P-M Interaction Equations for Design of CFT Beam-Columns

Abstract

Concrete-filled steel tube (CFT) beam-columns are categorized as compact, noncompact or slender depending on the tube slenderness ratio (width-to-thickness b/t or D/t ratio, λ) and the slenderness limits (λ p , λ r , and λ limit ) specified by the AISC 360-10. The behavior and strength of CFT beam-columns depend on the tube slenderness ratio (λ), material strength ratio (F y /f' c ), axial load ratio (α) and member length-to-depth ratio (L/B or L/D). In previous articles, the authors have presented results of numerical investigations evaluating the effects of these four parameters, and developed P-M interaction equations for designing noncompact and slender CFT beamcolumns. This paper evaluates the applicability of the developed design equations for: (i) designing compact CFT beam-columns, and (ii) estimating the available strengths of CFT members in frames designed using the direct analysis method. The applicability for designing compact CFT beam-columns is evaluated by comparing the P-M interaction curves resulting from the developed equations with those calculated using the plastic stress distribution method specified by AISC 360-10. The applicability for the direct analysis method is evaluated by comparing the available strengths estimated using the developed equations with demands calculated by conducting second-order elastic analysis using MASTAN2 (V3.3).