## 4 Discussion
### Rotational Capacity and Ductility Ratio  
The rotational capacities of the specimens are presented in Table 4 and compared in Figure 17a. The European standard Eurocode-8 [27] specifies a connection rotation [\(\theta_{\mathrm{f}}\)] of 0.025 rad for the  
Figure 16: Classification of the specimens. (**a**) Specimens SJ-1 through SJ-5; (**b**) Specimens SJ-6 through SJ-8.  
connections in frames of medium ductility class (DCM). Table 4 shows that for all tested joints, the \(\theta_{\text{f}}\)-values are greater than 0.025 rad. To meet the ductility requirement for seismic design, the American standard FEMA-350 [28] suggests that \(\theta_{\text{f}}\) should not be less than 0.03 rad. Only the \(\theta_{\text{f}}\)-value of specimen SJ-4 is less than 0.03 rad, due to the premature brittle fracture of bolt. A comparison with the rotational capacity requirements in various codes shows that the anchored blind-bolted joint can exhibit a good rotational capacity and can satisfy the relevant earthquake-resistant requirements of both Eurocode-8 and FEMA-350.  
Ductility is a main parameter reflecting the post-yield deformation capacity and is also an important index in the seismic design of composite structures. The angular ductility coefficient (\(\mu_{\theta}\)) is used in this study to analyze the deformation capacity of the anchored blind-bolted joint under seismic loading, and this coefficient is expressed as follows:  
\[u_{\theta}=\frac{\theta_{\text{f}}}{\theta_{\text{y}}} \tag{5}\]  
where \(\theta_{\text{f}}\) is the failure connection rotation and \(\theta_{\text{y}}\) is the yield connection rotation.  
Figure 17b shows a comparison of the ductility coefficients of the specimens. The specimens with failure mode I (due to the outward deformation of the steel tube wall) and the specimens with failure mode III (due to the local buckling of the steel beam) both have good ductility. The specimens fail in mode II have good ductility if they have thin end-plates and weak beams but have poorest ductility if they only have thick end-plates, just like specimen SJ-4. However, for the specimens that fail in mode I, significant concrete crushing and local buckling of the steel tube faces occurred, and the blind bolts could not reach their ultimate strength, resulting in a low flexural resistance.  
The ductility coefficient of specimen SJ-5 was the highest among all specimens, indicating that the local strengthening of the connection without plug welding and the anchorage method without an anchor nut could greatly increase the ductility. A certain gap could be formed between the C-channel and the steel tube wall, and the absence of an anchor nut caused the bolt to slip to a certain extent, ultimately resulting in the maximum specimen ductility.  
The ductility coefficient of specimen SJ-8 with stiffeners was 24.3% higher than that of specimen SJ-6. This phenomenon mainly occurred because the stiffeners resulted in more pronounced local buckling at the root of the beam. It is indicated the use of stiffener can change the deformation pattern of the joint to improve the ductility and ensure the high strength and stiffness.  
The effect of the end-plate thickness on the ductility is related to the failure mode. For specimens SJ-1 and SJ-2, which exhibit failure mode I, the ductility of the two specimens was nearly equal because the joint failure was mainly governed by the steel tube wall. For the specimens that exhibit failure mode II, the ductility coefficient of specimen SJ-3 with thin end-plates was 47.6% higher than  
Figure 17: Comparison of the (**a**) Rotational capacity and (**b**) Ductility coefficient of the specimens.  
that of specimen SJ-4 with thick end-plates, indicating that the reduction in the end-plate thickness significantly enhanced the ductility of the joint.