Numerous studies have indicated the existence of a size effect on the axial compressive behaviour of stirrup-confined concrete columns. However, most of these studies have focused on nominal compressive strength. Investigations of the size effect on the axial strain (at peak load) and of the descending branch are limited. In this study, the size effect behaviour of square stirrup-confined concrete columns under axial compression was explored, using a three-dimensional mesoscale simulation method. Based on the numerical and available experimental results, the influence of specimen size on peak axial stress (i.e. compressive strength), corresponding strain and softening rate was explored. Moreover, quantitative relationships between specimen size and peak axial stress, corresponding strain and softening rate for circular and square stirrup-confined concrete columns were derived. Finally, considering the size effect on the peak axial stress, corresponding strain and softening rate, a novel stress–strain model describing the axial compression behaviour of stirrup-confined concrete was developed. The proposed model was verified by comparison with the available experimental results and existing models.
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