Bond behaviour between concrete-filled steel tubes and reinforced concrete plinths

This manuscript investigates the bond behaviour between concrete-filled steel tubes and reinforced concrete plinths. The study addresses a practical need: evaluating the performance of foundations in cases where the steel tube lacks shear connectors (stubs). In such configurations, when the foundation is subjected to bending, the composite tube may experience tensile forces. Under these conditions, the bond between the tube and the surrounding concrete becomes the only mechanism for force transfer. While extensive research has been conducted on the bond resistance of embedded steel bars (both ribbed and smooth) as reflected in guidelines such as the Model Code 1990, significantly less attention has been given to the bond behaviour when the embedded element is a steel tube rather than a bar. In this context, the manuscript presents the results of an experimental test carried out at the University of Salerno, in which a concrete-filled steel tube was embedded in a reinforced concrete plinth and subjected to axial tensile loading. Following the experimental phase, a numerical model of the specimen was developed using the OpenSees software and validated against the experimental data. According to the provisions of the Model Code 1990, a non-linear bond behaviour between steel and concrete was implemented using a lumped plasticity approach at the nodes of a refined mesh, which formed the basis of the mechanical model. The results demonstrate that starting from the rules provided by Model Code 1990 provisions for smooth bars, a model able to accurately predict the bond behaviour observed in the tests involving steel tubes can be achieved. Finally, a parametric study was performed, which led to the formulation of a bond resistance equation.