Experimental Research And Theoretical Analysis On Semi-rigid Composite Joints To Square CFDST Columns

Concrete-filled double skin steel tubular(CFDST)column as a novel composite member is composed of two concentric steel tubes and the concrete poured between the two layers of steel tubes.Compared with the traditional CFST column,the CFDST column possesses higher bending rigidity,lighter weight,better cyclic performance and greater fire resistance capacity.Recently,lots of experimental and analytical investigations have been conducted on the CFDST columns,but researches on CFDST column joints have just started due to the closed section of double-layer steel tube.It is difficult to connect steel beam and CFDST columns by common methods.Moreover,the existing rigid connection methods could not ensure that the load transmitted from the beam ends to the column.However,High strength blind bolts could be easily inserted and fastened from only one side which could solve the aboved problem.In addition,using high strength blind bolts colud save construction time and cost that satisfied the needs of the development of prefabricated construction.Thereforce,a novel connection to CFDST column using high strength blind bolts and end plates was proposed in this paper.A series of experiments and finite element analyses were carried out to investigate the seismic performance and failure mode of this type of joint.The main achievements of this paper are summarized detailedly as follows:(1)The pseudo-static experiments of four interior joints of the composite frame consisting of square CFDST column and steel-concrete composite beam have been carried out to investigate the failure mode and hysterical behavior etc.In the test,the cyclic loads were applied on the column end and the main parameters included the hollow ratio and the end plate types.The test results indicated that this type of joint possessed high bearing capacity and initial stiffness,good ductility and energy dissipation capacity,and the joints behaved in the semi-rigid manner according to the EC3 and EC4.(2)The finite element(FE)models of the CFDST column composite joints under cyclic loading were developed by ABAQUS according to the experimental study.The reliability and accuracy of FE models were verified by comparing with the test results.In addition,the nonlinear full-range analyses of the typical CFDST column composite joints were conducted to investigate the stress distribution and failure mode of the joint components at the characteristic points and to reveal the mechanism of this type of joint under l cyclic loading.(3)The parameter analysis models of the square CFDST column composite joints were established to explore the effects of of sixteen parameters,such as hollow ratio,bolt diameter and reinforcement ratio etc.(4)Based on the experimental research and FE analysis,the moment capacities calculation formulas of this type of joint were proposed.Moreover,some reasonable structural measures were studied through FE analysis.The aboved researches showed that the CFDST column composite joint performed favourable seismic behavior.The experimental and analysis results could provide the references for the design and application of semi-rigid composite joints to the CFDST columns.