Load-carrying mechanism of thin-walled hybrid double-skin tubular columns subjected to axial compression

Rapid development of offshore infrastructure towards increasingly large dimensions and deep water highlights the need to seek promising alternatives to hollow steel tubular (HST) columns, which are suffering from corrosion-induced structural degradation and high constructional costs caused by the increasing wall thickness. As a variation of hybrid fibre reinforced polymer (FRP)-concrete-steel double-skin tubular columns (DSTCs), thin-walled hybrid DSTCs (TW-DSTCs) can largely retain HST columns’ main advantages of light self-weight and easy construction while they possess improved ductility and durability. Containing a thin inner steel tube and a thin concrete layer confined by FRP, TW-DSTCs may exhibit particular behaviour compared to ordinary hybrid DSTCs, whereas currently there is still a lack of understanding on their structural performances and load-carrying mechanism. To this end, this paper presents an experimental study on the behaviour of TW-DSTCs subjected to axial compression. Specimens with different inner tube strengthening strategies were tested and the experimental results were comprehensively discussed to reveal the working mechanism of the column. It is found that the failure of TW-DSTC specimens were induced by the large inward local deformation of inner tube and concrete crushing. Axial load-deformation relations of specimens were largely influenced by the strengthening strategies and levels. The inner tubes in TW-DSTC would not completely lose the load-carrying ability when the initial buckling occurs while good integration of the interface between the steel and concrete was important to improve the ductility of the column. Hybrid strengthening (HS) was a recommended strategy that combined distinct strengthening effects of T-section steel stiffeners and headed stud connectors. A simplified method developed based on existing theoretical model could reasonably predict axial load-deformation curves of specimens with rigid/hybrid strengthening whilst it led to discrepancies in predictions of those with flexible/no strengthening.