Sakino and Hayashi 1991
The axial load-longitudinal strain behavior of CFT stub columns with circular cross-sections was analyzed and compared to experimental tests. The effect of different D/t ratios and different concrete strengths was investigated. Studies were also conducted regarding the ratio of hoop strains to longitudinal strains in the steel tube. The main objective was to estimate the effects of both strain hardening of the steel tube and the triaxial confinement of the concrete core. The introduction to the paper presented a concise summary of the nature of stresses in the components of a CFT as the load increases.
Experimental Study
Annealed steel was used such that no residual stresses existed in the tube specimens. Hoop and axial strains were carefully measured to study the volumetric expansion, or dilatancy, of the concrete at higher strains.
Analytical Study
The plastic theory developed in the paper was based on the following assumptions to calculate the relationship between axial force and longitudinal strain in the column. The concrete was isotropic, had an elastic-perfectly plastic stress-strain curve, and adhered to a nonlinear constitutive law in the elastic range; the steel stress-strain curve was represented by a five-line curve simulating test results; the steel tube was in a biaxial state of stress and the concrete was in a triaxial state where the two principal stresses, hoop stress in the steel and radial stress in the concrete, were equal; octahedral stress components were used for the concrete and the von Mises yield criterion defined the elastic limit for the steel; the associated flow rule was adopted for both the steel and the concrete; and isotropic hardening of the steel was assumed.
Comparison of Results
The analytical and experimental results agreed quite well except for the specimens containing 'high-strength' concrete (6.5 ksi) and having a large D/t ratio. To predict the behavior of high-strength concrete in large deformation regions, the effect of work softening must be considered. The observed maximum axial load capacities were 1.12-1.25 times the analytical capacity, with the effect of strain hardening ignored. The theoretical and experimental results showed that the ratio of the hoop strain to the longitudinal strain became greater than 0.5 under large strains, indicating the concrete dilates in the plastic region. The hoop strain to longitudinal strain ratio increased with an increase in the D/t ratio and increased slightly with an increase in the concrete strength. However, the theoretical values were less than the experimental, prompting the authors to suggest the need for an alternative to the assumed associated flow rule.
Reference
Sakino, K. and Hayashi, H. (1991). “Behavior of Concrete Filled Steel Tubular Stub Columns Under Concentric Loading,” Proceedings of the Third International Conference on Steel-Concrete Composite Structures, Wakabayashi, M. (ed.), Fukuoka, Japan, September 26-29, 1991, Association for International Cooperation and Research in Steel-Concrete Composite Structures, pp. 25-30.