Bridges, Tunnels and other Transportation Structures
This paper describes the findings of a preliminary design study conducted for FRP composite bridge girders intended for use in extremely aggressive environments around the state of Florida. The target span length for the girders ranged from 30 ft. to 75 ft. and the design space was limited to simply-supported spans with cast-in-place reinforced concrete decks. Existing FRP-girder bridges constructed between 1997 and 2007 around the U.S. were used as benchmarks and the responsible maintenance entities were contacted to assess the current status of the bridges and to identify any potential durability concerns. From there, three basic girder geometries were selected for further study: (1) pultruded double-web I-beams (a proprietary system manufactured by Strongwell, Inc.), (2) concrete-filled FRP tubes, and (3) FRP U-girders. A basic design framework was established using existing guidelines for FRP materials from ASCE, ACI, and AASHTO. Four applicable limit states included (1) concrete compression for creep control, (2) live-load deflections, (3) fatigue and creep rupture of the FRP, and (4) ultimate strength. Two general design approaches were also investigated. For the FRP U-girders, an optimization algorithm based on output from finite element models of the entire bridge was used to obtain the most efficient girder dimensions for each span length. These results were compared to girder geometries based on the AASHTO distribution factor method for shear and moment. Results indicate that the hybrid FRP/concrete-filled U-girder is the most efficient and cost-effective alternative. The girder cost per square foot for the U-girder alternative is estimated at just over $40 per square foot of bridge deck for a 30-ft span to $140 per square foot for the 75-ft span. These estimates include the costs for materials and manufacturing of the FRP U-girder and the cast-in-place concrete required to fill the girder on-site. Additional costs associated with placing the girders and the cast-in-place reinforced concrete deck are not included.