Conventional continuous steel bridges only exhibit composite behavior in the positive moment region. Similar composite action may also be achieved in the negative moment region by casting a bottom concrete slab between the points of inflection. Such a section is referred to as "double composite" since it is composite in both the positive and negative moment regions. Savings in double composite bridges arise because expensive steel is replaced by inexpensive concrete to carry compressive loads. Although double composite bridges have been designed and constructed since at least 1978 there has been limited research. Thus, current designs rely on existing provisions for designing conventional 'single' composite bridges. This fails to fully exploit the advantages or recognize the weaknesses, if any, of double composite action. This thesis presents findings from a cooperative research project involving USF/URS/FDOT in which full-scale tests and theoretical analyses were carried to develop appropriate limit state rules for designing double composite bridges. A 4 ft. deep, 48 ft. long, 16 ft. wide box girder bridge representing the entire negative moment section at a support of a continuous full-size box girder bridge was fabricated and tested at FDOT's Structural Research Center, Tallahassee under fatigue, service and ultimate loading. Based on the findings from these tests and non-linear finite element analyses conducted by USF, URS proposed new design rules. This thesis focuses on the applications of these rules to develop a model design example for use by bridge engineers. The example was specifically selected from AISI so that a cost comparison with conventional design could be made. For completeness, an overview of the experimental results is also included in the thesis.

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