Analysis and Design of Noncompact and Slender CFT Beam-Columns

Abstract

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, λ) 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 (λ), material strength ratio (Fy/fc′), member length-to-section depth ratio (L/D), and axial load ratio (P/Po). The parametric analyses indicate that for L/D ratios up to 20, the P−M interaction curves are governed by the relative strength ratio (csr=AsFy/Acfc′). The parametric analysis results are used to propose revisions to the current standard’s interaction equations for designing noncompact and slender CFT beam-columns.