Analytical Modeling Of Fire Behavior Of Slender Concrete-filled Tubular Columns

Concrete-filled still tubes(CFT) have been widely used in the construction of many tall buildings. This infatuation for this kind of structure is explained by the fact that its good compressive load carrying capacity, good ductility and convenient construction. In this paper, the temperature field of composite columns under standard fire were calculated by numerical analysis software ABAQUS. The working mechanism of slender CFT were discussed at room temperature and high temperature, the analysis of ultimate load capacity and ductility of CFT were carried out under the normal temperature, and parametric analysis of fire resistance time of CFT when exposed to fire were conducted. The main research work and achievements include:(1) This paper explores the thermal parameters of steel and concrete, steel-concrete interface thermal contact resistance, emissivity and convective heat transfer coefficient, and the effects of these parameters on temperature distribution of CFT in fire. By comparing with extensive experimental data, the accuracy and universality of the numerical analysis of temperature field were verified.(2) A sequentially coupled thermal-stress finite element model was developed using the program ABAQUS, featuring heat transfer analysis and stress analysis. The mechanical responses of composite columns under constant load were calculated, and verifying the accuracy of the results by comparing with experimental results. In order to get ideal results of finite element simulation, lots of sensitivity analysis which a iming at the main factors affecting the accuracy were conducted. Results show that: Coulomb friction model is suitable for accurate simulation of contact between steel and concrete, and coefficient of friction has little effect on results; Influence of initial deflection on mechanical properties are mainly in the descending of axial displacement, and larger the initial bending, axial displacement fast drop, fire shorter; Using the finite element analysis and calculation, adopting reasonable pseudo dynamic energy can effectively avoid artifacts appear sharply change; By using the thermal expansion coefficient of steel recommended by Japanese specification, small platform phenomenon in the decline of the axial displacement- time curve is not appeared, and this is more close to actual situation.(3) 26 accurate finite-element model are established in this dissertation, parameter analysis are conducted on mechanism of slender composite columns under high temperature: different slenderness ratio of the ultimate bearing capacity and ductility at room temperature, standard and finite element calculation of fire resistance time, steel tube diameter to thickness ratio, full and half fire analysis, loading rate effect, axial stiffness. Results showed that: at room temperature, with the decrease of regularization of slenderness ratio and ductility of CFT room temperature gradually increased; Three kinds of boundary conditions for F-F,P-F and P-P, with boundary constraints reduced, ductility declined; For columns with the same boundary conditions, model has better ductility of steel thick; For half and full fire, increases of thickness of steel tube improve the fire resistance of composite columns, and it is more obvious when subjected to half of the fire; For different load ratios, thickness of steel tube can also improve the fire resistance of composite columns, and as the load increases, increased less clear-cut; With the strengthening of borders, half and full fire endurance of the model were increasing; for the same boundary conditions, half fire resistance times were longer than a comprehensive fire resistance; For the different components with different boundary conditions, as the load increases, fire resistance of CFT times were reduced with different degrees; When the axial stiffness ratios k? =0.1,0.05,0.03 or 0.005, failure temperature of CFT is around 500?; When k? =0, the failure temperature of steel tube is close to 600 ?. For axial displacement, with the decrease of axial stiffness, axial expansion increases, and the faster the deformation of CFT when buckling occurred.