The first prestressed concrete bridge in the United States was built in the early 1950s. Since then, several typical sections have been developed for use in bridge construction including I-beams, deck slabs, box beams, double tees, etc. In bridges under aggressive environments, corrosion deterioration of prestressing strands and stirrups has occurred creating challenges associated with determining the strength of deteriorated existing bridge sections. The MassDOT LRFD Bridge Manual includes provisions to estimate strength of corrosion deteriorated prestressed concrete box beams allowing engineers to calculate the load rating of these types of bridges. The provisions are based on the observed condition of the bridge, particularly with regard to estimates of strand area reductions to estimate residual strength. In bridges with adjacent box beams or deck slabs, corroded reinforcement is difficult to identify because only the top and bottom surfaces of the superstructure elements are accessible. The goals of this research are to evaluate the existing strength calculation procedures and to provide recommendations on how to properly evaluate the reduction in strand area based on the observed condition of the bottom surface of the prestressed box or deck beams.