## 1 Introduction  
In recent decades, cold-formed steel (CFS) shear wall systems are considered to be one of the most feasible approaches for constructing residential and commercial low-rise and mid-rise buildings because this system has numerous benefits such as environmental-friendly and recyclable materials, lightweightness, high performance, cost-effectiveness, dimensional modularity and stability, and convenience for construction. CFS products have the highest overall recycling rate of 66% [1]. A series of studies have been performed on CFS shear walls [2], including board-sheathed CFS shear walls [3; 4; 5], steel-sheathed CFS shear walls [6], and CFS shear walls sheathed with double-layer wallboards [7]. These shear wall systems were usually used in low-rise buildings, however, such systems are inadequate for mid-rise buildings, because only wallboards supply lateral-resistance. Therefore, researchers have looked for more methods of strengthening such shear walls, such as addingsteel straps and trusses to the wall [8]. Strap-braced CFS walls were proposed by [PERSON] et al. [9] to verify the seismic design provisions of the American Iron and Steel Institute (AISI). A series of cyclic tests on CFS diagonal strap-braced stud walls was performed by [PERSON] et al. [10]. Knee-braced CFS shear walls were introduced by [PERSON] and [PERSON] [11]; both cyclic tests and numerical analyses were performed to study their seismic performance. In addition, framed stud walls with strap braces and steel sheathing [12] and other composite walls were investigated sequentially. However, the mechanical behavior of CFS shear walls was not changed though there were various improvements aforementioned in lateral stiffness and shearing strength. That is because the end studs of such shear walls are disconnected between the upper and lower stories, thus the lateral deformation of the walls is discontinuous between the layers. The walls would sway in a great lateral amplitude if/when subjected to earthquakes, which increases the collapse risk due to the P-delta effects. Therefore, in order to mitigate the disadvantageous influence of P-data effects and enhance the collapse-resisting capacity of the shear walls, [PERSON] and [PERSON] [13] and [PERSON] et al. [14] suggested that both continuous end studs and rigid beam-column joints should be adopted to constitute a defensive frame, which would supply a second defense to resist seismic actions. This idea has been accepted by many researchers recently [2; 15; 16]. An X-shaped built-up CFS beam was used by [PERSON] et al. [14] to prevent local buckling occurring in the flange of the beam. In addition, the beams and the overhanging plate of the end studs were connected by bolts to achieve a rigid beam-column joint. [PERSON] [13; 17] proposed a reinforced cold-formed steel (RCFS) shear wall system, in which a concrete-filled CFS tube was used as an end stud to prevent local buckling, and a reinforced block beam-column joint was presented in order to achieve a rigid connection. In addition, seismic performance tests on single-story [13], and two- and three-story [17] RCFS shear walls were performed by [PERSON] et al. A series of tests were conducted by [PERSON] et al. [18; 19] to investigate the mechanical behavior of concrete-filled CFS tubes, and the test results showed that such members exhibit better strength, stiffness, and ductility in comparison with the built-up CFS studs. Therefore, the RCFS shear wall provides the possibility of realizing multiple means of earthquake fortification for mid-rise CFS structure. In recent years, the seismic failure mechanism of CFS shear walls has remained a hot topic. A series of cyclic tests was performed by [PERSON] et al. [20], and three typical damage states were defined as follows: DS\({}_{1}\), superficial damage; DS\({}_{2}\), local damage; and DS\({}_{3}\), severe damage.