Displacement ductility of staged construction-steel tube-reinforced concrete columns

The steel tube-reinforced concrete (ST-RC) column is consist of a concrete-filled steel tube embedded in reinforced concrete. As an innovative type of composite structures, there is still a lack of information on the behavior and ductility of ST-RC columns, particularly on staged construction-steel tube-reinforced concrete (SC-ST-RC) columns. This paper studied the mechanism of two types of ST-RC columns in a 15 storey building. A finite element (FE) model with proper material constitutive relationship was proposed for SC-ST-RC columns subjected to combined axial compressive force and lateral loading. The material nonlinearity and the interaction between steel tube and concrete were considered. The proposed FE model was able to predict the lateral stiffness, strength and deformation capacities of SC-ST-RC columns with a reasonable level of accuracy. Then, the effects of different parameters on displacement ductility were discussed in detail. Finally, a simplified formula for calculating the displacement ductility of SC-ST-RC columns was developed based on the parametric analysis, the predictions of the proposed formula fitted well with a large number of test results. Utilizing the proposed formula, the demand of displacement ductility under different seismic design grade in current ST-RC specification was presented, which may provide a useful reference for the seismic design of the SC-ST-RC structures.