“In most cases, concrete and FRP decks are there to support people or vehicles. The steel structure really carries the load to the ground,” says Reeve. “In the North Bank pedestrian bridge, aside from supporting people the deck actually helped connect all the steel pieces together and transfer load within the superstructure.” Composites made this possible: The deck achieves higher structural properties with the addition of more – and thicker – fiberglass skins. Plus, more fibers are oriented at ± 45° angles.

Bridge Project: Wolf Trap National Park
Location: Vienna, Va.
Challenge: Fully-assembled truss bridges needed to be erected rapidly to reduce road closure time.

wolf-trap-web

The superstructure of the Wolf Trap pedestrian bridge was a steel
truss with longitudinal beams spaced 4.25 feet apart. Mechanical
fasteners and Z clips provided the deck-to-beam connection.

Wolf Trap National Park for the Performing Arts hosts a variety of shows at its outdoor amphitheaters, ranging from jazz concerts to dance recitals. To walk from one facility to another, thousands of spectators often filed along a narrow curb over a vehicle bridge. The park put an end to this potentially dangerous situation by completing construction of a pedestrian bridge in April 2012.

The bridge, which crosses over a 10-lane highway leading to Dulles International Airport, was designed by the Federal Highway Administration (FHWA). The FHWA required accelerated construction to reduce lane closures that would hamper access to the busy airport. Composite Advantage’s FRP bridge decks offered the ideal solution. The deck panels were prefabricated, including the wear surface, curbs and expansion joints. This reduced on-site construction time. More importantly, the FRP bridge deck is 80 percent lighter than a concrete deck, making it faster to lift, move and place by crane.

Composite Advantage supplied 57 prefabricated deck panels measuring 15.5 x 8.3 feet. Each panel weighed only 880 pounds. The panels were assembled on three spans ranging from 138 to 170 feet long alongside the highway. Installation took place on three consecutive nights at 1 a.m., when traffic was light. Each assembled truss – including the steel, FRP deck and fencing – was towed to its position on the adjacent vehicle bridge. Then the contractors stopped traffic and two cranes lifted the truss into place, where it was bolted down. The highway was only closed for 15 minutes each evening. For two weeks following installation, construction workers finished the detail work, such as sealing joints, completing the railing and finishing the ends of the fencing.

The contractor on the project, Shirley Contracting Company LLC, Lorton, Va., never worked with composite decks before. It experienced some hiccups, such as matching the curbs and filling the joints, but was able to correct those issues after the bridge was in place. But the benefits outweighed the glitches. “We were able to install the entire deck in about three days. A traditional concrete deck, given the location, would have taken at least four weeks,” says John A. Majane III, senior structural project manager for Shirley Contracting Company. “[Another benefit] was the ability to assemble the deck off site, which reduced costs and the risk associated with working over traffic.”

Majane admits that cost is a factor when it comes to using FRP bridge decks. But he would consider composites for future projects. “This pedestrian truss bridge over traffic was a perfect application for the product,” he says.