Experiment investigation of the influence of reinforcing bar buckling on seismic behavior of RC columns

The inelastic buckling of longitudinal steel bar is one of the major damage states of reinforced concrete (RC) column under seismic loading, which may result in a reduction of section moment capacity and ductility. However, the relationship between bar buckling and deterioration in seismic behaviors of RC column has normally been proposed based on the theoretical analysis, direct experimental verification is scarce, and the differences between the buckling behavior of individual steel bar and that of longitudinal bar in RC column remain unclear. In this paper, a series of experiments on cantilever RC columns and individual bars were con-ducted to study the buckling behavior by means of the measured lateral buckling displacements of the bars. A new method was proposed to measure two components of the lateral buckling displacements of longitudinal bars and individual bars in orthogonal directions, the reliability and accuracy of the proposed method have been identified by the comparison of measured buckling displacement of corner bars in two identical RC specimens. Based on the measured lateral buckling displacements of reinforcing bar in RC columns and corresponding individual bar, the onset of buckling and the buckling direction of the two kinds of steel bars were analyzed. It is found that buckled main bars in RC columns behave differently with individual steel bars due to the constraint effect of cover concrete. The equivalent average stress of main bar was determined by creating a mapping between the buckling displacement of bar in RC section and the average stress of individual bar. On the basis of the buckling displacements and the equivalent average stress, the effects of bar buckling on the post-peak bearing capacity and the displacement ductility of RC column were investigated. The results show that the degradation of the post-peak lateral force of RC column is generally weaker than the strength deterioration of buckled steel bar due to the load redistribution between longitudinal reinforcement and surrounding core concrete.