 6,000 psi, the arches were rotated into a vertical position with a gantry system and strand jacks and shifted into storage. Each arch took approximately four weeks to complete.
Arches were loaded onto two self-propelled modular transport trailers, one on each end of an arch, using wing dollies. The transports were slowly driven by remote control across the bridge. Because of the original bridge's age, the twin Liebherr 1400 cranes with superlift were stationed just off the bridge, with one in the Trinity River flood plain and one on 7th Street, just east of the span. Together the cranes lifted the first arch into place.
Each subsequent arch required the same careful planning and staging to allow for successful placement along the new foundations located alongside the original structure.
Some of the most sophisticated, high-performance concrete ever used by TxDOT went into construction of the arches. Each arch is a frame that must be able to withstand shrinkage and thermal differentials, which could cause cracking during curing.
To minimize these problems, a number of techniques were used on the mix, including limiting the maximum cementitious materials content to 700 lb per cu yd, setting a water-to-cementitious materials ratio between 0.34 and 0.43, requiring the concrete mix to utilize optimized aggregate gradation and including the use of a shrinkage-reducing admixture.
A self-consolidating concrete mix was used to cast the 102 floor beams in order to create a smooth finish and minimize vibration and finishing efforts. The 12-hour compressive strengths of the beams averaged 5,600 psi, allowing a one-day turnaround on the casting bed.
General Contractor Sundt Construction Inc., San Antonio
Owner/Lead Design Texas Dept. of Transportation, Austin