Equivalent stress-strain models of components in high-strength concrete-filled circular steel tube columns

Abstract

Nine circular concrete-filled steel tube (CFST) columns with 109 MPa concrete were tested under eccentric axial loading to derive the equivalent axial stress-strain models of the steel tube and concrete. Based on the measured strain data and the theory of elasticity and plasticity, the progressions of the axial and hoop stresses at different locations of the steel tube section were obtained. For the compression side of the steel tube, the hoop stress was developed at or after yielding; then, the ratio of the axial stress to the effective stress, σsa/fe, gradually decreased to a value of around 0.77. For the tension side of the steel tube, σsa/fe continued to increase after yielding and kept constant when σsa/fe reached a value of approximately 1.15. Using the developed axial stress-strain model of the steel tube, the equivalent axial stress-strain relationship of concrete was determined by matching the axial load-vertical displacement curve obtained from the fiber model analysis to that from the test. The confinement index, ξ, mainly influences the peak compressive stress and strain of concrete, whereas the eccentricity ratio has some influence on the descending branch of the stress-strain curve. The fiber model with the proposed stress-train models of steel and concrete can well simulate the behavior of circular CFST columns with high-strength concrete under compression-flexural loading when 0.35 ≤ ξ ≤ 0.85, 0.14 ≤ e/D ≤ 0.68, and 3.8 ≤ Le/D (Le is the effective length of a column) ≤ 10.6.