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The flexural failure mode of concrete beams reinforced with glass fiber-reinforced polymer (GFRP) bars changes from GFRP rupture to concrete crushing as the reinforcement ratio increases. Due to the uncertainties of material strengths, assumptions made in analysis, and variations in locations of reinforcements and dimensions of sections, there is a transition region where both flexural failure modes are possible. An iterative procedure is required when GFRP rupture governs the design. To avoid this iteration, the current American standard adopts a simplified but conservative procedure. In this study, the upper bound of the reinforcement ratio for beams in the transition region is revised. Moreover, a simplified yet rational design equation for calculating the flexural capacity of under-reinforced beams is proposed based on rigorous sectional analyses. Also, alternative design equations based on regression analyses …