Effective stress-strain relationships for analysis of noncompact and slender filled composite (CFT) members

• Develops effective stress-strain relationships for noncompact and slender CFTs.
• Conducts comprehensive parametric analyses using benchmarked FEM models.
• Uses parametric analysis results to develop the effective stress-strain relationships.
• Includes the effects of imperfections, local buckling, hoop stresses, and confinement.
• Evaluates the effective stress-strain relationships using fiber-based analysis models.

Fiber-based section analysis methods are widely used to model and predict the fundamental axial force-bending moment-curvature (P-M-ϕ) behavior and the strength interaction (P-M) of concrete-filled steel tube (CFT) members. The accuracy of these predictions is governed by the uniaxial effective stress-strain relationships assumed for the steel tube and concrete infill of the CFT section. Prior research has developed these effective stress-strain relationships for compact CFT members. This paper presents the development and verification of effective stress-strain relationships for the steel tube and concrete infill of noncompact and slender CFT members. These relationships are developed using results from comprehensive 3D finite element analyses of CFT members with a wide range of geometric and material parameters. The 3D finite element models, which were developed and benchmarked previously by the authors, accounted explicitly for the effects of steel yielding and tube local buckling, concrete cracking and compression inelasticity, and the transverse interaction leading to steel hoop stresses and concrete confinement. As a result, the developed effective stress-strain relationships also accounted (implicitly) for these complexities (yielding, local buckling, confinement, etc.) of behavior. These effective stress-strain relationships are implemented in a nonlinear fiber analysis (NFA) macro model, and used to predict the behavior of noncompact and slender CFT members in the experimental database. The conservatism of the predictions using the effective stress-strain relationships is evaluated.