Code requirements for column confinement with a view on drift

Abstract

Confinement provided by transverse reinforcement plays an essential role in reinforced concrete columns. When proportioned judiciously, confinement provides longitudinal bar restraint, contributes to shear resistance, and can maintain the integrity of the concrete core. Its role is most critical in applications requiring toughness, such as in columns subject to seismic demands. As a result, the New Zealand NZS3101-06 and United States ACI318-19 design provisions for structural concrete specify minimum confinement requirements for earthquake applications. Both design codes intend to provide columns with the toughness to sustain inelastic displacements without a considerable loss in strength. Moreover, United States code provisions explicitly state that design expressions are intended to result in columns capable of sustaining 3% drift without considerable loss in strength. In this investigation, rectangular reinforced concrete column data compiled by ACI committee 369 was used to vet the efficacy of both design provisions from the perspective of drift capacity. The results indicate no clear correlation between expected drift capacity and minimum code confining requirements, contrary to current expectations. In addition, these requirements often lead to inefficient and non-constructible designs. Herein, the authors suggest an urgent revision of current practice regarding minimum confinement requirements. Not doing so may lead to consequential and unintended performance of columns during future earthquakes.