Ductility of symmetrically reinforced concrete columns

A theoretical ductility study based on full-range flexural behaviour is carried out for symmetrically reinforced concrete columns cast of normal- and high-strength concrete, with the axial load covering both tension and compression. The analysis employs the actual stress-strain curves and takes into account the stress-path dependence of concrete and steel reinforcement. From the complete moment-curvature curve obtained, the moment capacity, flexural ductility, balanced failure load, variations of neutral axis depth and steel stress are studied. In particular, the effects of axial load and confinement are evaluated. Generally speaking, the higher the axial load, the steeper is the descending part of the moment-curvature curve and the lower is the flexural ductility. Confinement is observed to help maintain the resisting moment up to a much larger curvature at the post-peak stage and therefore it benefits the flexural ductility. Its benefit to the moment capacity is, however, minor. The effect of concrete tensile strength is only observed for columns under relatively low axial load at service stage and has virtually no effect on moment capacity and flexural ductility.