Analysis and Design of Noncompact and Slender CFT Beam-Columns

Concrete-filled steel tube (CFT) beam-columns are categorized as compact, noncompact, or slender depending on the governing slenderness ratio (width-to-thickness $b/t$ or $D/t$ ratio, $\lambda$) of the steel-tube wall. The current AISC specification recommends the bilinear axial force-bending moment ($P-M$) interaction curve for bare steel members for the design of noncompact and slender CFT beam-columns. This paper compiles the experimental database of tests conducted on noncompact and slender CFT beam-columns, and demonstrates the overconservatism of the AISC $P-M$ interaction curve. This paper also presents the development and benchmarking of detailed 3D finite-element models for predicting the behavior and strength of noncompact and slender CFT members. The benchmarked models are then used to evaluate the fundamental $P-M$ interaction behavior of CFT beam-columns, and the influence of material and geometric parameters such as the tube slenderness ratio ($\lambda$), material strength ratio ($F_y/f_c^{\prime }$), member length-to-section depth ratio ($L/D$), and axial load ratio ($P/P_o$). The parametric analyses indicate that for $L/D$ ratios up to 20, the $P-M$ interaction curves are governed by the relative strength ratio ($c_{sr}=A_s{F}_y/A_c{f}_c^{\prime }$). The parametric analysis results are used to propose revisions to the current standard’s interaction equations for designing noncompact and slender CFT beam-columns.