This study investigated the behavior of short and slender square reinforced concrete (RC) columns with inadequate interior ties under concentric and eccentric loading. An eccentricity of 30 mm was adopted to evaluate the influence of load eccentricity on column behavior. Inadequate interior ties refer to insufficient transverse reinforcement provided to laterally support intermediate longitudinal bars. Twelve 1/4-scale square RC columns, comprising six short and six slender specimens, were tested with and without adequate interior ties. Specimens with inadequate interior ties were strengthened using full carbon fiber-reinforced polymer (CFRP) wrapping with a CFRP volumetric ratio of approximately 0.9%. The results showed that inadequate interior ties reduced the axial strength of short columns by approximately 27% under concentric loading, whereas only a marginal reduction of about 1% was observed in slender columns. Brittle failure occurred in both short and slender columns with inadequate interior ties under concentric compression. Under eccentric loading, the corresponding strength reductions were approximately 11% and 4% in the short and slender columns, respectively, indicating a less pronounced influence of inadequate interior ties. Full CFRP wrapping prevented brittle failure and improved the structural performance of the strengthened columns, increasing displacement ductility by up to 22% in short columns and 11% in slender columns. The beneficial effect of CFRP confinement was more pronounced in short columns than in slender columns. These findings demonstrate that CFRP confinement is an effective strengthening technique for square RC columns with inadequate interior ties.
