Longitudinal axial strain in plastic hinge regions of reinforced-concrete columns

Energy dissipation of columns in the post-yielding range of deformation plays a significant role in preventing the total collapse of reinforced-concrete (RC) structures. However, such plastic deformations cause elongation in plastic hinge regions of RC columns if the axial force ratio is low. This study investigated the elongation behaviour of RC columns by testing five groups of 20 specimens subjected to three different loading patterns. Experimental results showed that the longitudinal axial strain of RC columns was quite different from that of RC beams because of the presence of axial force. The longitudinal axial strain of RC columns reduced with increasing axial force. Axial strain was also a function of loading pattern and lateral reinforcement ratio. Based on these observations, an analytical model was developed to predict the axial strain behaviour of RC columns. The proposed model includes four path types: a loading region, an unloading region, a slip region and a repeated loading region. The model was verified against 32 test results, including specimens tested in this study and reported in the literature. The longitudinal axial strain predicted using the proposed model agreed reasonably well with the experimental axial strains results.