Mechanical Properties Of UHPC-filled Double Steel Tube Reinforced Cement-based Composite Columns

With the continuous development of modern building structures,ordinary concrete filled steel tubular structures have been unable to meet the high standards and requirements of current buildings.In order to meet the requirements of modern civil engineering for the bearing capacity,deformation capacity,and seismic performance of composite structures,based on the advantages of concrete filled steel tubular composite structures,this paper introduces ultra-high performance concrete(UHPC)and high ductility fiber reinforced cement based composite materials(ECC),and proposes a new type of built-in UHPC double steel tube ECC composite column.This type of composite column has high bearing capacity,good ductility,and seismic performance,It can be seen that this type of composite column is widely used in future building structures,so it is of great significance to study the mechanical properties of this type of composite column.Based on the ideal elastic-plastic model of steel,the basic structure model of fiber-reinforced cement considering constraints and the constitutive model of ultra-high performance concrete considering constraints,the finite element model of the fully-scale built-in UHPC double-steel tube ECC(EUCFDST)composite column is established by using the large-scale finite element analysis software ABAQUS.The load-displacement simulation curves of the existing test columns are obtained and compared with the test curves.Verify the rationality of the material constitutive model and boundary conditions.In order to study the axial pressure performance of full-scale EUCFDST composite columns,24 full-scale EUCFDST composite columns were designed by taking ECC fiber content,UHPC cylinder compressive strength,strength of inner and outer steel tubes,thickness of inner and outer steel tubes,diameter ratio of inner and outer steel tubes and slenderness ratio as main parameters.The finite element models of 24 full-scale axial compression specimens of EUCFDST composite columns were established,the axial compression load-displacement curves of the composite columns were obtained,and the failure mechanism,initial stiffness,ductility coefficient and stress-strain relationship of core concrete were analyzed.By introducing slenderness ratio coefficient,the full-scale calculation formula of ultimate bearing capacity of new EUCFDST composite column under axial compression is obtained by statistical regression.To study the eccentric pressure performance of full-scale EUCFDST composite column,35 full-scale EUCFDST eccentric pressure composite column specimens were designed with ECC fiber content,UHPC cylinder compressive strength,inner and outer steel tube strength,inner and outer steel tube thickness,inner and outer steel tube diameter ratio,eccentricity and slenderness ratio as main parameters.The finite element model of 35 full-scale EUCFDST eccentric composite columns is established,the eccentric load-mid-span deflection curves of the composite columns are obtained,the failure mechanism,ductility coefficient and stiffness degradation of the composite columns under different parameters are analyzed,and the assumption that the section of the composite columns meets the flat section is verified.By introducing eccentric correction factor and slenderness ratio correction factor,the calculation formula of ultimate bearing capacity of EUCFDST composite column under eccentric load is obtained by statistical regression.In order to study the seismic performance of full-scale EUCFDST composite columns,23 full-scale EUCFDST composite columns were designed with the main parameters of axial compression ratio,ECC fiber content,UHPC cylinder compressive strength,strength of inner and outer steel tubes,thickness of inner and outer steel tubes,diameter ratio of inner and outer steel tubes and shear-span ratio.The finite element models of 23 full-scale EUCFDST composite columns under horizontal reciprocating load were established,and the hysteretic curves and characteristics of the composite columns were obtained.The effects of different parameters on the seismic behavior of the specimen skeleton curve,ductility coefficient,stiffness degradation and energy dissipation were investigated,and the failure mechanism of the full-scale EUCFDST composite columns under horizontal reciprocating load was defined.Based on the simulation data,a full-scale load-displacement elastic-plastic restoring force model of EUCFDST composite column is established,which provides technical support for the promotion and application of this new type of composite column in practical engineering.