Research on Design Methods of Load-Carrying for Circular Concrete Filled Stainless Steel Tube Beam Columns

The concrete-filled stainless steel tubular (CFSST) structure combines the good mechanical properties of conventional concrete filled carbon steel columns and superior durability. Not only is the construction cost relatively reduced because of its infusion of concrete, but also when it is applied to offshore platforms, seaside buildings, bridges and super high-rise buildings, the later maintenance costs are also significantly reduced compared to conventional concrete——filled carbon steel tube structures. The CFSST structures have been used in actual engineering projects such as Stonecutters Bridge in Hong Kong and Hearst Tower in New York.
The existing specifications for the load bearing capacity of concrete filled steel tubular (CFST) columns, mainly due to the fact that the strain hardening characteristic of stainless steel is not beneficially considered, are all conservative. In order to make sure that the CFSST structures can be accurately evaluated in actual engineering projects, combined with the loading conditions of the circular CFSST beam-columns, the design method of calculating bearing capacity for circular CFSST beam columns is analysed. The existing Chinese national standard GB 50936-2014 Technical Specifications for Concrete Filled-Steel Tubular Structure, the local standard of Fujian Province DBJ/T 13-51-2010 Technical Specifications for Concrete-Filled Steel Tubular Structure, European Code Eurocode 4, and American Steel Structure Association Code ANSI/AISC 360 were used to calculate the bearing capacity of the collected 40 circular CFSST beam columns, and the calculated bearing capacity and the measured bearing capacity are compared:the average and variance of the calculated bearing capacity and the measured value of GB 50936 are respectively 0.871 and 0.105, the average and variance of the calculated and measured values' ratio of the bearing capacity in DBJ/T are respectively 0.868 and 0.073, the average and variance of the calculated and measured ratios of the EC4 specifications are 0.832 and 0.067, respectively. The average and variance of the calculated and measured values of the bearing capacity of the AISC specification are 0.612 and 0.122, respectively.
In view of the fact that the bearing capacity results of circular CFSST beam columns calculated by the above existing codes or specifications are all conservative, using finite element software, a finite element model of circular CFSST beam-column was established. On the basis of verifying the correctness of the model, the influence of grade of stainless steel, section steel ratio, core concrete strength and slenderness ratio on the axial force-bending moment correlation curve was conducted. The results show that the larger stainless steel yield strength and section steel ratio, the smaller abscissa and ordinate values of the equilibrium point in the axial force-bending moment correlation curve; the greater the strength of core concrete, the larger abscissa and ordinate values of the equilibrium point in the axial force-bending moment correlation curve. As the slenderness ratio increases, the the axial force-bending moment correlation curve tends to a straight line.
Finally, on the basis of the relative axial force-bending moment strength correlation equation of the circular CFST beam-columns recommended by the DBJ/T specification, the relationship between the equilibrium point coordinate value and the constraint coefficient is derived to apply for calculating bearing capacity of circular CFSST beam-columns.The simplified calculation formula of the circular concrete-filled stainless steel tubular bending capacity is more appropriate than the calculation formula of the existing codes or specifications, the calculated bearing capacity is closer to the actual measured value, and provide a reference for the engineering design of CFSST, and also provide a basis for the compilation of relevant codes or specifications.