Xu, Zhou, and Lee 1991
The experimental behavior of CFT members under combined compression and torsional loads was discussed. The authors examined the effect of parameters including the ratio of compressive to torsional load, the ratio of steel to concrete, the strength of the constituent materials, the loading path, and oiled versus non-oiled inner steel surfaces. The experimental results were rather abbreviated. Forty-seven specimens were tested under nonproportional monotonic axial force plus torsion loading and six were tested under axial force plus nonproportional cyclic torsion loading.
Experimental Study, Results, and Discussion
The tested specimens showed good torsional resistance and exhibited axial bearing capacity even at the member's ultimate torsional strength. This indicated that as the tube bulges and creases, the concrete rapidly fills the voids created between the materials, keeping both materials in an active role in resisting the axial load. Failure of the specimens began with a cracking of the concrete. Torsional resistance at this point was still influenced by the interlock of the aggregate particles as well as the friction caused by the axial load. Therefore, the authors found that up to a point (it varied with section properties -- usually about one half of the ultimate axial load) an increase in axial load produced a corresponding increase in the torsional moment the section was able to resist. This increase in torsional moment was not observed in tests of plain concrete or hollow tubes.
Reference
Xu, J., Zhou, J., and Lee, G. (1991). “Experimental Studies on Concrete Filled Steel Tubular Medium and Long Columns Under Compression and 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. 159-164.