EXPERIMENTAL STUDY ON CYCLIC BEHAVIORS AND RESTORING FORCE CHARACTERISTICS OF CONCRETE FILLED STEEL TUBE COLUMNS USING ULTRA-HIGH STRENGTH STEEL

H-SA700 is a type of new ultra-high-strength structural steel that is more environmentally friendly and more suitable for mass production than conventional high-strength steel. The objective of this paper is to contribute to the practical application of CFT columns using such ultra-high-strength as a means to provide a significant increase in the seismic capacity of building structures. In this paper, the cyclic behavior of concrete filled steel tubes (CFTs) using H-SA700 was investigated experimentally. H-SA700 offers an approximately three time higher yield strain than that of conventional steel, and therefore, the CFT columns using H-SA700 reached their elastic limit when the infilled concrete sustained compressive strength, while the steel tube remained elastic at the moment. In such a case, it is unsafe to evaluate the strength by using the current design formula based on the superposed strength method. Due to the relatively low ductility of H-SA700, steel is not allowed to yield in the design of CFT columns. Nevertheless, the experimental results revealed that, comparing with CFT columns using conventional steel, the CFT columns using H-SA700 exhibited a substantially large capacity of plastic deformation, smaller axial deformation associated with local buckling, and a similar capacity of energy dissipation.