Confined Columns under Eccentric Loading

The characteristics of confined-concrete columns under strain gradient were examined. Twelve columns were tested under two different levels of end eccentricity. The test parameters included the arrangement, spacing, and volumetric ratio of confinement reinforcement. Columns with well-distributed longitudinal reinforcement, laterally supported by closely spaced transverse reinforcement, showed extremely ductile behavior, developing inelastic displacements exceeding 4% of the drift ratio without a significant loss of strength. Those with a low volumetric ratio and wide spacing of transverse steel developed strength decay immediately after the peak load. A confined concrete model, developed on the basis of column tests under concentric loading, was used to compute analytical moment-curvature relationships of the critical column section. The analytical relationships are compared with those recorded experimentally, for columns with different eccentricity of loading and parameters of confinement. The comparisons indicate that the flexural behavior of confined-concrete columns can be computed reasonably accurately by a confined-concrete model developed for concentric loading, provided that the relevant parameters of confinement are incorporated in the model, and that strain hardening in reinforcement is considered.