Kitada and Nakai 1991 & Kitada 1992
A review of the ultimate strength and ductility of circular and rectangular concrete-filled steel tubes under a number of different types of loading was presented. Included in the review were discussions on columns, beam-columns, beams, members subjected to shear, and members subjected to torsion. Both monotonic and cyclic loading were examined. The authors cited a number of advantages of using CFTs: the buckling of the outer steel tube may be prevented by the concrete core; the concrete core's strength is augmented by the confining effect of the steel; the combined effect of the latter two advantages affords an increase in the ultimate load carrying capacity of the section. The CFT also has a greater ductility than steel or reinforced concrete.
Theoretical Discussion
Short Axially Loaded Columns. The authors compared three cases: a composite section with uniform loading of the steel and concrete, a composite section with only the concrete loaded, and a hollow steel tube alone. They showed that the compressive load of the two composite sections gradually increases (to the limit of their test) with increasing axial displacement, even in the region of large displacements, while the steel section shows a decrease after its smaller ultimate load is achieved. The composite section was shown to have 20% more strength than the sum of the individual steel and concrete components. Rectangular sections will not exhibit the increased strength of the circular sections, but will, however, still exhibit a substantial ductility.
Beams. CFT sections, both circular and rectangular, show substantial increases in strength and especially ductility over comparable steel sections, both with slip between the concrete and steel restricted and unrestricted. With the slip at the ends of the beam restricted, a significant gain in ductility can be achieved. Curves examining cyclic behavior showed that the decrease in strength with subsequent cycles in hollow square tubes could be alleviated by filling the tube with concrete. The CFT hysteresis loops were much more stable and showed little strength deterioration.
Short Beam-Column. The authors illustrated the advantage of using a circular CFT cross-section. The circular section has the ability to sustain a larger moment combined with an axial load. Also, the rectangular section loses its ductility under combined axial load and bending.
Shear. Graphs were presented to illustrate a short CFT's superior ductility over a comparable reinforced concrete section under the effect of a cyclic shear load. Again, the circular section exhibited a somewhat greater ductility than the rectangular section.
Torsion. The ultimate torsional moment of a CFT section is about 1.2 times the sum of the individual torsional resistance of the steel and concrete. The ultimate torsional moment can, however, be accurately predicted by assuming that the ultimate shearing stress of the concrete is equal to f'c/2. Both steel and composite specimens with rectangular cross-sections show good ductility. The circular CFT section, however, shows a superior ductility over the hollow steel tube and a much larger ultimate torsional moment. The behavior of the circular section is decidedly different from the rectangular section. In the circular section, the components behave independently. Therefore the torsional rigidity will rival that of the steel tube until yield, at which point the two materials behave compositely.
Reference
Kitada, T. and Nakai, H. (1991). “Experimental Study on Ultimate Strength of Concrete-Filled Square Steel Short Members Subjected to Compression or Torsion,” 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. 137-142. Kitada, T. (1992). “Ductility and Ultimate Strength of Concrete-Filled Steel Members,” Stability and Ductility of Steel Structures under Cyclic Loading, Fukomoto, Y. and Lee, G. C. (eds.), CRC Press, Boca Raton, Florida, pp. 139-148.