Research On The Design Method Of CFST-RC Transfer Columns

Concrete filled steel tube(CFST)members have the advantages of high bearing capacity,good plasticity and toughness,and convenient construction when the behavior of steel tube and core concrete are fully utilized.However,compared with reinforced concrete(RC)structures,the cost of CFST structures is relatively high.CFST-RC transfer columns generally appear at the upper part of a structural building to achieve the change of structural form.In general,the structures with RC columns are used below the CFST-RC transfer columns,and meanwhile the structures with CFST columns are adopted above the CFST-RC transfer columns.As a result,the behavior of RC and CFST structures can be fully explored,and the economic benefits and architectural effects of structures with CFST-RC transfer columns are better than the structures with only RC or CFST columns.However,the CFST-RC transfer columns are easy to become the weak zone of the building structures because of their sudden change in stiffness.This paper deeply investigates the hysteretic behavior and design method of CFST-RC transfer columns using the theoretical analysis method,and the results may be referenced for the design and analysis of practical structures with transfer columns.Main research works in this paper are as follows:(1)Review of the research status of related topics,including behavior of steel reinforced concrete(SRC)-RC and CFST-RC transfer columns at home and abroad,bond slip behavior between steel reinforcement and concrete,and bond slip behavior between steel tube and concrete,is carried out.(2)Nonlinear finite element analysis(FEA)models built using the large general purpose software ABAQUS are used to simulate the hysteretic performance of typical circular RC columns,circular CFST columns and one CFST-RC transfer column.The simulated results of FEA models are generally in good agreement with the measured results in the tests,and thus the rationality of the nonlinear FEA models is verified.(3)Based on the verified FEA model,the impact of physics,geometry and load parameters on the behavior and mechanism of CFST-RC transfer columns is studied.In addition,the quantifiable parameter that measures the effectiveness of CFST-RC transfer columns is determined,i.e.the elongation factor of reinforcement subjected to the maximum stress.(4)Based on the results of the systematic parameter analysis,combined with the regression analysis method,the simplified formulas for calculating the elongation factor of reinforcement in the CFST-RC transfer columns is put forward.The calculating results of the simplified formulas generally agreed well with the FEA results.