Why This Project Was Different
FiberSPAN bridges provided an ideal solution to the unique demands of constructing a shared-use path on the region’s rocky uneven slopes for several reasons:
A robust alternative to concrete and asphalt, the key value add for the NDOT was Composite Advantage’s ability to fully assemble the lightweight, fiber reinforced polymer (FRP) bridge spans prior to panel placement. Installation had to be completed quickly and at night to minimize traffic impact on the lake’s only road. Each of the 32 bridge spans was prefabricated using two steel stringers, two steel cross beams, FRP deck and steel railing. Typical deck length was 40 ft. long. This project also required FRP to take on a larger role in terms of performance. In most FRP bridge applications, the steel support beams provide the primary load carrying structure. For this project, CA designed the FRP sections to act as the shear diaphragm for the stringers. The connection between the deck and steel beams was bonded with structural methyl methacrylate adhesive to permit complete shear transfer between the steel and FRP. This method of connection is atypical for most FRP bridge deck applications. Generally FRP decks are joined to superstructure supports with mechanically fastened clips that capture the underside of the beam flange.
Design
FRP composites are orthotropic materials so the properties in each deck panel direction are different. By placing the desired amount of fiber in any orientation, these properties can be engineered to achieve design goals. This allows the engineer to tailor properties like strength and stiffness in the direction needed. For the State Route 28 Shared-Use Path Project decks, a higher percentage of +/- 45 layers provided higher shear properties. Sandwich construction gives the FRP bridge decking the most efficient structural configuration for applications that require high stiffness and low weight. Panel depth and laminate thicknesses are easily tailored to a project’s requirements. FRP decks for Lake Tahoe were 134 in. wide with a 5-in. structural thickness at the thinnest section and a 1 percent cross slope. Panels were coated with a 1/8-in. non-slip polymer aggregate surface. FRP panels weigh just 8.8 psf. Deck weight with wear surface, curbs and railing was rated at 12 psf. Specification requirements called for a uniform live load of 90 psf pedestrian loading. Mid-span deflection of the deck was limited to L/500 and the mid-span deflection of the superstructure was limited to L/360. Vehicle loading was rated at H-5 for a rear axle load of 8,000 lbs. Uplift load was rated at 30 psf. The client selected custom colors for the decking to blend with the natural surroundings.
Installation
Each bridge span section was delivered as a prefabricated unit of steel beams, FRP deck and railing.
To prepare for the FRP decks, Granite Construction built intermediate piers supported by 3-in. micropiles driven into the shoreline. Steel piers were installed on this foundation so that the bridge assemblies could be bolted down. CA worked with Granite Construction iteratively because some of the piles had to be relocated a few feet requiring dimension changes to the FRP panels. For example, one of the panels was originally a 40-foot long rectangle. Because immovable objects like granite rocks changed the piling locations, the panel had to be adjusted to 42 ft. long with a skew on the end. CA made adjustments to 31 panels based on as-built conditions. Thirty-two 40-foot bridge span sections were installed using a lightweight crane. The spans were grouped to create five distinct bridges.
Feedback
FRP’s ability to accommodate a range of design variations, its’ 100-year life cycle and its capacity for use in difficult terrain make it an attractive option for projects like the State Route 28 Shared-Use Path for Lake Tahoe. The response has been so positive that there are prospective plans to extend the path created by FRP pedestrian decking. The NDOT and the Tahoe Transportation District opened the new Tahoe East Shore Trail in June 2019.
