Mechanical equivalence of circumferential and spherical-cap debonding morphologies in CFST columns

Steel–concrete debonding in concrete-filled steel tube (CFST) members reduces composite action by delaying or suppressing the mobilization of confinement. Debonding is commonly described through scalar geometric parameters, such as debonded area ratio or arc-length ratio, although different gap morphologies may induce different confinement mechanisms. This paper investigates the mechanical equivalence between two representative debonding morphologies in circular CFST short columns: circumferential debonding gaps (CDG) and spherical-cap debonding gaps (SDG). An ABAQUS finite element model is first validated against experimental databases comprising both CDG and SDG specimens. The validation confirms accurate predictions, with a mean ratio N u , FEM / N u , EXP = 0.999 and a mean absolute percentage error of 3.21%. The model is then used to compare paired CDG and SDG specimens with identical debonded area ratio or identical debonding arc-length ratio. The results show that equal debonded area does not imply equal mechanical degradation: CDG specimens exhibit lower ultimate loads, earlier peak deformation, sharper post-peak softening, and a more extensive loss of strongly confined concrete than in SDG specimens. At A D = 1.30 % , the strongly confined area is about 40% for CDG and 70% for SDG. Conversely, when the debonding arc-length ratio is identical, CDG and SDG responses remain nearly equivalent up to a critical value. For the reference specimen, this threshold is R D , cr ≈ 0.12 ; at R D = 0.20 , the capacity reduction factor decreases to about 0.89 for CDG and 0.86 for SDG. Beyond this threshold, the SDG morphology becomes more detrimental because the larger local gap thickness delays steel–concrete contact recovery. Parametric analyses show that the threshold generally remains within 0.12–0.13, supporting the use of debonding arc length as a robust equivalence parameter.