# Numerical Simulation Analysis of Concrete-Filled Circular CFRP-Steel Middle Long Columns
## 6 Conclusions  
This study investigates circular CFRP-steel tube confined concrete intermediate columns, elucidating their working mechanisms and identifying key factors influencing axial compressive performance, thereby filling a critical research gap in this field. Combined with theoretical analysis, the calculation method of its bearing capacity is found, which can provide a reference for design and construction. Furthermore, by numerical simulation of ANSYS, the error of theoretical calculation will be reduced. The following conclusions can be obtained:  
(1) It can be seen from the axial compression test of concrete-filled circular CFRP-steel middle long columns that when the CFRP fractures, the bearing capacity of the column reaches the limit, and the circular steel tube bends obviously after the load continues to be applied. Finally, the instability failure of specimens occurs. (2) Experimental investigations demonstrate that both enhanced concrete strength and increased steel tube wall thickness contribute significantly to improved load-bearing capacity, while additional CFRP layers, though effective in strengthening confinement, exhibit diminishing reinforcement efficiency with progressive layering. Conversely, the bearing capacity displays a marked reduction as the slenderness ratio increases. (3) From the axial compression test results, the hoop force of concrete, the hoop tensile stress of CFRP, and the restraint effect of steel tube were analyzed theoretically. According to the three stages of analysis of the specimens in the state of compression--the elastic stage, the elastic-plastic stage, and the failure stage--the ultimate bearing capacity formula of concrete-filled circular CFRP-steel middle long columns was obtained. Comparing the theoretical calculation results with the test results, the error is less than 5%, which is in good agreement. It indicates that the ultimate bearing capacity formula can be applied to engineering applications. (4) ANSYS finite element analysis software simulation error is less than 4%, which can better simulate the test parameters. The axial load versus displacement of mid-span curves can directly reflect the elastic-plastic relationship of materials, which provides a reliable reference for the analysis of test results. (5) The number of CFRP layers, the strength grades of the concrete, the thickness of the circular steel tubes, and the slenderness ratio have a certain influence on the ultimate bearing capacity of concrete-filled circular CFRP-steel middle long columns. Among them, the number of CFRP layers has the greatest impact on the ultimate bearing capacity; the first layer of CFRP can be increased by more than 40%, and the second layer and the third layer of CFRP can improve the ultimate bearing capacity in a smaller range. So, the effect of single-layer CFRP on the ultimate bearing capacity of concrete-filled circular CFRP-steel middle long columns is the best, and the effect of multi-layer is relatively small, with a poor economic benefit. The strength grades of the concrete and the slenderness ratio have a relatively small effect on the ultimate bearing capacity. The influence of the thickness of circular steel tube is the least. Additionally, from the engineering economic point of view, increasing the number of CFRP layers to improve the ultimate bearing capacity of concrete-filled circular CFRP-steel middle long columns is economical and applicable. Methodology, S.W. and J.L. ; Formal analysis, S.W. and J.L. ; Data curation, S.W. ; Project administration, J.L. ; Funding acquisition, C.Z. and J.Z. All authors have read and agreed to the published version of the manuscript.