Abstract
Concrete filled steel tubes (CFT) are widely used in Japan as beam-columns for high-rise building frames because of the high stiffness, strength and deformability in comparison with the other structural members such as shaped-steel sections or vacant steel tubular sections. In this study, a CFT member with a pre-tensioned steel wire is proposed. In the member, an asphalt thin layer is placed on between steel tube and filled concrete, so that the no bond effect between those is expected. A pre-tensioned steel wire si placed at the center of a CFT section, which introduces the compressive pre-stress into the filled concrete. The bending test has been done, where the vertical load applies monotonically at the mid span of a simple supported beam composed of a CFT member with steel wire. The test parameters are (1) the diameter-to-thickness ratio (Dlt); and (2) the ratio of the pretension force to the axial strength of CFT section. The test revealed that the strength increased when the proper pre-tension force was introduced; the elastic flexural stiffness of the member was adequately evaluated by a simple summation of the concrete stiffness and the steel tube stiffness; and the prediction formulae specified in the present Japanese codes under-estimated or over-estimated by Dlt both the yield strength and the maximum strength of the specimen. Therefore, new strength prediction formulae have been proposed based on the test results.
