Strengthening of steel beams using fasteners was recently introduced by a few researchers to overcome the undesirable de bonding failure of the adhesively bonded FRP-steel beams. This paper investigates numerically the performance of steel beams strengthened by FRP sheets anchored using steel fasteners without a bonding agent. Detailed numerical models of fastened FRP-steel beams were developed using ANSYS software and verified against published experimental results. The calculated error between the experimental and numerical ultimate loads ranged from 2.6 to 10.2% proving the accuracy of the developed model. The verified model was used to evaluate the effectiveness of applying the fastening technique in enhancing the load capacity of FRP-steel beams subjected to one-point and two-point loads while adopting various FRP thicknesses of 3.175, 6.35 and 12.7 mm. Ultimate load improvements of 7.8, 20.2 and 25.8% were obtained for beams subjected to one-point load and fastened by the 3.175, 6.35 and 12.7 mm thick FRP sheets, respectively. Meanwhile, beams subjected to two-point loads showed 18.7, 21.5 and 25.2% enhancements in the ultimate load for the three thicknesses, respectively. Increasing the thickness of the FRP sheet boosted the ultimate load of the fastened FRP-steel beam; however, the ductility of the system was reduced.