## 3 Results and Analysis of Prefabricated Lightweight CFST Framework-Composite Slab Structure
### Characteristic Points of the Skeleton Curves  
For the experiments, the characteristic points were selected in accordance with several rules. The cracking status referred to the loading and displacement observed as minor cracks started to appear on the slab. The ultimate status and failure status were determined based on collected data, whereas the yielding status was calculated using the energy equivalence method. The frameworks of specimens did not yield when the specimens yielded, except for specimen FM. Considering this fact, the yield of the frameworks was defined as the status when the strain in the lower part of framework columns reached the yield strain. Table 6 showed the loads and displacement angles corresponding to the characteristic points of the skeleton curves of the tested specimens. Displacement angle \(\theta\) (%)--displacement at feature point (\(\triangle\)/mm)/distance from the embedded end of the frame column to the loading point (h/mm) \(\times\) 100%. An analysis of Table 6 revealed that the yield load, ultimate load, and yield load of the framework of specimen FM-S were 1.72, 1.80, and 2.03 times those of specimen FM, respectively. The ultimate loads of specimens FM-SW and FM-SD were 85.2% and 79.6% of the ultimate load of specimen FM-S, respectively. The yield loads of specimens FM-SW and FM-SD were 103.6% and 103.2% of the yield load of specimen FM-S, respectively, which indicates that an opening in the sandwich slab \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Specimen No.**} & \multicolumn{2}{c}{**Yield**} & \multicolumn{2}{c}{**Ultimate**} & \multicolumn{2}{c}{**Failure**} & \multicolumn{2}{c}{**Yield of the Framework**} \\ \cline{2-9}  & \(\mathbf{F_{y}}\) & \(\mathbf{\theta_{y}}\) & \(\mathbf{F_{max}}\) & \(\mathbf{\theta_{max}}\) & \(\mathbf{F_{u}}\) & \(\mathbf{\theta_{u}}\) & \(\mathbf{\theta_{u}}\) & \\  & **(kN)** & **(\%)** & **(kN)** & **(\%)** & **(kN)** & **(\%)** & **(\%)** & **F\({}_{yy}\) (kN)** & \(\mathbf{\theta_{fy}}\) (\%)** \\ \hline FM & 64.20 & 1.23 & 98.09 & 2.68 & 94.68 & 3.68 & 62.39 & 1.23 \\ FM-S & 110.42 & 0.35 & 176.83 & 3.49 & 170.50 & 3.96 & 126.78 & 0.88 \\ FM-SW & 114.43 & 0.59 & 151.75 & 3.50 & 146.25 & 3.98 & 132.75 & 0.85 \\ FM-SD & 113.94 & 0.82 & 144.08 & 2.48 & 140.68 & 3.44 & 116.45 & 0.87 \\ FM-SS & 186.27 & 1.28 & 202.90 & 1.68 & 153.78 & 3.47 & 155.51 & 1.00 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Characteristic points of the skeleton curves of the tested specimens. contributed to a smaller ultimate load of the specimen, but did not have a clear effect on the yield load of the specimen. The area of the window opening along the edges of the sandwich slab of specimen FM-SW was 2.86% smaller than the door opening where the sandwich slab was broken at the bottom of specimen FM-SD. However, at a displacement angle of 1%, the load borne by the first specimen was 8.27% greater than that of the second specimen. Moreover, the ultimate load of the first specimen was 5.32% greater than that of the second specimen, which indicates that the type of opening in the sandwich slab had some degree of impact on the ultimate load of the structure, with a greater impact of the door opening on the ultimate load. The frameworks of specimens FM-S, FM-SW, and FM-SD yielded a similar displacement angle, which indicates that an opening in the sandwich slab had no impact on the yield deformation of the framework. The yield load, ultimate load, and yield load of the framework of specimen FM-SS were 2.90, 2.07, and 2.68 times those of specimen FM, respectively, which indicates that the concealed steel-truss slab contributed to a much higher load-bearing capacity of the lightweight CFST framework. Although the sandwich wall was formed by connecting multiple sandwich panels with tongue-and-groove joints, the working performance of the wall structure was similar to a seamless wall under loads smaller than the yield load.