In-plane bar buckling in RC columns in extreme seismic events

A clear understanding of resistance mechanisms in reinforced concrete (RC) columns under extreme earthquakes is crucial as column failure can lead to structural collapse. To enhance such understanding, seven full-scale reinforced concrete columns were tested under distinct monotonic and cyclic loading protocols, similar to those that RC columns experience during large earthquakes. The columns were representative of columns in mid- or high-rise buildings, located in high seismic zones, and were designed according to seismic provisions of ACI 318-11 [1]. Transverse ties were placed at close spacing along column height to prevent reinforcing bar buckling and to enhance ductility. However, during the tests, longitudinal bars were observed to buckle in-plane (i.e., parallel to column compression face); a failure mechanism that has not been addressed in previous RC column tests. While transverse ties could prevent or postpone out-of-plane bar buckling, they were found to have no effect in restraining the bars from in-plane buckling. The observed in-plane bar buckling phenomenon is investigated analytically using three-dimensional (3D) nonlinear finite element (FE) models of the tested columns. The FE model was validated with experimental results and was used to study the effect of several parameters that can affect in-plane bar buckling including concrete compressive strength (f_c), longitudinal bar size and spacing, transverse tie spacing, and cross sectional size. The analysis shows that RC columns that feature a larger cross section with larger longitudinal bar sizes and are built with a lower strength concrete are more susceptible to in-plane bar buckling.