REINFORCED-CONCRETE COLUMNS CONFINED BY CIRCULAR SPIRALS AND HOOPS

Twenty-seven short concrete columns reinforced with longitudinal steel and circular spirals or hoops were tested to failure under monotonic axial compression. Effects of different variables, such as amount and type of lateral steel, lateral steel spacing, and specimen size, on the behavior of columns were investigated The relation between lateral pressure on concrete and concrete strength enhancement, and the variation of spiral steel stress and confinement effectiveness coefficient k with respect to the amount of spiral steel were also investigated. Requirements of the ACI 318-89 Building Code related to the minimum volumetric ratio of spiral reinforcement and the maximum spiral pitch of 80 mm (3 in.) were critically examined. An increase in the volumetric ratio of spiral steel was found to significantly improve strength and ductility of confined concrete, the effect on ductility being more pronounced. The maximum effect of spiral steel spacing was observed for the amount of spiral steel, which was approximately equal to that required by the ACI code. The specimen size appeared to have no significant effect on the behavior of similarly confined columns of different sizes. In well-confined specimens, the confinement effectiveness coefficient k corresponding to the maximum concrete force was between 2.1 and 4.0.