Confined concrete model for columns under combined axial load and bending

A stress–strain curve model for confined concrete, which is essential for generating moment–curvature relationships for columns under constant axial loading, was developed by modifying Mander's confined concrete model. Two main assumptions adopted in Mander's model (45° arch action and yielding of tie bars) were tested and modified according to the theories of elasticity and strain compatibility, respectively. It was shown from the proposed confined concrete model that, although the 45° arch action assumption seems to be appropriate for confining effectiveness, tie bar stress may be far less than yield stress. A computer-based program was prepared to simulate moment–curvature relations of confined square concrete columns subjected to constant axial loading using the proposed concrete model. Numerical results from the proposed model were validated by comparing them with available laboratory test results; good agreement was found, especially in post-peak parts of moment–curvature relations.