| Concrete-filled steel tube(CFST)is a kind of structural member formed by filling steel tube with concrete,these two materials work together to make it has good mechanical properties,so it is widely used in building structures.Special-shaped CFST represented by T-shaped,L-shaped and cross-shaped sections have appeared in recent years,it can be designed to be hidden in the four corners of buildings to provide prominent architectural functions,so it is very popular among designers and we need to conduct in-depth research on it.Theoretical analysis and experimental investigation methods are mainly used in this paper,key issues such as constitutive models of steel and concrete in CFST,the restraint of steel on concrete,the interface slip are disscussed,formula of in-plane stability of CFST compression-bending members is presented,approximation formulas of interaction surfaces when T-CFT and L-CFT under biaxial bending and axial force are proposed,also experimental investigation on seismic behavior of special-shaped concrete-filled steel tubular frames is conducted.Main contents of the dissertation are summarized as follows:(1)Base on the experimental results in the literature and theoretical analysis,design formula of axial compression bearing capacity for circular concrete-filled steel tubular short columns is obtained,a new calculation method of the stress-strain constitutive relation models for steel and concrete core is proposed.By comparing the theoretical analysis results of hoop effect of steel tubes in circular section and square section,formulas for calculating the equivalent hoop coefficient and reduction coefficient are proposed and also the situation of rectangular section is considered.(2)Eccentric compression tests of 12 rectangular CFST flat columns are carried out,the influences of slenderness ratio,hoop coefficient and eccentricity ratio on failure process,failure modes and mechanical properties are analyzed,suggestion of calculating the bearing capacity is proposed by comparing the experimental results with the satandard calculation results.A finite element model is developed to predict the eccentric compression loaded behavior of specimens,the rationality of constitutive relation models and finite element model are verified(3)The limitation and deficiency of the in-plane stability formula in the current code of CFST are analyzed,a unified formula for calculating the strength of rectangular and circular CFST compression-bending members is proposed.Based on strength theory,formula of in-plane stability of CFST compression-bending members is finally proposed through parameter correction,the results of the formula are compared with the numerical results obtained by the fiber model method and the exisiting test results,the accuracy of the formula is verified.(4)The slip stiffness of CFST is proposed,the axial force,interface shear stress and slip formulas under uniformly distributed and concentrated load are obtained,the value of interface slip stiffness is also obtained based on the experimental results in the literature.The CFST column model is established by ANSYS to evaluate that the cap on top is important to guarantee the composite action between the steel section and the concrete core.(5)Concrete-filled T-shaped/L-shaped multi-celled steel tube sections comprise of 4/3 rectangular sections are proposed,the ultimate bearing capacity under uniaxial eccentric load is studied the formulas to calculate axial force and bending are proposed.Interaction surfaces of this two cross sections under biaxial bending and axial force are analyzed,approximation formulas are finally proposed and good accuracy is verified.(6)Pseudo-static tests of 8 special-shaped concrete-filled steel tubular frame specimens are carried out,including 4 T-shaped specimens and 4 L-shaped specimens.Section size of columns and axial compression ratio are considered as experimental parameters and failure process and modes are observed,hysteretic curves and skeleton curves are obtained from the test results.The bearing capacity,ductility and energy dissipation capacity of specimens are analyzed,the results of the analysis are adopted to evaluate the seismic behavior and the engineering application of this kind of frame structure system.(7)Axial behavioral tests of 22 short CFST columns with through bolts are carried out,including 20 rectangular specimens and 2 special-shaped specimens,influences of bolt distance,bolt diameter and hoop coefficient are considered.Based on the load-average longitudinal strain curve,bearing capacity and ductility of the specimens are analyzed,also the experimental results are compared with the theoretical calculation results and the experimental results in the literature. |
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