Mechanical Behavior of Slender CFST Columns with Debonding under Axial and Eccentric Compression

Debonding between the steel tube and concrete core is a critical defect in concrete-filled steel tube (CFST) columns, especially in slender members where accidental load eccentricities are likely to occur. Although both debonding and eccentricity are known to reduce structural capacity, their combined effects and potential interaction in slender CFST columns remain largely unexplored in the literature. This study addresses this gap by experimentally investigating the compressive behavior of slender CFST columns with slenderness ratios λ=16, 24, and 32, subjected to concentric (e=0) and eccentric (e=0.5) ratio axial loads. A total of 20 debonding and non-debonding specimens with varying steel and slenderness ratios were tested. Debonding was introduced by placing removable PVC sheets during casting, producing well-defined voids confirmed through nondestructive testing and internal inspections. Results showed that debonding and load eccentricity reduced axial capacity almost independently, with no relevant interaction observed. These findings suggest that for slender CFST columns, debonding can be treated as a separate strength reduction mechanism, independent of eccentricity. To interpret the experimental results, a simplified analytical model was adopted, assuming that the total axial capacity is the sum of the individual contributions from the steel tube and the concrete core. The model confirmed that in slender CFST columns, concrete confinement is limited, and axial capacity can be reasonably predicted without enhancement factors, especially when debonding and eccentricity are absent.