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New Steel Arch, Stress Ribbon Bridge Opens
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Phyllis J. Tilley Memorial Bridge
A new pedestrian footbridge was dedicated this month in Forth Worth, Texas. The first bridge of its kind in North America, the $3-million Phyllis J. Tilley Memorial Bridge features a 163 ft steel deck arch topped with a stress ribbon—two structural systems that are not typically combined. © Rosales + Partners

A newly dedicated pedestrian bridge in Forth Worth, Texas, that combines a steel deck arch with a stress “ribbon” is the first of its kind in North America. 

September 4, 2012—The $3-million Phyllis J. Tilley Memorial Bridge, which extends across the Trinity River in Forth Worth, Texas, is the first bridge of its kind in North America. Incorporating two structural systems—a steel deck arch and a stress “ribbon”—that are not typically combined, the 368 ft arched bridge pays homage to the curved lower chord of the nearby historical Lancaster Avenue Bridge, a truss bridge that carries vehicular traffic. The new bridge, which places no footings in the river, is designed to withstand a 500-year flood event while increasing the existing floodwater elevations by no more than 1 in.

The bridge, which connects the city’s largest and oldest park to a new trail, was designed to be a thin and elegant structure that complements the “serene setting and the gently sloping banks of the river,” according to Miguel Rosales, AIA, the president of Rosales + Partners and the architect who designed the bridge. Rosales wrote in response to written questions submitted by Civil Engineering online.

The bridge’s 163 ft steel arch supports a stress ribbon footbridge that extends across the top of the arch; the two meet only the arch’s apex. Precast concrete panels that are anchored directly to a pair of steel plate “ribbons” act as the deck surface, according to Rosales. The steel plate ribbons tie directly into abutments located on either side of the river.

The unusual bridge design allowed the architect to create a bridge that appears “to float over the landscape,” Rosales said. “This unconventional structural combination was selected as a result of the site conditions, visual objectives, and the desire of the City of Fort Worth to build a special structure uncommon in the United States.”

Because the banks of the Trinity River do not have competent rock near the ground’s surface, a characteristic that is typically necessary for stress ribbon bridges due to the substantial horizontal forces on their abutments, the hybrid design was developed, Rosales said. “To counteract the large horizontal tensile forces of the stress ribbon, we decided to add a steel arch as an integral part of the overall design,” Rosales said. “The steel arch supports the stress ribbon at mid-span, reducing the loads ,and provides compressive thrust reactions in the opposite direction, which offsets the stress ribbon tensile reactions.” 

Trinity River side view of the Phyllis J. Tilley Memorial Bridge 

The 368 ft arched pedestrian bridge crosses the Trinity River,
connecting a new trail to the city’s Trinity Park. The curve of the
bridge pays homage to the curved lower chord of the nearby
historical Lancaster Avenue Bridge, a truss bridge that carries
vehicular traffic. © Rosales + Partners
 

The forces of the two systems thus balance one another, and as a result, smaller foundations were able to be used in the 20 ft deep clay and sand layer that forms the banks of the river. Four 60 in. diameter drilled shafts that extend 15 ft in depth are located beneath both of the arch’s termination points, and four 30 in. drilled shafts—two of which extend 15 ft in depth and two of which extend 24 ft in depth—are located under each of the stress ribbon’s abutments. The foundations are connected to one another with tie beams.

One of the program requirements of the new bridge was that it could not increase the existing floodwater elevations by more than 1 in. despite the fact that in a maximum credible flood event—a 500-year flood—the entire bridge would be submerged, according to Rosales. “In order to minimize the flow disruption during floods and gain approval from the U.S. Army Corps of Engineers, the new bridge needed to have an extremely thin profile,” he said. The arch and stress ribbon combination does not contain any columns located in the water and did not require the vertical struts needed in a traditional deck arch, which helped reduce the horizontal loads created by periodic river flooding, according to Rosales. The pedestrian railings were also designed with visual transparency and minimal horizontal loading in mind.

The resulting bridge is both structurally and hydraulically efficient, as well as being “visually compelling and memorable,” Rosales noted. “We expect that it will become a new symbol for the City of Fort Worth and help fulfill the community vision for a truly accessible river.”

A $2.3-million federal grant that was administered by the Texas Department of Transportation made the bridge possible, according to Rosales. Additionally, the City of Fort Worth provided $500,000 and Streams and Valleys, Inc.—a nonprofit organization that helps to protect and enhance the Trinity River and was cofounded by Phyllis J. Tilley—provided an additional $200,000 that was raised through private donations.

Freese & Nichols, headquartered in Forth Worth, and Schlaich Bergermann und Partner, headquartered in Stuttgart, Germany, performed the structural engineering on the project. Contractor Rebcon, Inc., based in Dallas, constructed the bridge, which was opened and dedicated earlier this month.


 

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