CFT Members Subjected to Torsion
As with shear, few tests of CFTs under torsional loading have been done. In the limited tests performed, concrete-filled steel tubes performed quite well under torsional loading. The nature of the steel tube itself is conducive to excellent torsional behavior. A closed section such as a tube has a much greater torsional resistance than a W-section with a similar area. Also, since torsional stresses increase with radial distance, the orientation of the steel (which has a much larger shear modulus, G, than the concrete) at the perimeter, where stresses are a maximum, idealizes the torsional resistance of the section.
Torsional failure in a CFT is not abrupt or distinct, but is characterized by a large increase in torsional rotation at a fairly constant load. The failure is due to a combination of spiral cracking in the concrete and tensile yielding of the steel. The effect of axial load on the torsional response is for the most part detrimental. Lee et al. (1991) found that for axial loads up to one-half of the ultimate axial load, an increase in the axial load resulted in a corresponding increase in the torsional resistance of the member. Xu et al. (1991), however, found that the greatest resistance of the section occurred in the case of pure torsional load and any increase in the axial load resulted in a decrease in the torsional resistance.