During the period 1930 to 1960, many composite concrete deck and steel beam bridges were constructed in Iowa. The bridges, designed in accordance with then-current AASHO bridge design specifications, had exterior beams smaller than interior beams. Because of changes in AASHO wheel load distribution fractions in 1957, which increased the design live load for exterior beams, and because of increases in Iowa legal loads in 1980, the bridges are currently understrength. The Iowa bridges can be strengthened economically by post-tensioning only the small exterior beams, providing that the design accounts for the distribution of a significant portion of the post-tensioning to the interior beams;In order to determine the post-tensioning distribution, a research program was initiated which involved construction and testing of a half-scale model bridge, strengthening of two Iowa bridges, and development of a design methodology. A SAP IV finite element model for a post-tensioned composite bridge was developed and verified with the experimental results from the half-scale model bridge. The finite element model was utilized separately, with simple support and with fixed support conditions, to bracket field test data for the two Iowa bridges strengthened by post-tensioning. The bridges tested in the field behaved as if partially restrained due to construction details at supports and attached steel guard rails;Results from the SAP IV composite bridge model were used to determine post-tensioning distribution for both axial forces and moments. Relatively simple formulas were developed for force fractions and moment fractions at midspin, by separate multiple linear regression analyses, for typical Iowa three-beam bridges and four-beam bridges;A design procedure for use with the AASHTO Service Load Design Method was developed and applied to a typical Iowa composite bridge. The design example indicated that post-tensioning the exterior beams would eliminate flexural stress deficiencies for all bridge beams without causing overstress, if integral curbs were neglected. The design methodology is valid for a variety of three-beam and four-beam composite bridges, including four-beam bridges with beams of equal size.