Research On Seismic Performance Of CFST Connections And Progressive Collapse Of CFST Frames

The concrete-filled-steel-tube (CFST) structure, which is a kind of composite structure, shows a variety of excellent mechanical properties and good resistance to extreme loading capacity. The CFST beam-to-column joint plays an important role when the structure subjected to extreme loading. In this paper, several kinds of reinforced concrete beam to CFST column joint has been investigated with both experimental and finite element methods, and the influences of these joints in seismic and structural progressive collapse has been analyzed. Moreover, the simplified progressive collapse analysis method is proposed.The main contents are as follows:(1) The seismic performance of three kinds of reinforced concrete beam to CFST column joints are studied with experimental methods. The joint types include welded joints, lap joints and new joint with reinforcement sleeve. The force-displacement hysteresis curves of these joints are obtained with experimental methods, and the influence of different bracket lengths are also studied as a parameter in the tests. The skeleton curves, ductility coefficient, viscos damping coefficient are obtained from the hysteresis curves. According to the test results, the welded joint and new joint with reinforcement sleeves show good performance in hysteretic loading and energy dissipation. However, the lap joints still meet the ductility requirement in seismic design, except for a great number of intial cracks in the beginning of the test, and it is not recommended for pratical use. Moreover, the failure mechanism of new joint with reinforced sleeves is discussed from the observation of the specimens during the tests, and the influence of steel bracket on the failure mode is also analyzed.(2) Cyclic loading on beam end and column top are carried on the same kind of joint. The hysteretic behavior obtained from the above two kinds of loading methods are compared and discussed. It is concluded that the loading on column top is more realistic than the other loading method, since the RC beam may subjected to tensile force when vertical load is applied on the beam end and the lateral displacement is restrained. The slippage of reinforcement is thus introduced obviously during the test. Accordingly, the experimental method with cyclic loading on column top is recommended in the future seismic experiments to replace the loading method with vertical loading on beam end. The previous loading method will provide more realistic joint response during earthquake, which will provide an important reference for practical design and for comparison.(3) Finite element analysis is carried on the specimens in the test, and the numerical results are in good agreement with the test results. A series of parametric analysis are conducted on the new joint with reinforcement sleeves, in which the beam, column, bracket and annular plate are investigated as geometry or material properties parameters. According to the FEA results, the design of column section, the thickness of steel bracket and the thickness of annular plate has little influence on the hysteretic behavior of the joints, while the design bending capacity of beam plays a more important role in seismic performance.(4) A three-stage skeleton curve model is proposed for the new joint with reinforcement sleeves. The calculation methods of the parameters in these stages are provided from the beam-column theory and limit equilibrium methods. FEA results are compared with the theoretical results, which show good agreement on the skeleton curves.(5) In addition to the great damage caused by the seismic hazard, the structural progressive collapse has also become a kind of severe structural damage reason in recent years. In this paper, several levels of simplified analysis methods are proposed for evaluating the potential of structural progressive collapse, and verified with finite element analysis method. Moreover, the simplified pseudo-static analysis method is also verified and developed in this paper, which is proven to be suitable for the situation with the membrane action of floor and the cases with several column deleted simultaneously. 2D floor membrane action amplification factor is thus proposed for the structures with floor.(6) The pseudo-static analysis method presented above are utilized to study the behavior of CFST structure with new reinforcement sleeve joint, and the influence of different factors on the potential of structural progressive collapse are analyzed based on the framework presented above.