Lateral reinforcement for high-strength concrete columns

The basic philosophy of the current ACI Code for confining concrete in earthquake design is that the increase of the strength of the core of the column due to confinement must offset the loss of strength due to spall ing of the unconfined cover. The equations given in the code are based on the assumption that when a reinforced concrete column is subjected to uniaxial load the maximum capacity of the confined core is reached when the unconfined cover starts spalling. It is not clear whether this assumption is applicable for high strength concrete. The strains at which the cover concrete and confined concrete ·will reach their maximum capacities will depend on their respective stress-strain curves. In this paper, based on several sets of experimental data, analytical expressions are proposed for the stress-strain curves of confined and unconfined high- strength cone rete. Using these analytical expressions, moment-curvature relationships are predicted. The predicted curves were compared with the experimental data of columns subjected to reversed lateral loading. Rased on the satisfactory comparison for normal strength concrete columns, the theoretical model is then applied to high strength concrete.