The need for upgrading understrength bridges in the United States has been well documented in the literature. In this thesis, two methods of strengthening are presented: post-tensioning of the positive moment regions of the bridge stringers and the addition of superimposed trusses at the piers. The use of these two systems is an efficient method of reducing flexural overstresses in undercapacity bridges. The objective of the research described in this thesis was to develop a design methodology to assist bridge engineers with designing a strengthening system to obtain the desired stress reductions. In addition, one such strengthening system was designed for use on a three-span continuous steel stringer bridge in the field;A design methodology was developed to simplify the design process for the strengthening system on a typical continuous-span composite bridge. As a result of the longitudinal and transverse force distribution, the design methodology presented in this thesis for continuous-span composite bridges is extremely complex. To simplify the procedure, a spreadsheet has been developed for use by practicing engineers. The force and moment distribution fraction formulas developed in this study are primarily for the Iowa DOT V12 and V14 three-span four-stringer bridges. These formulas may be used on other bridges if they are within the limits stated in this study. Use of the distribution fraction formulas for bridges not within the stated limits is not recommended;The bridge selected for strengthening was in Cerro Gordo County near Mason City, Iowa on County Road B65. A strengthening system composed of post-tensioning and superimposed trusses was designed to remove overstresses that occurred when the bridge was subjected to Iowa legal loads. The strengthening system was installed in the summer of 1992. Instrumentation was installed in the summers of 1992 and 1993. In the summer of 1993, the bridge was load tested before and after the strengthening system was activated. The load test results indicate that the strengthening system was effective in reducing the overstress in both the positive and negative moment regions of the stringers.