Influence of Confinement on Nominal Capacity and Curvature Ductility of Spun Piles

Axial force-bending moment (P-M) interaction diagrams are indispensable in the design of spun piles. From the interaction diagrams, the capacity of the section to resist axial force and bending moment can be obtained based on the results of a cross-sectional analysis of strain and stress distributions. The construction of an interaction diagram generally requires manual neutral axis trial and error calculations, which are time-consuming. Hence, a time-saving and more accurate auxiliary program was developed using MATLAB. Confinement can change the shape of concrete stress-strain curves, as seen from increasing compressive stress in the cross-section at certain ultimate strains. Accounting for the effect of confinement will increase the ultimate strains, leading to a more ductile structure. The confinement models used include Mander et al., El-Dash and Ahmad, and Kusuma and Tavio. The variations included the effect of prestressing force, spiral spacing, and the impact of confinement reinforcement area. The results of the confinement variables affect the cross-sectional curvature ductility. If the spiral spacing is closer or the area of reinforcing bars is increased, the curvature ductility increases.