Practical Performance Model for Bar Buckling

A practical model has been developed to predict, for a given level of lateral deformation, the likelihood that longitudinal bars in a reinforced concrete column will have begun to buckle. Three relationships linking plastic rotation, drift ratio, and displacement ductility with the onset of bar buckling were derived based on the results of plastic-hinge analysis, moment-curvature analysis, and the expected influence of the confinement reinforcement. These relationships, which account for the effective confinement ratio, axial-load ratio, aspect ratio, and longitudinal bar diameter, were calibrated using observations of bar buckling from cyclic tests of 62 rectangular-reinforced and 42 spiral-reinforced concrete columns. A version of the drift ratio relationship is proposed for earthquake engineering applications. The ratios of the measured displacements at bar buckling to the displacements calculated with the proposed model had a mean of 1.01 and a coefficient of variation of 25% for rectangular-reinforced concrete columns. The corresponding mean and coefficient of variation for spiral-reinforced columns were 0.97 and 24%, respectively.