Seismic behavior, analysis, and design of high strength square concrete filled steel tube (CFT) columns

The behavior of concrete filled steel tube (CFT) columns made from high strength materials was investigated experimentally and analytically. The effects of the width-to-thickness (b/t) ratio, yield stress (σ_y) of the steel tube, and the axial load level on the force-deformation response of the high strength CFT beam-columns were studied. Twenty three-fourth scale CFT specimens, which included four stub column specimens, eight monotonic beam-column specimens, and eight cyclic beam-column specimens were tested. The experimental results indicate that the load capacity of CFT columns is reached as inelastic behavior of the steel tube and concrete infill is combined with local buckling of the steel tube and crushing of the concrete.; Three-dimensional finite element method (FEM) models were developed for the CFT stub column specimens and analyzed under axial loading. The results from the FEM analyses compared favorably with the experimental results and offered significant insight into the behavior of square CFT columns. The results from the FEM analyses indicate that the concrete infill changes the local buckling mode of the steel tube, delaying local buckling, and the steel tube offers confinement to the concrete infill. However, the confining forces act primarily from the corners of the square cross-section causing triaxial compression in a core region and biaxial compression in the non-core region of the concrete infill. The steel tube is subjected to tensile hoop stresses, which reduce its longitudinal stress capacity.; Fiber-based models were developed for the monotonic and cyclic beam-column specimens. The results from the fiber analyses are governed by the stress-strain relationships for the fibers. Therefore, FEM-based effective stress-strain curves were developed for the steel and concrete fibers in compression. These effective stress-strain curves implicitly account for the effects of local buckling and biaxial stresses in the steel tube, and the effects of confinement of the concrete infill. The results from the fiber analyses compare favorably with the experimental results. The fiber-based models along with the method used to derive the effective stress-strain relationships are recommended for modeling high strength square CFT columns for the purpose of conducting static or dynamic analysis of structural frame systems.