Ductility Analysis of Reinforced Concrete Beam Sections under Simple Bending According to Strain Domains

Abstract

Ductility is a central requirement in the flexural design of reinforced concrete beams because it governs the capacity of a section to undergo inelastic deformation before losing resistance. The aim of this narrative review is to examine the relationship between neutral-axis depth, strain compatibility, reinforcement condition and curvature ductility in reinforced concrete sections subjected to simple bending. Peer-reviewed analytical, numerical and experimental studies were analyzed qualitatively, distinguishing conventional steel reinforcement from fiber-reinforced polymer (FRP), hybrid steel–FRP, ultra-high-performance concrete (UHPC) and steel-fiber systems. The literature shows that under-reinforced sections, in which the tensile reinforcement yields before concrete crushing, provide a more ductile response than over-reinforced sections. As the neutral axis deepens and the compression zone becomes dominant, the available deformation capacity decreases and failure becomes more brittle. Compression reinforcement, confinement, fiber-reinforced cementitious materials and hybrid steel–FRP reinforcement can modify this response, although the conventional yield-based definition of ductility must be applied with caution when non-yielding FRP reinforcement is used. Neutral-axis depth is a useful design indicator but should not be treated as an isolated or universal measure of ductility, because material properties, reinforcement ratios, confinement, geometry and the adopted definition of ultimate curvature also govern sectional behavior. The findings support a strain-compatible, mechanism-based assessment of ductility rather than reliance on reinforcement strength alone. From a sustainable-construction standpoint, these mechanisms inform durability-oriented detailing, material-efficient reinforcement selection and service-life considerations, but the reviewed studies do not establish comparative life-cycle environmental benefits.