Prestressed concrete has become the predominant construction method in highway bridge girders. This research aims to solve one of the most troublesome problems in prestressed concrete beams, namely the shear problem. In fact, there is at present no rational model to predict the shear behavior of prestressed concrete structures and the various modes of shear failures. Because of this deficiency, all the shear design provisions, such as those in the ACI Codes and AASHTO Specifications, are empirical, complicated and have severe limitations. In this research, five full-scale prestressed concrete I-beams were tested to study the effect of three variables, namely, the shear-span-to-depth ratio, the transverse steel ratio, and the draped strands, on the two types of shear failure capacities, namely, the web-shear and the flexural-shear. The results from these tests, together with those found in literature, were used to develop a rational, yet simple, equation for calculating the shear strengths of prestressed concrete beams. The new shear strength equation proposed in this research significantly reduces the unwarranted conservatism of the ACI Code, and guards against the unsafe nature of the AASHTO specifications for high strength concrete.