Study on axial compression bearing capacity of novel concrete-filled square steel tube columns

The way that strengthening the bond effect between the concrete and the inner steel by welding technology can effectively improve the axial bearing performance of the concrete-filled square steel tube (CFSST) column. However, this method requires high welding quality. In order to simplify the construction and reduce the difficulty of construction, two kinds of novel section forms of CFSST column without welding have been proposed. One is CFSST column reinforced with rhombic stirrups (specimens in SSSC group), the other is double-skin CFSST column (rhombic inside & square outside) (specimens in SSC group). A total of 21 specimens were fabricated for axial compression test, including 10 specimens in SSSC group, 7 specimens in SSC group, 1 CFSST column reinforced with inclined I-steel, 1 CFSST column reinforced with I-steel, 1 CFSST column reinforced with circle steel tube, and 1 CFSST column. The axial load-displacement curves of the specimens were obtained, and the failure modes and the influence parameters of specimens with different sections were analyzed and compared. The results show that the failure modes can be divided into local buckling failure and instability failure during plastic loading according to the aspect ratio. The specimens in SSC group have the highest average peak load, and the best residual bearing capacity, and the most eximious plastic strengthening characteristics in the later stage than other specimens. The decreasing rate of unit peak load of SSSC group was the slowest when the steel content was increased, which improves the bearing capacity overall. Increasing the core concrete area (inner steel tube side length) can enhance the bearing capacity of the specimens with longitudinally continuous build-in steel (reinforced with steel tube). While, it is not economical to improve the peak load of the specimens with large section (side length >200 mm) by increasing the thickness of the outer steel tube when the build-in steel is longitudinally discontinuous (reinforced with rhombic stirrups). Based on the result analyses, the calculation formulas considering the stability of axial compressive bearing capacity of specimens in SSSC and SSC group have been derived by superposition method. And the calculated results are in good agreement with the test results.