## 3 Results and Analysis of Prefabricated Lightweight CFST Framework-Composite Slab Structure
### Analysis of Strains  
Higher strains developed at the lower part of the framework columns and the ends of the specimen beam. Thus, an analysis of the cross-sectional strains in the lower part of the columns and at the ends of beams was performed on the typical specimens, FM, FM-S, and FM-SS. Figure 22 showed the cross-sectional lateral load-strain curves of the lower part of the columns of the typical specimens. The strains shown in the figure were measured at the middle of the steel-tube wall. Points 1 and 2 in the figure correspond to the yield loads, points 3 and 4 correspond to the ultimate loads, and points 5 and 6 correspond to the failure loads of the specimens.  
Figure 22a,b showed that, before the specimens yielded, the steel tubes basically retained an elastic stage, which was followed by a stage when the strains in the steel tubes rapidly increased with the lateral load. The specimen then directly entered a plastic strengthening stage, exhibiting no clear yield point. More specifically, the compressive (\(-\)) and tensile (\(+\)) strains of specimen FM under the ultimate load were 2000 \(\times\) 10\({}^{-6}\) and 2500 \(\times\) 10\({}^{-6}\), respectively, whereas those of specimen FM-S were 7000 \(\times\) 10\({}^{-6}\) and 2000 \(\times\) 10\({}^{-6}\). This was because the steel tubes of specimen FM-S, supported by the sandwich slab, were capable of a greater deformation or were capable of bringing the greater deformability of steel into full effect.  
Figure 22: Cross-sectional lateral load-strain curves of the lower part of the columns of typical specimens.  
Figure 22c showed that, under loads smaller than the yield load, specimen FM-SS exhibited an elastic strain. Under the yield load, the steel tubes exhibited a compressive strain close to \(2500\times 10^{-6}\). Under loads greater than the yield load, the compressive strain in the steel tube developed rapidly, because the columns of the framework acted as chords of the lightweight steel-truss at this stage and were, therefore, subjected to higher lateral loads.  
Figure 23a-c showed the lateral load-strain curves at the ends of the framework beams. The strains shown in the figures were measured at the flange of the beams.  
Figure 23 showed that, under the failure load, all strains of the specimens measured at the flange of the beams were smaller than the yield strain of \(1500\times 10^{-6}\). This was because the vertical load of the floor and slab on the framework beam acting on the steel H-beam were neglected for the test, and only the effect of the deformation of the framework columns on the steel H-beam was considered. This resulted in a smaller bending moment on the steel H-beam.  
The typical specimen, FM, was selected to verify whether or not the cross-sectional strain in the lower part of the columns were consistent with the plane cross-section assumption. The cross-sectional strain in the framework columns could be expressed with reference to the coordinate system defined in Figure 24.  
Figure 24: Cross-sectional location of the framework columns.  
Figure 23: Lateral load-strain curves at the ends of the framework beams.  
Figure 25 showed the distribution of the cross-sectional strain in the lower part of the columns. Under loads smaller than the yield load, the cross-sectional strain in the lower part of the framework columns of specimen FG were consistent with the plane cross-section assumption. Under loads higher than the yield load, the strain along the outer sides of the cross-section in the lower part of the framework column clearly increased, and the distribution of the cross-sectional strain was no longer consistent with the plane cross-section assumption.