## 1 Introduction  
In recent years, load-bearing cold-formed steel (CFS) walls sheathed with gypsum plasterboard on either side have been increasingly used in low- and mid-rise buildings. Screw fasteners that connect the CFS frame and gypsum sheathing have become the main connection method for this wall system. Due to the lateral constraints provided by the screw connections, the bearing capacity of a CFS frame can be significantly enhanced. To prevent failure of the sheathing-fastener-stud sheathing, the maximum nominal load for the screw connection was given for the all-steel design method of CFS wall studs in the design manual of American Iron and Steel Institute (AISI) [1]. Moreover, screw connections make the walls behave similarly to diaphragms resisting in-plane lateral loads (wind and seismic loads) [2]. Hence, the mechanical behavior of the screw connections plays an important role in the design of CFS walls. Several experimental investigations [3; 4; 5; 6; 7; 8; 9] have already been conducted on the shear response of screw connections with gypsum sheathing or other board materials at ambient temperature. Gypsum sheathing does not have a preferential material response in a specific direction [7], and the failure of screw connections with single-layer gypsum sheathing was mainly identified as breaking or bearing of the loaded sheathing edge [6; 7; 8]. Furthermore, the effect of the loading rate, steel thickness, loaded edge distance and loading protocol were also discussed with explicit conclusions [5; 6; 7; 8; 9]. Some mathematical models, such as the Foschi model [10], Pivot model [9; 11] and Pinching 4 model [7], were used to describe the monotonic or cyclic load-displacement characteristic of connections at ambient temperature. However, previous research [3; 4; 5; 6; 7; 8; 9] mainly focused on screw connections with only a single layer of sheathing material at ambient temperature. In mid-rise buildings, double layers of gypsum sheathing are necessary on both sides of load-bearing CFS walls to satisfy the fire resistance rating of at least 60 min [12; 13; 14]. The lateral resistance performance of CFS walls with double-layer gypsum sheathing on both sides has been shown to be different from that of walls with single-layer sheathing [15; 16]. As described in the recently issued AISI standard [17], insufficient research exists to provide a definitive solution for the design of CFS walls with multiple layers of sheathing on an individual face of a wall. In addition, no literature was found regarding the shear response of screw fasteners with double-layer gypsum sheathing at ambient temperature. A few experiments were conducted on the mechanical behavior of CFS sheeting-fastener-sheeting connections at elevated temperatures [18; 19; 20]. However, investigation on the screw connections with board sheathing materials at elevated temperatures is still limited, so the mechanical contribution of gypsum sheathing in the previous theoretical and numerical investigation of CFS walls under fire conditions remains unknown [21; 22]. According to the previous full-scale experiments [13], the loaded CFS wall assembly lined with double layers of gypsum plasterboard on both sides was subjected to the ISO834 fire exposure of 71 min, and the flexural deflection of steel studs increased rapidly towards the fire side at the final two minutes of fire exposure. Post-fire inspection showed that the fire-side gypsum plasterboard had good deformation compatibility with the CFS frame, and only few cracks were observed in the field of fire-side gypsum plasterboard. In addition, due to the shrinkage of gypsum and degenerated mechanical behavior of screw connections, the breaking of the loaded sheathing edge was identified along the periphery of the fire-side gypsum plasterboard, leading to the opening of vertical board joints.