## 4 Discussion
### Moment-Rotation Envelope Curves  
The moment-rotation envelope curves for the test specimens, which are averaged for comparative purposes, are compared in Figure 11. The key parameters in moment-rotation relationship of joints are defined in Figure 12. According to the yield moment defining method in reference [21], equivalent yield point B is determined in the diagram and the corresponding moment and rotation are the yield moment (\(M_{\rm y}\)) and yield rotation (\(\theta_{\rm y}\)), respectively. The moment and rotation at the peak point are the measured flexural resistance (\(M_{\rm max}\)) and corresponding connection rotation (\(\theta_{\rm max}\)), respectively. According to Chinese specification JGJ3-2010 [22], the moment equal to 0.85\(M_{\rm max}\) is defined as the moment (\(M_{\rm f}\)) at failure, and \(\theta_{\rm f}\) is the corresponding connection rotation used to represent the rotation ability of a connection in this study. The main results and key parameters of the specimens are summarized in Table 4.  
Figure 11: Moment (M)–rotation(\(\theta\)) envelope curves. (**a**,**b**) Effect of steel tube thickness; (**c**,**d**) effect of end-plate thickness; (**e**) effect of anchorage method; (**f**) effect of beam section.  
Figure 10: Moment (\(M\))-rotation (\(\theta\)) hysteresis curves.  
By the comparison of the moment-rotation envelope curves shown in Figure 11a,b,e, it is immediately apparent that the flexural resistances of the failure mode I specimens were much lower than those for the failure mode II specimens; additionally, while the initial stiffnesses of the failure mode I specimens were slightly reduced, their ductilities were greatly improved.  
Figure 11a,b illustrate that the initial stiffness and flexural resistance of specimen ST-3 increased by 40% and 47%, respectively, compared to those of specimen ST-1 and that the initial stiffness and flexural resistance of specimen ST-4 increased by 22% and 84%, respectively, compared to those of specimen ST-2. The test results indicate that an increasing steel tube wall thickness had an improved effect on the stiffness and strength. As seen from parts (c) and (d) of Figure 11, the effect of end-plate thickness on the initial stiffness is similar to the effect observed due to the tube wall thickness, where the initial stiffness of specimens ST-2 and ST-4 increased by 42% and 26%, respectively, compared to those of specimens ST-1 and ST-3. However, in terms of the flexural resistance, when the connections failed in mode I, the influence of the end-plate thickness was negligible, as the flexural resistance was determined by the CFST columns; when the connections failed in mode II, a greater end-plate thickness increased the flexural resistance. Compared with specimen ST-1, the flexural capacity of specimen ST-2 increased by only 11%, while the flexural capacity of specimen ST-4 was 35% higher than that of specimen ST-3. Figure 11e shows that compared with those of specimen ST-5, the flexural resistance of specimen ST-4 increased by 22%, but the initial stiffness increased by only 7%. These results demonstrate that the use of the anchor nut in specimen T4 resulted in an increase in the flexural resistance as the failure mode changed from mode I to mode II but slightly increased the initial stiffness of connection. Figure 11f indicates that using a weak beam in specimen ST-6 resulted in a 19% reduction in the flexural resistance and an 8% reduction in the initial stiffness; however, significant improvement in the ductility of connection was observed.