Confinement effects on axial compression resistance of concrete filled double skin (CHS inner and SHS outer) aluminum tube stub columns

This paper presents an experimental and analytical study on the axial compressive behavior of concrete-filled double-skin aluminum tube (CFDAT) stub columns (with CHS inner and SHS outer tubes). Since existing methods for steel CFDST columns are not directly applicable due to material differences, axial compression tests were conducted on twenty-seven CFDAT specimens. The key parameters investigated were the width-to-thickness ratio of the outer aluminum tube, the concrete strength, and the diameter-to-thickness ratio of the inner aluminum tube. The results show that the confinement effect decreases with higher concrete strength for a given cross-section. Reducing the outer tube's width-to-thickness ratio changes the failure mode from buckling to shear failure and from ductile to brittle behavior. Higher concrete strength increases stiffness but reduces ductility, while thicker outer tubes lead to abrupt cracking failure rather than gradual post-peak deformation. Smaller inner tube dimensions improve both stiffness and deformation capacity. A theoretical model based on the arching effect theory is proposed to calculate the confinement stress on concrete, which is then incorporated into the Richart equation to predict the axial compressive strength of the confined concrete core. The proposed calculation method yields an average error of the calculated stub column strength to the experimental results of 3 %, with a standard deviation of 0.057. In comparison, using the existing method for CFDST calculates average error and standard deviation values of 20 % and 0.079, respectively.