Ductility of Square-Confined Concrete Columns

A brief review showed considerable differences in the quantity of rectangular hoops required by various seismic design codes in the potential plastic hinge regions of rectangular reinforced concrete columns. Tests conducted on four nearly full size reinforced concrete columns with a 550 mm (21.7 in) square cross section, various axial load levels, and various quantities of rectangular hoops, are described. The hoops were designed according to the recommendations of the draft New Zealand Concrete Design Code. The columns demonstrated very ductile behaviour when loaded by simulated seismic loading up to displacement ductility factors of at least 6, confirming the adequacy of the proposed design provisions. Considerable enhancement of flexural strength was observed over that calculated by normal code design procedures, but the enhancement could be accounted for a refined flexural strength analysis which included the effect of strain hardening of the longitudinal steel and the increase in strength and ductility of the confined concrete. Maximum compressive concrete strains in the range 0.016 to 0.026 were measured on the surface of the confined concrete core at the peak displacements.