Circular reinforced concrete columns strengthened by combined square steel tube and concrete jackets subjected to eccentric load

An experimental and numerical investigation on circular reinforced concrete columns strengthened by combined square steel tube and concrete jackets subjected to eccentric load is presented. Totally 39 specimens were tested, including 3 un-strengthened circular RC columns, 27 eccentrically loaded strengthened columns, and 9 axially loaded strengthened columns. The study showed that the brittle failure mode of the un-strengthened columns was improved after strengthening, with the strengthened columns exhibiting ductile failure. As the eccentric ratio increased from 0 to 0.16, 0.32, and 0.48, the ultimate loads decreased by averages of 10.9 %, 26.7 %, and 40.8 %, respectively. Reducing the width-to-thickness ratio of the steel tube improved its confining effect. This enhanced the strength of the original concrete but did not improve the strength of post-cast concrete. However, it did improve the ductility of the post-cast concrete. Increased eccentricity weakened the confining effect of the square steel tube at both the yield and ultimate states of the strengthened column, reducing its strengthening effect. Finally, a high accurate N-M interaction model was proposed to calculate the eccentric bearing capacity of the strengthened columns.