To investigate, by using numerical analysis, the flexural behavior of concrete beams strengthening by external prestressing. The studied parameters were prestressing steel area, prestressing stress, tendon profile, tendon eccentricity, number and location of deviators. Tendon was placed under a beam and was not placed under the beam more than two times of beam depth. The beams used in this study were the bridge girders, e.g. RC SLAB, PC PLANK GIRDER, PC BOX GIRDER and PC I GIRDER. Results of this study indicated that an increase in prestressing steel area led to an increase in flexural strength and stiffness of the beams but led to a reduction in ductility of the beams. Prestressing stress level in prestressing steel had no effect on beam's flexural strength; however, an increase in prestressing force level in prestressing steel resulted in an increase in stiffness and cracking load of the beams. An increase in tendon eccentricity led to an increase in beam's flexural strength, but it has no effect on beam's ductility. In beams strengthen with large eccentricity, high level of prestressing stress led to the reduction in the stiffness of the beams, while low level of prestressing stress led to an increase in the stiffness of the beams. A decrease in number of deviators had less effect on the loss of tendon eccentricity. In casse of decreasing number of deviators, beams strengthen with large eccentricity led to less decrease in flexural strength than beams with small eccentricity. Tendon profile affects the flexural strength, stiffness and ductility of the strengthen beams. Load patterns on beam and increase in tendon stress are the major parameters in determining the tendon profile. Trapezoidal profile of tendons made beams more stiff, more ductile and higher flexural strength than the draped profile. It was found that behavior of beams strengthening by external prestressing in this study is similar in term of flexural strength, stiffness and ductility.
