Axial Compressive Behaviour of Intermediate-Length Concrete-Filled Steel Tubular Columns with Different CFRP Wrapping Configurations

Abstract

The strengthening of intermediate-length concrete-filled steel tubular (CFST) columns using CFRP has attracted growing attention, but the effect of fibre orientation on the axial compressive behaviour—particularly for members where stability rather than strength governs—has not been fully clarified. This study therefore addressed the issue through a combined experimental, numerical, and analytical approach. Specimens were divided into four groups: unstrengthened controls, those with four longitudinal CFRP layers, those with four hoop layers, and a hybrid group with two longitudinal plus two hoop layers. All were tested under monotonic axial compression. The ultimate load-carrying capacity was enhanced by 23.0%, 20.5%, and 8.4% for the longitudinal, hybrid, and hoop-only CFRP wrapping schemes, respectively. These findings indicate that the strengthening efficiency is likely governed by the wrapping configuration under the conditions examined in the present test programme. Unstrengthened columns exhibited local buckling of the steel tube and crushing of the concrete core, a brittle failure mode. Longitudinal CFRP participated directly in load resistance and raised the flexural stiffness, thus effectively suppressing global buckling—the critical mode for intermediate-length columns. The hybrid configuration provided a reasonable strength gain and exhibited a relatively gentle descending branch together with favourable deformation compatibility. Hoop-only confinement, however, contributed little to stability improvement because it only provided lateral restraint without affecting bending resistance. A three-dimensional finite element model was constructed in ABAQUS, and its predictions of failure modes, load–displacement curves, and ultimate loads agreed well with the experimental data, confirming its reliability. Parametric studies using this model further revealed that the axial capacity decreased markedly with increasing slenderness ratio, whereas higher concrete and steel strengths both led to steady improvements. Finally, a simplified design formula was derived for the axial compressive capacity of CFRP-confined circular CFST intermediate-length columns. This formula includes a confinement efficiency coefficient that reflects the wrapping pattern and a stability coefficient suitable for intermediate-length members. Comparisons with both experimental and numerical results demonstrated that the proposed formula yields predictions within ±5% of the present dataset. The formula is applicable to circular CFST intermediate-length columns with 40 ≤ λ ≤ 80, concrete grades of C30–C50, steel grades of Q235–Q460, and the three wrapping configurations examined herein. Extension to slenderness ratios below 40 requires further validation, since the present tests and parametric study did not cover this range. Further independent validation is warranted prior to its adoption in general design.