# Behavior of Two-Chord Steel-Concrete Composite Columns under Axial Compression
## 1 Introduction  
Existing reinforced concrete (RC) structures are often in need of improvements to their strength, capacity, and structural response. Many RC structures have insufficient resistance to meet the increased demands made by new seismic codes. Moreover, the need to strengthen an RC structure may arise from enlarged loads, the extension of buildings, or even overspacr insights in the design or construction process. A concrete-filled steel tube (CFST) was successfully used as a solution to improve the strength and ductility of an existing RC column as it was presented in [1]. RC structures strengthened with CFST have an improved load-bearing capacity for additional permanent, variable, accidental, and seismic loads. Columns built from CFST take advantage of the coupling effect formed between the concrete fill and the steel tube; this allows for the formation of hoop stress in the steel tube wall, which can raise the ultimate capacity of the column [2, 3]. The formation of tangential hoop stress will cause the development of a biaxial (plane) state of stresses in the tube and the development of a 3D stress state in the concrete infill. Furthermore, the concrete infill will also prevent the buckling of the tube's wall in an inward direction. The solution to improving the RC structure investigated in this research is the use of a built-up two-chord column made from vertical steel tubes (chords) filled with concrete. Twovertical circular tubes are sparsely interconnected with hollow horizontal tubes (battens). Two-chord columns could be used in existing RC structures as additional columns or a bracing system, or they could be used as the main load-bearing elements in different structures, such as residential or industrial buildings, bridges, and stadiums. Since the chords are primarily subjected to axial forces, they can take full advantage of the improved compressive properties of a CFST element, and slender CFST chords with relatively small cross-sections could be used. Latticed CFST models are lighter in weight and have increased flexural stiffness compared with singular CFST models with an equal ultimate capacity. In CFST columns, the most influential property of the concrete is the compressive strength. When the steel tubes are filled with normal-strength concrete (30-50 N/mm\({}^{2}\)), the concrete weight can represent a considerable addition to the intrinsic weight of the structure. High-strength and ultra-high-strength concrete infills provide the same beneficial effects as a steel-concrete composite with a significantly lower weight increase. The cross-section used for CFST latticed columns usually consists of two, three, or four chords, among which the four-chord ones are mostly used in practice. [PERSON] et al. [4; 5; 6] conducted experiments to determine the behavior of circular two-legged CFST latticed member loaded with cyclic lateral force. The results indicated that the deformability and load-bearing capacity of composite member were significantly better than those of the equivalent steel member owing to the presence of the concrete fill in the chord elements. Over time, researchers successively investigated the behavior of three- and four-legged CFST latticed member [7; 8; 9; 10; 11; 12; 13]. The factors considered in the investigations included axial capacities, the strength of the concrete in the chords, slenderness ratios, diameter-to-thickness ratios in the tubular battems and chords, the arrangement of the battems or braces, and seismic excitations. The applicability of high-strength steel and high-strength concrete as materials for composite columns was investigated in [14; 15; 16; 17]. High-strength materials yield the potential for weight and material reduction as a way to produce efficient structures. The utilization of high-strength steel enhances the overall behavior of structural elements.