Study of the concrete in reinforced concrete-filled steel tube column under axial loading

Abstract

Experimental and analytical studies are performed to determine the response of axially loaded reinforced concrete-filled steel tube stub columns (R-CFSTs) to the strength of concrete infill. The concrete strength was set as variable in the test, and six strength levels were considered. Three specimens were fabricated for each concrete strength, resulting in a total of 18 R-CFST specimens, along with another 18 concrete-filled steel tube (CFST) specimens for comparison. Based on the experimental results, the improving mechanism of reinforcing bars on the overall performance of R-CFST was investigated. The effects of the concrete strength on the load transfer performance, steel tube, reinforcing bar, failure mode, ductility, and strength of R-CFST were examined in comparison with the corresponding values associated with CFST. The strength prediction equations proposed for R-CFST were validated using experimental data from previous research. Further, analytical studies were performed to confirm the experimental results and proposed equations. A constitutive model for the concrete core of R-CFST is proposed, and a corresponding analytical method is introduced. Finally, specific analyses are performed on the steel ratios of R-CFST and CFST. The results indicate that R-CFSTs outperform CFSTs with respect to post-peak response, ductility, strength, and brittle fracture resistance. For the same steel ratio basis, R-CFSTs with thinner tubes perform well relative to CFSTs with thicker tubes. Recommendations to configure reinforcements to guarantee good performance by CFSTs when filled with high-strength concrete are provided. Furthermore, the proposed equations are recommended for more accurate prediction of the strength of R-CFSTs.