Nie, Bai, and Cai 2008

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A new connection system for composite columns (concrete section with embedded concrete-filled steel tube) and reinforced concrete beams is proposed. Axial compression tests on six specimens and reversed cyclic loading tests on three interior column specimens and three corner column specimens were conducted to evaluate the proposed connection. The bearing strength of composite column and the effective confining radius in the stiffening ring are proposed.

Experimental Study, Results, and Discussion

This paper proposes a new connection system for CFT composite columns and RC beams. The steel tube is cut off and the steel reinforcing bar in the RC beam is continuous at the floor, and the transferring of the moment and the shear can be resolved. Concrete is confined by multiple lateral hoops in this area. The key to this new system is that the continuous RC beams are achieved by the interruption of the steel tube, which can be compensated and reinforced by the stiffening ring.

Two series of specimens with varying parameters including the ratio of reinforcement in the stiffening ring, the area ratio of column cross section to stiffening ring, different column sections and different thickness of steel tube, were tested under axial compression. Seismic performance and energy dissipation capacity of the system used for internal columns was investigated through reversed cyclic loading tests. Three specimens with identical column cross sections were designed on a 1:4 scale. The ultimate strengths of the columns were taken from the previous axial compression tests. A constant compressive axial force was applied to the column while four actuators at the beam ends exerted cyclic forces. The ends of the column were pinned. Another set of tests was conducted on three specimens designed as corner columns on a 1:2.7 scale. The loading pattern for these tests was the same as the pattern for internal column tests.

Testing demonstrated that the loss of confinement due to the interruption of the steel tube was fully compensated by the stiffening ring. Two failure modes, bending failure and shear failure, were observed in the two series of cyclic loading tests. Based on two series of cyclic tests, the “strong joints” can be achieved in both cases. Satisfactory energy dissipation and ductility can be obtained for both interior and corner column joints. Enough anchoring effects can also be obtained for corner columns, though some details should be avoided.

An effective confining radius based on the Lamé theory is proposed; this can be used to determine an effective domain of stiffening ring for this new joint. Stress and strain analyses are performed to determine bearing strength of composite columns and the joint in axial compression. A method to estimate the bearing capacity of concrete confined by multiple transverse hoops is obtained, which can be directly used to calculate the bearing capacity of joints confined by the stiffening ring. The bearing capacity of the composite column under axial loading is obtained based on the deformation compatibility of the internal CFT and external reinforced concrete. The thick cylinder model is very useful for evaluating materials under a confining status, based on which a further result is given to estimate the local bearing capacity of concrete, which compares well with experimental data. Based on the reversed cyclic loading tests, the Clough hysteresis loop is used to model the responses of specimens. It shows that the theoretical results are in good agreement with the experimental data for central column specimens.


  • Bai, Y., Nie, J., and Cai, C. S. (2008). “New Connection System for Confined Concrete Columns and Beams. II: Theoretical Modeling.” Journal of Structural Engineering, 134(12), 1800–1809. doi:10.1061/(ASCE)0733-9445(2008)134:12(1800)
  • Nie, J., Bai, Y., and Cai, C. S. (2008). “New Connection System for Confined Concrete Columns and Beams. I: Experimental Study.” Journal of Structural Engineering, 134(12), 1787–1799. doi:10.1061/(ASCE)0733-9445(2008)134:12(1787)