Experimental study on the tensile behavior of square concrete-filled steel tubes

• The tensile stiffness of SCFTs is 31.8% higher than that of hollow tubes.
• The confine-stiffening effect reveals the mechanism of this stiffness enhancement.
• The transverse-to-longitudinal strain ratio proves the confine-stiffening effect.
• The fractal cracking phenomenon is observed and quantitatively studied.

This paper presents an experimental study on the mechanical behavior, particularly the tensile stiffness, of square concrete-filled steel tubes (SCFTs) under axial tension, from both the force/displacement-level and strain-level perspectives. A group of SCFT and hollow tube specimens are tested with extensive data of the steel tube strains measured. From the force/displacement-level perspective, it is found that the tensile stiffness of SCFTs is 31.8% higher than that of hollow tubes on average. It is proposed that tensile stiffness enhancement is a combined result of the confine-stiffening effect and tension-stiffening effect. The confine-stiffening effect is because of the biaxial tensile stress state of the steel tube due to the concrete-to-steel tube confinement of SCFTs. From the strain-level perspective, the existence of the concrete-to-steel tube confinement is proved by comparing the measured transverse-to-longitudinal strain ratio of the steel tube between SCFT and hollow tube specimens. Besides, the evidence for the tension-stiffening effect in SCFTs is also provided by the non-uniformly distributed longitudinal steel strain of the steel tube. In addition, the fractal cracking phenomenon is experimentally observed and quantitatively measured, indicating the interaction of the concrete-to-steel tube confinement and tension-stiffening effect. It is concluded that the tensile stiffness of SCFTs will be underestimated if the SCFTs under tension are simply treated as equal to the hollow tubes by neglecting the concrete-to-steel tube confinement and tension-stiffening effect; this underestimation may cause non-conservative inaccuracies in calculating the component internal forces, structural natural frequencies, and seismic effects in the design and analysis of structures.