Seismic Behavior and Design of High-Strength Square Concrete-Filled Steel Tube Beam Columns

The behavior of high strength square concrete filled steel tube (CFT) beam columns subjected to constant axial load and cyclically varying flexural loading was investigated experimentally. The effects of the width-to-thickness $(b/t)$ ratio, the yield stress $\left({\sigma }_y\right)$ of the steel tube, and the level of axial load on the behavior (stiffness, strength, ductility, and energy dissipation) of high strength square CFT beam columns were studied. Eight cyclic beam-column specimens were tested. Crushing of concrete and cyclic local buckling in the steel tube affected the cyclic strength and stiffness of the CFT specimens. The elastic section flexural stiffness under cyclic loading decreases rapidly due to tension cracking of the concrete infill and local buckling of the steel tube. The total energy dissipated by the cyclic specimens is dominated by the flexural energy dissipated in the failure segment (plastic hinge region). Both the current American Concrete Institute code provisions and the modified Architectural Institute of Japan method predict the moment capacities of the high strength square CFT specimens accurately. The cyclic curvature ductility $\left({\mu }_{\varphi -c}\right)$ decreases significantly with an increase in the axial load level. At higher axial load levels, the steel tube $b/t$ ratio and ${\sigma }_y$ have a small influence on ${\mu }_{\varphi -c}\text{.}$ However, at lower axial load levels, increasing the steel tube $b/t$ ratio or ${\sigma }_y$ reduces ${\mu }_{\varphi -c}\text{.}$