Effect of Debonding on Axial Response of Concrete Filled Steel Tubular Columns

The axial strength and ductility of concrete filled steel tubes is significantly improved due to two major factors namely, firstly due to confining effect provided by steel tube, and secondly due to the synergistic interaction between steel tube and concrete core. Sometimes, the contact between concrete core and steel tube is vanished and debonding between concrete core and steel tube (normal referred as gap) occurs owing to various reasons such as improper compaction, shrinkage in concrete, lack of curing of encased concrete, and due to non-uniform distribution of load at column top. Due to occurrence of gap, the composite action between concrete core and steel tube no longer remains effective the concrete core becomes unconfined in the initial phase of loading, consequently, the axial load carrying capacity of the concrete filled steel tubular (CFST) column decreases. The aim of this paper is to numerically investigate the effect of the gap between concrete core and steel tube on the axial load-deformation response and to Comparisons the predicted column strengths according to the Eurocode-4. In the present study, the gap between square section concrete core and encasing steel tube is considered on all the four sides with the magnitude of gap as 1 mm and 2 mm. Moreover, in order to investigate the effect of the gap on different size of square section CFST columns, keeping the length of column and steel tube thickness as constant 540 mm and 3.8 mm respectively, the side length of square section CFST column has been varied between 140 mm to 200 mm at an interval of 20 mm. The nonlinear finite element analysis of the CFST columns has been performed using the Abaqus where the C3D8R element has been used to discretize the concrete core as well as steel tube and the Drucker-Prager model has been used to simulate plastic behaviour of concrete. It has been observed that the presence of gaps between concrete core and steel tube significantly reduces the axial strength square CFST column.