Bridge engineers are continually faced with the challenge of providing efficient and cost-effective structures. In particular, the Florida Department of Transportation has recognized the need to develop economical bridge configurations in the medium-span range of 200- to 400-ft (60.96-to 121.92-m) spans and authorized a research project at the University of South Florida to identify and develop innovative design concepts within this span range. The study identified the concept of a steel bridge with double-composite action as an innovative bridge concept with the potential for significant cost savings compared with conventional modes of construction. This bridge type has been used with good success in Europe, but to the authors’ knowledge it has not been used in the United States. In addition to a composite concrete top slab, the double-composite bridge concept utilizes a composite concrete bottom slab in the negative moment regions. The result is provision of a design meeting compact requirements throughout, increased stiffness with corresponding decrease in fatigue stress range and deflections, savings in cross frames, and savings in flange material. The design implications of this system are examined, including redistribution effects due to creep and shrinkage, implications of different construction sequences, and strength and serviceability issues. Trial designs are presented, including both plate and box girder type structures, and design considerations are discussed. A prototype structure is identified for further development of the double-composite concept.
