## 5 Conclusions  
This paper reported a detailed study of 200 monotonic tests of CFS screw connections with gypsum sheathing at both ambient and elevated temperatures. Three loaded edge distances and single- or double-layer gypsum sheathing were taken into account in the experiments. The following conclusions are drawn from this work:  
1. The failure characteristic of the screw connections with double-layer gypsum sheathing in shear was different from that of single-layer gypsum sheathing connections at ambient temperature, and it could be described as the breaking of the loaded sheathing edge combined with significant screw tilting and loaded sheathing edge flexing fracture. The screw tilting and loaded sheathing edge flexing fracture disappear gradually at elevated temperatures.
2. Compared to the shear strength at ambient temperature, the shear strength of screw connection decreased sharply at 150 \({}^{\circ}\)C and 200 \({}^{\circ}\)C due to the gypsum dehydration and gradually declined from 250 \({}^{\circ}\)C to 500 \({}^{\circ}\)C.
3. The initial screw connection stiffness seems irrelevant to the loaded edge distance of more than 10 mm and single- or double-layer gypsum sheathing at ambient temperature. The shear strength and absorbed energy of the screw connection were significantly enhanced by increasing the loaded edge distance from 10 mm to 20 mm or replacing the single-layer gypsum sheathing with double-layer sheathing at both ambient and elevated temperatures.
4. The shear strength of screw connection could not be linearly superposed by the number of layers of gypsum sheathing because the shear strength of screw connection with double-layer gypsum sheathing is less than twice the screw connection strength with single-layer gypsum sheathing at ambient and elevated temperatures.  
Figure 17: Comparison of the predicted load–displacement curves to the experimental results: (**a**) screw connection with single-layer gypsum sheathing and a loaded edge distance of 15 mm; and (**b**) screw connection with double-layer gypsum sheathing and a loaded edge distance of 15 mm.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sheathing** & **Edge Distance (mm)** & \(A\) \\ \hline Single-layer gypsum & \(d=10\) & 4 \\  & \(d\geq 15\) & 18 \\ \hline Double-layer gypsum & \(d=10\) & 10 \\  & \(d\geq 15\) & 18 \\ \hline \hline \end{tabular} Note: If \(d\) lies between 10 and 15 mm, the values of \(A\) can be obtained by linear interpolation.  
\end{table}
Table 5: Values of the exponent \(A\) in Equation (13).  
* A unified design formula for the screw connection shear strength at ambient and elevated temperatures was proposed with sufficient accuracy, and it takes into account the effect of the loaded edge distance and double-layer gypsum sheathing. In addition, a simplified load-displacement model with the post-peak branch was developed to evaluate the load-displacement response of the screw connection with different loaded edge distances and single- or double-layer gypsum sheathing at ambient and elevated temperatures.  
This research was supported by the National Natural Science Foundation of China (Grant No. 51508088); Natural Science Foundation of Jiangsu Province of China (Grant No. BK20150605); the Priority Academic Program Development of Jiangsu Higher Education Institutions, Key Laboratory of Building Fire Protection Engineering and Technology of MPS (Grant No. KFKT2015 ZD05); Jiangsu Key Laboratory of Environmental Impact and Structural Safety in Engineering (Grant No. JKL2014K04); and Jiangsu Key Laboratory of Structure Engineering (Grant No. ZDI1402). The authors thank [PERSON] and [PERSON] for their kind assistance during the experiments.  
[PERSON] contributed to design and conduct the experiments, perform the data analyses and write the manuscript; [PERSON] contributed to the conception of the study; [PERSON] contributed to provide the test device and discuss the results.  
The authors declare no conflict of interest.