| The concrete-filled steel tube X-column(CFST X-column)is a kind of composite column that is formed by two crossing CFST columns,which can induce an axial force to resist the lateral load and overturning moment when the structure suffering wind or earthquake,maximizing its high strength.However,the “X” form leads to the changing of the shape and size of the cross-section along the height direction,causing some differences in interaction between its steel tube and core concrete from conventional CFST column,and the mechanism of interaction needs to be clarified.The previous research on the mechanical properties of such component is not yet systematic or in-depth.Therefore,the theoretical study,experimental study and finite element analysis on the mechanical property of the CFST X-column were conducted in this study;in addition,the application of the CFST X-column in the cooling tower structures was studied.The main research work and the results achieved are as follows:(1)The study on the mechanical property of the CFST X-column under axial compression: Firstly,an experimental study on CFST X-column under axial compression was conducted,the test results show that the failure of the specimens occurs in the cross area,and the separation between the steel tube and the concrete is observed near the internal corner.Then,the finite element(FE)model of CFST X-column under axial load was established.The behaviour of the CFST X-column under axial load is studied deeply,including the working mechanism,development of the stress of concrete at different sections and the effect of internal corner on the confinement effect.The parametric study was conducted to learn the effect of cross angle and material strength,etc.on the behaviour of the CFST X-column.At last,the calculation formulae of axial compression strength and capacity of CFST with irregular cross-sections with internal corners are deduced based on the unified theory.Considering the CFST X-column as a single member,the calculation formula of the axial compression capacity of the CFST X-column is proposed.(2)The study on the mechanical property of the reinforced CFST X-column under axial compression: Firstly,a joint strengthening method was proposed for the CFST X-column,and the strengthening law of each component of the joint strengthening method was studied.The calculation formula of axial compression capacity of reinforced CFST X-column was proposed through theoretical study.Then an experimental study on reinforced CFST X-column under axial compression was conducted.The test results show the failure of the specimens occurs in the short column area and the reinforced area of the rectangular connecting plate,and the average axial compression bearing capacity is 85%higher than that of the specimen without strengthening.At last,the FE model of reinforced CFST X-column under axial load was established.The behaviour of the reinforced CFST X-column under axial load is studied deeply with the FE method,including the working mechanism,distribution of the capacity and development of the stress.The parametric study was conducted to learn the effect of some parameters on the behaviour of the reinforced CFST X-column.The proposed calculation formula was verified by the results of the tests and FE analysis.(3)The study on the stability of the CFST X-column under axial compression: Firstly,the calculation method of the stiffness of the various sections and the stability capacity of the CFST X-column were proposed through theoretical study.Then,an experimental study of the stability performance of through-type CFST X-column and reinforced CFST X-column was conducted.The test results show the through-type specimens have the largest out-of-plane deformation on the middle section,while the reinforced specimens exhibit overall outward deformation in the cross area.At last,the FE model of slender CFST X-column under axial load was established to deeply learned the working mechanism of the progress of the instability failure of CFST X-columns.And the parametric study was conducted to learn the effect of some key parameters on the behaviour of slender CFST X-column.The proposed calculation formula to calculate the stability of CFST X-columns was verified by the results of the tests and FE analysis.(4)The study on the overall stability of CFST X-column system: the FE model of overall stability analysis of the CFST X-column system was established,and the buckling mode and the deformation of the ring beam were obtained.The buckle modes of the system are found to be antisymmetric overall bending buckling and symmetric overall bending buckling.The effect of the ring beam on the overall stability was learned,a nd it is found that with the greater the stiffness ratio of the ring beam to the column,the more likely the system is to undergo symmetrical overall bending buckling.And the overall stability capacity of the system can be improved by increasing the stiffness ratio of the ring beam to the column and the column inclination.Moreover,the FE model of the CFST X-column system consisting of the upper structures(cooling tower)was established,taking into account material,geometric non-linearity and initial defects,to study the overall stable performance of the CFST X-column system under a combined load of wind and gravity.(5)The application of the CFST X-column system was discussed.According to the structural characteristics and load forms of cooling towers,a FE model of the whole cooling tower was established,and the structural response of cooling tower structures with CFST X-column system and traditional reinforced concrete X-column system were compared under different load combinations,and some suggestions of design and selection of the crosssection of CFST X-column were put forward.A construction plan for a CFST Xcolumn system was proposed,with an analysis of its cost and duration. |
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