Experimental behaviour of concrete-encased square concrete-filled steel tubular (CFST) hybrid structure subjected to axial compression

Concrete-encased square concrete-filled steel tubular (CFST) hybrid structure is a kind of hybrid structure with inner square CFST component and an outer RC component. Although square steel tubes provide slightly weaker confinement than circular counterparts, they offer distinct advantages in improved connection reliability, simpler fabrication, and larger bending stiffness. However, there is currently a lack of research on the fundamental axial compressive mechanism of the concrete-encased square CFST hybrid structures. To address this, 15 axial compressive specimens were studied considering steel tube grade, steel tube ratio, fabrication process, and cross-section type. Emphasizing the material properties and fabrication process of the square steel tubes, a full-range analysis of this hybrid structure under axial compression is conducted. The failure modes, force-deformation relationships, and strain distribution are investigated. The influences of key factors, such as the strength of the square steel tube, welding residual stresses, and cold forming residual stresses, are revealed on the structural strength and deformation capacity. Moreover, the contribution of each component to the overall structural strength is clarified, demonstrating that the hybrid action can be fully mobilized. Based on the findings, a full-range analytical model for predicting the axial compressive behaviour of the concrete-encased square CFST hybrid structures is proposed, explicitly incorporating fabrication effects.