Structural Behavior And Design Methods Of Rectangular Steel Tube-reinforced Concrete(ST-RC) Columns

Steel tube-reinforced concrete(ST-RC)column is a novel type of steel-concrete composite structure,which is composed of core Concrete Filled Steel Tube(CFST)and peripheral Reinforced Concrete(RC).Due to their excellent struc tural performance,ST-RC columns have been widely applied in practical projects,including high-rise buildings,station platforms and bridge piers in earthquake-prone areas.As a structural form widely used in practical engineering,the mechanical properties and design methods of ST-RC columns under axial compression and combined compression and bending are still lacking.The calculation formula and design methods of axial bearing capacity and combined compression and bending bearing capacity in existing ST-RC columns specifications also need to be further optimized and analyzed.Based on the existing theories,this paper focuses on the design method,numerical analysis and simulation of ST-RC columns under axial compression and combined compression and bend ing.The research contents of this paper can be summarized as follows:(1)Considering the secondary confinement effect of peripheral stirrups,an improved superposition method is proposed to accurately predict the axial bearing capacity of ST-RC columns.The confinement mechanism model of ST-RC columns under axial compression is established.The prediction formulas of axial bearing capacity considering secondary confinement effect is derived.The calculated results are in good agreement with a large number of published experimental data.(2)Considering the secondary confinement effect and the deformation compatibility between the core CFST and the peripheral RC,the analytical model of the axial force-strain full curve of ST-RC columns is established.The shortcomings of the conventional confined concrete theory in simulating the axial compressive performance of ST-RC columns are analyzed.The calculated results of the model are in good agreement with a large number of published experimental data,particula rly in terms of initial stiffness,peak strength,peak displacement,softening stiffness and ductility.The formulas for calculating the ultimate strength and ductility of ST-RC columns under axial force are also presented.(3)A numerical model of staged construction ST-RC columns based on fiber element analysis was established.By calculating the initial strain difference caused by stage construction process on the core CFST part,the influence of stage construction process is simulated.The comparison be tween the numerical model and the experimental results shows that the analytical model has satisfactory accuracy.By defining the efficiency coefficient and quantifying the key factors affecting the mechanical properties of the ST-RC columns,the influence of the critical parameters of the ST-RC columns on the actual engineering is evaluated.(4)A simplified formula for calculating the combined compression and bending capacity of ST-RC columns is presented based on the existing methods in ST-RC design provision.The main design idea and the possible problems of current provision CECS 188 are elucidated.The influence of common used parameters on the combined compression and bending of ST-RC columns is analyzed.Some reasonable suggestions based on the existing design provisions are presented,which may provide a useful reference for the design and application of ST-RC columns under combined compression and bending.