Evaluation the effects of stirrup spacing and buckling of steel reinforcing bars on the capacity of RC columns

The capacity of reinforced concrete sections highly depends on the interaction between reinforcing bars and surrounding concrete. The buckling of longitudinal steel bars have an important role in the nonlinear behavior of RC sections. In order to nonlinear analyzing of the bar’s buckling in RC columns, two types of column and joint elements are modeled and separate degrees of freedom are used for the reinforcing bars and concrete. Buckling effect is considered in the numerical modeling of bars and the ability and reliability of the numerical method is assessed through the verification of numerical and experimental results. The effect of stirrup spacing on the Axial-Force–Moment (P-M) interaction curve of the RC column is calculated using the nonlinear model of bar's buckling and compared with the ACI criteria. Results show that the numerical capacity in a RC section subject to pure axial compressive load and pure bending moment, with bar's buckling effect, are only about 6% and 9% lower than those obtained without buckling effect, respectively. Also, although ACI criteria does not takes bar's buckling effect into account in the estimation of ultimate strength capacity of RC section, the ultimate capacity recommended by ACI code is acceptable and conservatively enough.