Unified calculation model of axial compressive capacity of steel tube and sandwiched concrete jacketed RC stub columns

The method of strengthening RC columns with steel tube and sandwiched concrete jackets takes advantage of the excellent mechanical characteristics of concrete-filled steel tubes, and has the advantages of high material utilization rate, minimal increase in section dimensions and easy for construction. However, there is no widely recognized or applicable calculation methods for the load capacity of the strengthened columns, which restricts the applications of this strengthening technology. Therefore, a unified model for calculating the axial load capacity of the strengthened RC columns were proposed. Firstly, considering the difference in the confinement effect on the concrete and the buckling characteristics of the steel tube, the different sectional forms of composite columns were transformed into equivalent circular sections for both inner and outer parts. Then, a unified model for calculating the load capacity of the strengthened columns was derived using the section force equilibrium equation, material constitutive relationship and circumferential deformation compatibility condition. The load capacities of 81 specimens with different section forms under axial loading were calculated and compared with the experimental results. The average value of the ratios between the calculated and the tested results is 0.947, and the standard deviation and the coefficient of variation are 0.077 and 0.081, respectively, exhibiting the accuracy as well as the safety of the unified model.