The 29th Street Bridge in Georgetown, Washington, DC is made of FRP composites. (photos: CA)
Published in Plastics, November 2013. Find original article here: http://www.plastics.gl/construction/let-composite-bridges-take-the-strain/
Corrosive salt, chemicals, moisture, fatigue and the wear and tear of dynamic vehicular loads are just some of the factors that chip away at the health and stability of bridges during their service life. Weather and environmental conditions dictate regular bridge and bridge deck maintenance for structures built with conventional materials. Eventually, bridge owners are faced with the need to replace or rehabilitate their concrete and steel spans.
In 2007, the District of Columbia Department of Transportation (DDOT) initiated a construction project to refurbish several ageing bridges spanning the C&O Canal in Georgetown, Washington DC. The final phase of the C&O Canal Bridges Rehabilitation Project focused on replacement of the 29th Street Bridge. Crumbling reinforced concrete and exposed, rusting steel rebar coupled with a requirement to reduce dead load on old stone canal walls demanded a lighter weight bridge deck solution that conventional methods could not solve. Logistics challenges associated with utility lines that ran beneath the bridge and crossed over gas, water, Pepco and telecommunications lines demanded a simple, fast deck installation.
The DDOT, looking to new technologies for a modern-day fix, chose fibre reinforced polymer (FRP) composites. The Federal Highway Administration issued funding through the Innovative Bridge Research and Design (IBRD) programme to cover the cost of the fibreglass composite bridge deck. The DDOT considered several products on the market before selecting Composite Advantage‘s FiberSpan FRP bridge deck. Based in Dayton, OH, USA the manufacturer produces large fibre reinforced composite parts, up to 15.8m (52ft), for structurally demanding applications and corrosive environments.
Following its selection, Composite Advantage (CA) was awarded a design subcontract from bridge designer KCI Technologies, Hunt Valley, MD, USA. In 2010, a construction contract for the project was awarded to Flippo Construction Company, Forestville, MD. Flippo contracted with CA to supply the bridge deck for the 29th Street Bridge.
“FRP composites’ corrosion resistant properties offered the DDOT a low maintenance solution that could help reduce life cycle costs,” says Scott Reeve, President of CA. “Our product’s flexibility supported design of a thin deck to meet fit and clearance requirements”. CA submitted a design to the DDOT based on calculations for strength, stiffness and safety, and drawings detailing deck fabrication including materials, layup and sizing.
“The challenge with this project was achieving the required stiffness for the deck in a thin section,” says Reeve. “Most vehicle bridge decks are 20.3cm (8in) deep. The requirement for this bridge deck was 12.7cm (5in). The configuration was chosen because bridge depth was very restricted. The bridge had to clear tour boats using the canal but match the existing street level. Using longitudinal steel beams for high bending stiffness, we designed the FiberSpan deck within an allowable depth of 12.7cm and tested it to demonstrate its ability to support required truck loads”.
CA used continuous fibreglass reinforcement to provide the necessary strength and stiffness properties. Vinyl ester polymer was used to protect the fibreglass from chemicals and bind the fibres together so the composite material could deliver superior properties. “Fibreglass reinforcements provide the best combination of strength, stiffness and cost compared to other reinforcement options,” says Reeve.
Five large FRP deck panels and one sidewalk panel were moulded using the vacuum infusion process. This process allows large parts to be manufactured with lower cost moulding tools. The tooling, moulding and finishing was performed at CA’s manufacturing facility in four weeks. CA also dry-fitted deck panels to verify dimension and assembly tolerances.
The FiberSpan bridge deck panels were trailered to the project site on a flatbed in 2012. Installation took just one day on a steel beam superstructure. It took only one more day to bolt the panels to shear studs welded to steel beams and add the FRP sidewalk. The sidewalk and granite curbs were bonded to the deck panels to match the granite curbs throughout DC.
The new FRP short span bridge, 11.9m (39ft) long by 9.8m (32ft) wide, is the area’s first fibreglass reinforced vehicle bridge deck. Its lighter weight also helps support the canal’s older stone walls and maintain their vertical position. The FRP sidewalk is a new feature CA made available to designers for this project.