Of the approximately 600,000 bridges in the United States, more than 110,000 are either structurally deficient or functionally obsolete. Causes of deterioration are attributed to the age of the structure, steady increase in legal load limits, increase in traffic density, deterioration due to environmental attack, and the lack of proper maintenance. A large number of these bridges are constructed out of structural steel. One significant cause of deterioration in these structures is corrosion due to extensive use of de-icing salts in severe winter weather. These bridges either have to be replaced or strengthened to extend their service life. In most cases, strengthening of these structures is more cost effective than replacement. Among the popular strengthening methods used in the past on steel bridges are post-tensioning and the bonding of steel plates to the tension flange of the girders. However, these methods are labor intensive and disturbance to the traffic flow is inevitable. This investigation focused on the behavior of steel composite beams strengthened with carbon fiber reinforced polymer (CFRP) plates attached to their tension side. In the experimental part of this study, a total of ten beams were tested under a four-point bending; six of the beams were in the undamaged state (two control beams and four strengthened), while the remaining four beams were damaged and repaired (one control beam and three repaired). Since this technique depends on the shear strength of the adhesive, extensive instrumentation was utilized to measure shear stresses along the CFRP plate in all load ranges (i.e. elastic and inelastic); this is something that was not emphasized in the previous research available. Another contribution of this work was the study of the effect of damage on the stiffness and strength of the section as well as the effect of repair schemes in restoring both the lost stiffness and strength of the section. The test results showed a significant increase in the strength and the stiffness of the strengthened/repaired beams. To help implement this strengthening technique, a nonlinear analytical procedure was developed to predict the behavior of the section/member in the elastic, inelastic, and ultimate states. A parametric study was also conducted to investigate the effect of some of the important variables on the behavior of the composite section. In addition, damage analyses of a bridge system were completed using grillage analysis to investigate the influence of the damage in the girders on load distribution. Based on the damage analyses results, an analysis/design methodology to determine the required repair or strengthening system was developed.