External post-tensioning, in which external tendons are installed on the outside of a beam, is an efficient strengthening method for concrete beams. In such cases, a direct approach to determine the tendon configuration for a desired increase in load-carrying capacity would be useful, and this paper develops just such an approach for simple-span beams. The installation of external tendons causes an increase in the beam capacity in two ways. First, the tendons contribute directly to the load-carrying capacity by balancing part of the increased loads. Second, the concrete compression zone is increased, which indirectly leads to an increase in the beam capacity.Two sets of equations—refined equations and simplified equations—are established for the determination of the increase in load-carrying capacity. The refined equations account for the contribution of both the increased concrete compression zone and the direct contribution of the tendons, while the simplified equations account for the direct contribution of the tendons only. A comparison with 124 simple-span beams from previous investigations showed that the increases in load-carrying capacity predicted by the refined equations are in good agreement with the test data, while those predicted by the simplified equations are conservative.
