Size-scale effects on interaction diagrams for reinforced concrete columns

The use of N-M interaction diagrams is well established in the design of reinforced concrete columns, when the second order effects can be neglected. According to the stress-strain constitutive laws usually adopted to compute the resistant domains, complex phenomena such as size effects and concrete confinement cannot be considered in practical applications. On the other hand, several experimental evidences, and some analytical models available in the literature, emphasize the influence of such effects. In the present paper, a numerical approach based on the integrated Cohesive/Overlapping Crack Model is applied to compute the interaction diagrams. Compared to classical approaches, different constitutive laws are assumed for concrete in compression and tension, based on Nonlinear Fracture Mechanics models, and a step-by-step analysis is performed instead of limit state analysis. The proposed model permits the size and the confinement effects to be predicted, according to the experimental results. Moreover, the obtained results completely agree with previous extensive applications of the model to plain concrete specimens subjected to uniaxial compression and reinforced concrete beams in bending.