Judges Award, Design – U.S. 17/U.S. 92 General Hutchison Pedestrian Bridge

Bridge Project Wins Top Honor for Design and Best Engineering Award

By Debra Wood

The $4.7 million U.S. Route 17-92 General Hutchison Pedestrian Bridge connects two portions of the Cross Seminole Trail separated by a busy central Florida thoroughfare.

Seminole County selected Ayres Associates and American Bridge, both of Tampa, to design and construct the 1,220-ft-long, S-shaped bridge at General Hutchison Road, linking Big Tree Park with the rest of the trail.

“It was natural and made sense for the setting,” says Hisham Sunna, project manager for Ayres. He explains that looking at the curvy trail it hardly seemed right to install a straight-span bridge to cross the road, even if the S-spanned main-span truss and the elevated curved approaches would cost more.

“By curving the entire approaches, we were able to avoid the mature trees in the area,” Sunna adds.

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The project lies in an environmentally sensitive area. Ayres kept clearing to a minimum, and all disturbed areas were replanted with natural vegetation.

“The architecture of the bridge is not imposing,” says Allen Dronko, project manager for American Bridge. “The owner wanted something that would blend in.”

The bridge’s geometry required three-dimensional computer modeling to understand the structural behavior and to derive the forces on the bridge members. Sunna says that structural behavior is not straightforward after twisting a truss and not covered in the engineering books. It acts more like a beam with a lot of torsion.

Ayres also modeled expansion and contraction to determine what direction it would take. A straight truss requires its bearings accommodate expansion in the direction of its length, but that does not occur with a doubly curved truss. Error in aligning the bearing could have resulted in excessive forces on the anchor bolts and foundation during thermal expansion.

The Florida Department of Transportation, responsible for U.S. 17-92, reviewed and approved the complex design and granted an air-space agreement. Part way through design, FDOT indicted it planned in the future to raise by about 5 ft the profile of U.S. 17-92 to solve expensive right-of-way and drainage issues on a different project.

“We had to quickly get back to the drawing board,” Sunna says. He adds that Ayres considered several options, such as adding more vertical curvature, but ultimately decided to raise the main-span columns and adjust the slopes, retaining walls and 977-ft long, cast-in-place approach spans. It revised the plans within one month.

Chicago Metal in Chicago rolled the main truss members and shipped them to Florida Structural Steel in Tampa for assembly. Tampa Tank of Tampa fabricated other portions of the truss. Ayres sent a weld inspector to witness the welding operations at the fabrication plant.

Ayres specified hollow structural-steel members, which allowed truss members with similar function to have the same outside dimensions while using thicker walls for members bearing higher stresses.

The size of the bridge and its curvature and camber required shipment in sections to stay within the legal transport limits for width and height. The team mapped the hauling route and checked to confirm it could accommodate the transfer.

American Bridge completed construction of the bridge in May. It began work on the curving approaches and pier foundations while Ayres continued to develop the main span.

Main column caps were detailed to allow for easy setting of the truss. The team preinstalled into the column cap one set of anchor bolts for the four bearing corners.

It assembled the truss in the median, bolting it together into one piece before lifting and setting the 208-ft, 135-ton serpentine-shaped steel box truss in one pick during a nighttime highway closure.

“The anchor bolts fell in the center of the holes,” Dronko recalls. “Everything was aligned perfectly.”

Ayres submitted renderings with different coloring for the bridge components and first 8 ft of the entryways, which give the appearance of a traditional covered bridge. Columns are dark brown, spans are cream and the truss is green. The team painted the main columns to match “The Senator,” a 2,500-year-old bald cypress tree located at Big Tree Park. All blend with the natural surroundings. A trex entryway, made of composite plastic, enhances the bridge’s entrance.

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