<p>Glass-fiber-reinforced polymer (GFRP) bars are widely applied due to their advantages over reinforcement steel bars. GFRP bars are the main adhesive reinforcement since they have a high strength capacity and corrosion resistance. From the literature review and due to the insufficiency of these studies concerning the repair and strengthening of box-section RC beams reinforced by GFRP bars using external GFRP strips, this study was done. This paper presents an experimental, numerical, and analytical study for strengthening box-section reinforced concrete RC beams reinforced by GFRP bars and stirrups using external GFRP strips. The studied parameters in this investigation are (1) the width of GFRP strips (S_f), (2) the centerline-to-centerline spacing of the strips (S_f) and (W_f/S_f) ratio, (3) the GFRP layers, and (4) the inclination of GFRP strips. The experimental study consists of nine specimens. The specimens are tested as simply supported RC box-section beams. All beams have dimensions of 400 * 600 * 2200 * 2000&#xa0;mm (width * depth * total length * span). The nonlinear finite element program ANSYS was used to verify and validate the numerical models. Verification models have been developed. Using the measured results as crack patterns, load–deflection curves, failure modes, and failure loads, it can be concluded that, when doubling the number of GFRP layers, the failure load increased by 82%. Due to increasing the spacing between strips, the ultimate load decreased by about 9%. The ultimate load increased by about 3% when reducing the spacing between strips. The capacity of all tested beams after repair and strengthening was calculated using the Egyptian and American codes. Both codes are unconservative in some cases and conservative in others. The numerical output is unconservative compared to the experimental results.</p> Graphical Abstract <p></p>