Design-oriented model for circular F-STC stub columns subjected to axial compression

A fiber-reinforced polymer (FRP)-steel composite tubed concrete (F-STC) column is formed by wrapping FRP sheets around a steel tubed concrete column (STC). Both the strength and deformability of the core concrete are improved by the FRP-steel composite tube and the steel tube is prevented from corrosion. A number of studies have been conducted on the prediction of stress-strain behavior of FRP-confined concrete columns and steel tube-confined concrete columns. However, few models focus on the concrete under combined confinement of FRP and steel tube. This paper presents an analytical study on the design-oriented model for F-STC columns. To determine the confining stress, the stress distribution of steel tube was firstly analyzed and the equivalent stress was obtained. In addition, a strain efficiency factor was proposed for the prediction of rupture strain of FRP. Then the peak condition and ultimate condition of F-STC columns were investigated and related predicting models for the load bearing capacity and deformation were proposed. Finally, design-oriented models were proposed for F-STC columns with different types of axial load-strain curves and generally coincided well with the existing test results.