Rational definition of the flexural deformation capacity of RC column sections

Traditional definitions of the ultimate displacement of RC structures are usually based on strength concerns in order to maintain a substantial proportion of the initial load-carrying capacity. This kind of definition does not reflect the true deformation capacity of structures. This work reveals the qualitatively different deformation stages of RC members through analytical and numerical studies, which is then used to define ultimate displacement and displacement ductility. The study provides a clear insight into the fundamental mechanism of concrete structural deformation and reveals the following three basic parameters that affect the deformation capacity of RC sections: ultimate strain of concrete material; axial load level; and cross-sectional depth. The longitudinal reinforcement is found to reduce the deformation capacity of RC sections at axial load levels that are lower than a critical axial load level, which is near to 30% of concrete crushing load, and to increase the deformation capacity at axial load levels that are higher than the critical axial load.