Strength and ductility of confined concrete columns under axial load and biaxial bending

The behaviour of prismatic reinforced concrete columns under increasing eccentric compression load was experimentally investigated and analytically modelled. Columns with distributed longitudinal reinforcement and closely spaced transverse reinforcement were tested up to failure, considering different values and directions of eccentricity. Preliminary tests were carried out under concentric compression in order to validate the choice of the stress-strain laws adopted for the confined concrete and the longitudinal steel bars in compression. In the paper, closed form expressions able to approximate the experimental results in the case of uniaxial bending are presented. Finally, a fibre numerical model is utilized for the cases of biaxial bending. The results show the reliability of the analytical models and suggest further studies to relate analytically the components of ultimate bending moment and curvature, in the cases of biaxial bending, to the values corresponding to two separate cases of uniaxial bending under the same level of compression.