Strength and ductility of laterally confined concrete columns

The objective of this study is to determine experimentally and analytically the magnitude of the strength enhancement of concrete confined by lateral ties. Sixty-five reinforced concrete columns with a 200 mm square cross section were tested. Experimental parameters include the compressive strength of concrete; the volumetric ratio, strength, and confinement type of rectilinear ties; and the distribution of longitudinal reinforcement bars. The confinement effects are explained by utilizing an effectively confined distance ratio. Tie stress caused by lateral concrete pressure is presented as a function of the volumetric ratio and configuration of ties, and the strength of concrete based on the experimental data and the nonlinear multiple regression method. Based on the empirical data and the nonlinear multiple regression method, this paper presents an empirical equation to determine the strength enhancement as a function of the tie stress, the effectively confined distance ratio, the volumetric ratio of ties, and the strength of concrete. The validity of the American Concrete Institute and Canadian Standards Association specifications for minimum tie spacing and the design of cross ties are examined.