When it opens this summer, the 186-foot Halls River Bridge in Homosassa, Fla., north of Tampa will likely have more composite elements than any vehicular bridge in the United States. While each of the composite technologies has been used in other projects nationally, the bridge, which will replace one built in 1954, will feature composites in its pilings, deck, retaining walls, abutments and traffic rails.
“There are some pedestrian bridges, but to our knowledge I don’t think there’s another vehicular bridge that’s used composites in every element to this extent,” says Steven Nolan, P.E., senior structures design engineer with the Florida Department of Transportation’s State Structures Design Office in Tallahassee. He cites pioneering work by the University of Miami’s College of Engineering, which built a pedestrian bridge on campus that extensively used composites and was a collaborator on the Halls River Bridge design.
The bridge replacement project began in 2012 when engineers at the FDOT chose the Halls River Bridge as a demonstration site for the state’s first bridge built with Hillman Composite Beams® by HCB Inc., Alpharetta, Ga. (HCBs were described by inventor John Hillman in a 2016 Composites Manufacturing article, “Making Inroads in Infrastructure,” as FRP boxes with a steel tension tie in the bottom flange resisting the thrust from a concrete arch inside the box. The FRP outer shell provides shear strength, the concrete arch offers compressive strength and the steel reinforcement running longitudinally provides tension capacity.)
While the project began with a focus on HCBs, as planning progressed the FDOT engineers saw opportunities to create proof-of-concept applications for other composite technologies, too. “Ultimately, we replaced every reinforced concrete element in the project with some form of composite material or hybrid – either carbon pre-stressed reinforced concrete, GFRP-reinforced concrete, or, in the case of the HCBs, a composite shell over a steel-reinforced concrete core,” says Nolan.
The FDOT opted for composites primarily because of the high costs of maintaining traditional steel-reinforced bridge elements in the state’s saltwater and wetland environments.
“The majority of long-term maintenance costs on our bridges are the result of degradation of the substructure, predominantly our pilings,” says Nolan. “We spend a major portion of our bridge maintenance budget on rehabilitating piles on bridges over saltwater crossings.” Rehabilitation includes replacement, repair, pile jackets and cathodic protection – all of which are expensive and require working within the body of water.