The Interaction of Section and Member Slenderness on the Behavior of High Strength Composite Filled Tube (CFT) Members

The use of concrete-filled tube (CFT) columns as part of the lateral force resisting system or the gravity system of high-rise buildings is of interest due their potential stiffness, strength and ductility in terms of deformation and strain capacity. The steel tube or box section can provide stability during construction activities, and serve as stay-in-place (permanent) formwork during concrete placement. The structural effectiveness of these filled composite columns can be improved by using higher strength materials, since the elastic stiffness of concrete increases with strength. The economic efficiency of filled composite columns can be improved by using thinner but higher strength steel tubes. Overall, the structural and economic efficiency can be harnessed by using higher strength filled composite (CFT) columns with non-compact or slender cross-sections. However, there is limited research on the behavior, analysis and design of such members, particularly the interaction of local and global slenderness on the behavior and strength of high strength CFT members.

There has been significant interest in the behavior of high strength CFT columns in the recent past, and researchers (including the authors) have proposed phenomenological (effective) stress-strain curves for the steel and concrete materials that can implicitly model the effects of yielding, local buckling, biaxial stresses, concrete confinement and crushing. These effective stress-strain curves have been used to investigate and evaluate the behavior of high strength CFT members, but the investigations have been limited to short members with little to no global slenderness effects. This paper addresses this significant limitation by conducting extensive numerical investigations to evaluate the effects and interaction of member (global) and local (section) slenderness on the behavior and design of high strength CFT members including columns, beams, and beam-columns.

The parameters included in the experimental investigations were the steel strength (80 - 100 ksi), concrete strength (10 - 15 ksi), the section slenderness (non-compact, slender to maximum permitted), section aspect ratio (1:1 to 1:2), and the member slenderness (length-to-depth ratio from 5 - 40). The models were developed and analyzed using the 2D nonlinear fiber element in OpenSEES software and the effective stress-strain curves for the steel and concrete materials developed by prior researchers. Over 12,000 different cases were analyzed to evaluate the behavior high strength CFT columns, beams and beam-columns, and the influence of various geometric and material parameters including the interaction of local and global slenderness on the behavior and strength. The results from these evaluations will be presented along with fundamental insights into behavior. The potential simplification of these findings into design equations that can be used in everyday practice will be discussed.

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