# Numerical Simulation Analysis of Concrete-Filled Circular CFRP-Steel Middle Long Columns
###### Abstract  
Based on the research on concrete-filled circular steel tubular columns, the influence of carbon-fiber-reinforced polymers (CFRPs) on the ultimate bearing capacity of concrete-filled steel tubes (CFSTs) was further explored in this paper. Ten different concrete-filled circular CFRP-steel middle long columns were made for an axial compression test, and the influence of the CFRP layers, the concrete strength grades, the steel tube wall thickness, and the slenderness ratio on the ultimate bearing capacity was discussed. Combined with theoretical analysis, the calculation method of ultimate bearing capacity of it was found. The load midspan deflection diagram was obtained by numerical simulation with finite element analysis software ANSYS2021R1, and the test results were compared. The results demonstrate that CFRP layers significantly enhance the ultimate bearing capacity of circular steel tube-CFRP confined concrete columns, with one to three layers increasing the capacity by 42.5%, 69.4%, and 88.4%, respectively, under identical conditions. In comparison, the concrete strength, the steel tube thickness, and the slenderness ratio showed lesser effects (\(<\)20% improvement), providing critical support for engineering applications of CFRP-confined circular steel tubular columns. Moreover, the error of ANSYS calculation results is small, which is in line with the test. This is of great significance to verify the correctness of the test of concrete-filled circular CFRP-steel middle long columns. steel tubes are mostly used in workshops, bearing columns, and arch ribs of bridges. For instance, the Yajisha bridge in Guangzhou is a half-through concrete-filled steel tube arch bridge, which is composed of six concrete-filled circular steel tubes forming a lattice arch rib. Similarly, the main tower of the Houhu stayed-cable bridge in Wuhan incorporates concrete-filled circular steel tubes. Carbon-fiber-reinforced polymer (CFRP) is a lightweight material with exceptional tensile strength [1; 2; 3]. It exhibits advantages such as corrosion resistance, fatigue resistance, and minimal creep. Applying CFRP to enhance the ultimate bearing capacity of CFST structures has become an inevitable trend. In this paper, the concrete-filled circular CFRP-steel middle long columns are made of circular steel middle long columns, which are poured with concrete inside and wrapped with CFRP on the outer surface of the steel tube in a circular direction. The axial compressive capacity of concrete-filled steel tubes is enhanced by the high strength tensile property of CFRP so as to strengthen the ultimate bearing capacity of concrete-filled steel tubes [4; 5]. Moreover, the bending stiffness and ductility of the CFST are improved, and the local buckling is prevented in the case of large lateral deflection. In the past two decades, many researchers have raised a variety of discussions in this field. For example, [PERSON] et al. [6] investigated the behavior of cold-formed thin-walled circular stainless steel tube stub columns fully wrapped with CFRP through axial compression tests and ABAQUS finite element analysis. Experimental and numerical results demonstrated that this strengthening method effectively enhances the ultimate compressive strength of columns. [PERSON] et al. [7] experimentally investigated the eccentric compression performance of RC columns strengthened with CFRP and steel fibers. Results demonstrated CFRP effectively enhanced both load-bearing capacity and ductility, while steel fibers mainly improved ductility. [PERSON] et al. [8] examined CFRP-strengthened circular CFT stub columns with initial imperfections. Using experimental tests and ABAQUS simulations with Riks nonlinear analysis, they found that horizontal imperfections more significantly reduce load-bearing capacity than vertical ones, while CFRP strengthening effectively improves capacity, alleviates stress concentration, and inhibits local buckling.