Behavior And Design Method Of Concrete Filled Circular Steel Tubular Structures

This paper presents theoretical analyses and experimental studies deeply on the behavior of concrete filled circular steel tubular (CFST) structures both at room tempera -ture and in fire. Strength criterion and constitutive model for concrete under multi-axial stress states both at room temperature and at high temperatures were proposed, the behavior of CFST stub columns under axial compression both at room temperature and at high temperatures were discussed, the theoretical models of CFST columns under eccentric compression at room temperature were developed, the experimental investigations on the behavior of CFST specimens at room temperature were presented, the nonlinear finite element analysis theories of CFST structures subjected to in-plane load both at room tempe -rature and in fire were founded, and the fire performance of a full scale CFST plane frame was analyzed. The main contents are listed as follows:(1) Strength criterion for concrete under multi-axial stress states both at room temperature and at high temperatures was founded, and axial stress-stain relations of laterally confined concrete under axial compression and hysteretic stress-stain relations of concrete under uniaxial stress states both at room temperature and at high temperatures were proposed.The total principal strain of concrete was classified into elastic principal strain and inelastic principal strain, the conception of damage Poisson's ratio was presented, supposing the inelastic principal strain was related to damage Poisson's ratio, and then the theory of damage mechanics and the theory of minimum energy dissipation rate were applied. The general formula of strength criterion for concrete under multi-axial stress states was proposed. The failure mechanism of concrete was illustrated. The equation for damage Poisson's ratio was determined basically based on the existing experimental data. Through analyzing large numbers of test results of uniaxial mechanical properties of various strength of concrete, the unified formulas for mechanical indexes, the unified method of uniaxial stress-strain relationship, the evolvement law of damage, damage constitutive model, and the hysteretic stress-stain relations for concrete both at room temperature and at high temperatures were proposed. Based on triaxial stress-strain relationship for concrete proposed by Ottosen, the axial stress-strain relations for laterally confined concrete under axial compression both at room temperature and at high temperature were developed, in which the expression of peak Poisson's ratio of concrete reflected the feature of damage Poisson's ratio. These models give a relatively reasonableestimate of the experimental behavior.(2) The elasto-plastic analysis method of CFST stub columns under axial compression both at room temperature and at high temperatures was proposed, and thus the confinement effect of CFST stub columns under axial compression was illustrated. Utilizing the axial stress-strain relations for laterally confined concrete under axial compression, based on continuum mechanics, the mechanical model of concentric cylinders of circular steel tube with concrete core was determined. The elasto-plastic analysis method of CFST stub columns both at room temperature and at high temperatures was proposed, and a computer program was developed. Through analyzing the behavior of CFST stub columns, it was pointed out that the confinement effect decreased when axial pre-stress in steel tube, and for the concrete confined steel tube, the confinement effect strengthened but the composite elastic modulus diminished.(3) Practical calculation formulas of load bearing capacity for CFST columns under axial compression, pure bending, and eccentric compression were proposed, and practical calculation methods of axial force-strain relations and axial force-moment-curvature relations for the composite section of CFST beam-columns were presented. Based on elasto-plastic analysis method, the numerical constitutive model for the concrete filled steel tubes was determined, the practical calculation formulas of composite axial stiffness and axial ultimate capacity for CFST stub columns were proposed, and the practical calculation methods of axial force-strain relations for the composite section were presented. Based on plane section assumption, layered method was applied to calculate the axial force-moment-curvature relations for the composite section of CFST beam-columns, the practical calculation formulas of composite flexural stiffness, flexural capacity, axial force-moment interaction equation were proposed, and practical calculation methods of axial force-moment-curvature relations for the composite section of CFST beam-columns were presented. Fiber model method based on partial sinusoidal shape was applied to calculate the load-deformation relations of CFST columns under eccentric loading with equal or unequal end-moment, and the practical calculation formulas of load bearing capacity for CFST columns under eccentric loading were presented. Good agreement is obtained between these predicted results and experimental results.(4) Experimental investigations of 67 CFST specimens at room temperature were presented, the behavior was discussed, and the calculation theories and methods by the author were validated.45 CFST stub columns under concentric compression, 4 CSFT members under pure bending, and 8 CFST columns under eccentric compression with equal end-moment wereinvestigated, the behavior of load capacity, load-deformation relations, and load-strain ratio relations of the specimens were discussed. Combined with the test results by other researchers, the reliability of the elasto-plastic analysis method, layered method, fiber model method, the practical calculation formulas and the practical calculation methods was verified.(5) The nonlinear finite element analysis theories of CFST structures subjected to in-plane load both at room temperature and in fire were founded, and finite element progra -m named NACFSTLF was developed.An appropriate numerical thermal-stress coupling constitutive model of both steel tube and concrete core and its calculation method in concrete filled circular steel tubes in fire were proposed. Based on the principles of continuum mechanics, the U.L. formula of element incremental equilibrium under fire condition was deduced. Through adjusting the strain at centroid of cross section and the curvature, the external nodal loads was in equilibrium with the internal stress, and thus the layered finite element method was perfected. A FORTRAN program named NACFSTLF was developed and the reliability of the program was tested by the experimental results of CFST columns and structures, and reinforced concrete columns both at room temperature and in fire, showing that the analysis results are in good agreement with the experiment results from references. The program reflects the confinement effect and the temperature-load path of CFST structures in fire, and holds the virtue of higher calculative function and efficiency.(6) Applying the program of NACFSTLF, the fire performance of full scale single CFST columns with both fixed ends condition, full scale reinforced concrete rectangular beams with both pinned ends condition, and full scale CFST plane frame consisting of circular CFST columns and reinforced concrete T beams was analyzed.The influence of initial defect of the single CFST column, axial force ratio, strengths of concrete, strengths of steel, steel ratio, and the outer diameter of steel tube on the fire performance of CFST columns with four surfaces exposed to fire was discussed. The fire performance of full scale reinforced concrete rectangular beams with three surfaces exposed to fire was investigated. The fire performance of a full scale CFST plane frame of six-storey (6×4.5=27 m) and three-bay (3×9=27 m) under storey fire condition was analyzed. From the result, it is indicated that: the worst condition of the full scale frame was the middle CFST column of ground floor when the fire happened in the floor, and the failure mode was stability failure due to decrease of load capacity; due to the redistribution of internal forces, the fire resistance of the frame would be slightly higher than that of single columns of both fixed end condition in fire, and the CFST columns in the structurealso need protection under fire condition.