# Mechanical Performance of Built-Up Columns
## 1 Introduction  
Galvanized lipped channel sections are mainly used as the bearing components in cold-formed steel (CFS) wall-stud systems, which are confined to low-rise buildings. To apply this system to taller buildings [1; 2; 3; 4], several lipped channel sections are usually combined to form built-up sections. However, the inherent open and thin-walled characteristics of lipped channel sections cause them to suffer from local buckling and distortional buckling. Further, these buckling modes increase the complexity of bearing capacity calculation and greatly cause the decrease of bearing capacity. Therefore, recent research has primarily focused on eliminating the occurrence of such buckling modes and improving the bearing capacity of cold-formed thin-walled built-up sections. The most common built-up section is the combination of two channels placed back-to-back. Either they were welded together and researched by [PERSON] [5; 6] and [PERSON] [7] or connected through self-tapping screws and tested by [PERSON] [8] and [PERSON] [9] Later the channel section is improved at the web stiffened lipped channel section by [PERSON] [10]. In this way, the local buckling of web could be avoided. [PERSON] and [PERSON] [11] combined two angle steel through battened plates, which are connected to the angles by bolt or welding. [PERSON] [12; 13; 14]increased the number of angles from two to four to form a box section. In addition, he investigated the effect of the relationship between the slenderness ratio of angles between batten plates and overall slenderness of the column on the compression stability of the built-up section. Similarly, [PERSON] [15] and [PERSON] [16] directly welded two C-shaped sections to form a box section and investigated the effect of weld spacing on the stability performance of the box section. However, according to the results obtained in the above literatures, these built-up sections formed by open sections cannot effectively eliminate the occurrence of the local buckling of elements. Therefore, [PERSON] and [PERSON] [17] made some improvements on the form of the built-up section, that is, they combined three flipped channel sections into a T-shaped section. The local buckling of elements was effectively eliminated, but the calculation of bearing capacity is still very complicated due to torsional buckling and flexural-torsional buckling. To avoid these defects of the open sections, [PERSON] and [PERSON] [18] proposed an innovative fabricated column made of steel plates and steel tubes whose sections were closed. The mechanical properties of these sections were also studied in the subsequent literature [19; 20; 21]. Although their bending and torsional stiffness are improved, the steel plates and steel tubes were connected by welding, which prevented rapid construction. In addition, the hollow flange section [22; 23; 24] is also proposed to replace the open sections. [PERSON] and [PERSON] [25] performed an investigation on the structural performance characteristics of intermittently screw/rivet fastened steel hollow flange sections which are structurally efficient compared to the conventional open channel sections. However, the built-up sections can achieve a good mechanical performance unless a very small river spacing (<20 mm) was used. Based on the above literatures, a novel built-up section is proposed, which is formed by four galvanized cold-formed thin-walled square steel tubes assembled through self-tapping screws. However, for the novel section, the great difference from the most frequently used built-up sections (built-up section composed of open channel sections) is that the component is a square tube with a small section size, which is generally 40 mm \(\times\) 40 mm with a thickness between 0.9-2 mm. These tubes are usually assembled by several connectors, and each joint is generally connected by two self-tapping screws. The cross-sectional height of connectors is preferably not less than the cross-sectional width of tubes.