## 4 Discussion
### Rotation Ability and Ductility Ratio  
The Chinese code GB50011-2010 [24] specifies an elastic layer angular displacement [\(\theta_{\text{e}}\)] of 0.004 rad and an elastic-plastic layer angular displacement [\(\theta_{\text{p}}\)] of 0.02 rad. As is shown in Table 4, for all tested joints, the yield connection rotation (\(\theta_{\text{y}}\)) is 1.55-3.13 times [\(\theta_{\text{e}}\)], and the failure connection rotation (\(\theta_{\text{f}}\)) is 1.62-5.13 times [\(\theta_{\text{p}}\)]. To satisfy the ductility requirement in seismic design, FEMA 350 [25] suggests a ductility limit of 0.03 rad. It is indicated by comparison that the anchored blind bolted connections show excellent rotation ability, satisfying the specified earthquake design requirements in both GB50011-2010 and FEMA 350.  
The concept of ductility is one of the important elements in the structural earthquake-resistant design. In this paper, the angular ductility coefficient (\(\mu_{\text{\theta}}\)) is adopted to define the ductility of the joint as follows:  
\[\mu_{\text{\theta}}=\frac{\theta_{\text{f}}}{\theta_{\text{y}}} \tag{4}\]  
where \(\theta_{\text{f}}\) is the failure connection rotation and \(\theta_{\text{y}}\) is the yield connection rotation.  
The ductility coefficients of the test specimens are greater than 3, and the test specimens show good ductility. These specimens that failed in mode I exhibited much greater ductility coefficients than those that failed in mode II. However, failure mode I is achieved by concrete crushing in the columns and the local buckling of the steel tube faces, and the full strength of the anchored blind bolts cannot be reached, resulting in a low capacity of connections. Additionally, the ductility coefficient of specimens ST-1 is 42% and 65% those of ST-2 and ST-5, respectively. These results indicate that the ductility coefficient increased with an increase in the end-plate thickness due to the high stiffness of the thick end-plate, which limited the rotation of the joint and slowed the decline of the bearing moment of the joints, resulting in a more ductile joint. Moreover, the ductility coefficient of specimen ST-5 is 3.4 times that of specimen ST-4 because using blind bolts without anchor nuts directly led to the change in the failure mode from mode II to mode I, resulting in a significant improvement in the ductility.  
Figure 13: Classification of tested connections.  
For the specimens that failed in mode II, due to the thicker end-plates used in specimens ST-4 and ST-6, premature fracturing of the bolts was observed, and the corresponding ductility coefficients of both connections were less than 4, demonstrating a lower ductility for these connections. However, with the weaker steel beam used in specimen ST-6, the ductility coefficient increased by 22% at the cost of a 19% reduction in the flexural resistance compared to that of specimen ST-4. The ductility coefficient of specimen ST-3 is 5.171, which is clearly greater than that of the other specimens, showing good ductility. Compared with specimen ST-4, the ductility coefficient of specimen ST-3 increased by 66%, indicating that the connection ductility clearly increased with a decrease in the end-plate thickness. However, the counterpart flexural resistance for specimen ST-3 decreased by only 26%. Therefore, with a thick steel tube, the use of thin end-plates and a weak steel beam could exhibit good ductility and seismic performance and ensure a high flexural resistance, which is in accordance with the \"strong column-weak beam\" design concept. In addition, a large bolt diameter and high bolt grade should be adopted to avoid premature brittle fracture of the bolt and fully develop the deformation performance of the end-plate and steel beam.