## 5 The Finite Element Model
### Validation  
The flexural behavior of the anchored blind bolted joints to CFST columns is analyzed, and the FE models are validated against the experimental results. The moment-rotation curves obtained by the finite element calculations are compared with the moment-rotation envelope curves obtained by the experiments in Figure 19. The curves of specimens ST-3, ST-4, and ST-6 approximately coincide. The curves of specimens ST-1, ST-2, and ST-5 approximately coincide in the early stage and exhibit few errors in the latter stage. Due to the influence of anchor nuts, anchoring bolts, and expansive sleeves, and considering the group anchoring effect and steel tube confinement, the multidirectional stress states of concrete between the steel tube wall and anchoring end were very complicated. Moreover, in the latter loading stage, specimens ST-1, ST-2, and ST-5 underwent severe concrete damage and bolt slippage, and many cracks would continue to occur until the core concrete in the panel zone was totally destroyed. Regardless of whether the ordinary concrete uniaxial pull-out model or the confined concrete model was used to simulate concrete behavior, it was difficult to fully reflect the true concrete behavior in the complicate state of stress. In addition, under cyclic loading, the cracking of core concrete would be aggravated. Therefore, the maximum moment of those specimens would be reduced to some extent, and the ultimate moment was hard to predict accurately. As shown in Table 6, only the initial stiffness value calculated using FE analysis for specimen ST-3 is underestimated. Through the comparison between FE analysis and the following theoretical formula derivation, this underestimation might occur because the test data of specimen ST-3 in the elastic stage had some measurement errors and were overestimated to a certain extent. The prediction of the initial stiffness obtained from the other FE models shows sufficient accuracy, with an average value of the FE/Test ratio of 1.088 and a standard deviation of 0.017. As shown in Figure 20, model validation is conducted by comparing the failure modes of the FE models with those observed in the experimental observations. The end-plates, the thick steel tube, the weak beam, and the bolts showed similar deformation in the test and FE models. The only differences observed between the experimental tests and FE models were for the thin tubes of specimens ST-1 and ST-2 due to the severe damage of the infill concrete, as mentioned above, which resulted  
Figure 18: 3-D finite element (FE) model and mesh of the connection components. in excessive outward deformation of the tube face. Therefore, the failure modes of the connections in both the experimental tests and FE models show good agreement. Through this validation of the tested connections, it is clear that the FE models could simulate the connection behavior successfully; in particular, the stiffness of the anchored blind bolted joints obtained in the early stage could be predicted by the models with sufficient precision. Therefore, the FE models are suitable for the following stiffness analysis. Figure 19: Comparison of test and finite element moment–rotation curves. (**a**) ST-1; (**b**) ST-2; (**c**) ST-3; (**d**) ST-4; (**e**) ST-5; (**f**) ST-6. Figure 20: Observed and predicted failure modes. (**a**) The flexural deformation of the thin end-plate in specimen ST-1. (**b**) The outward deformation of the thick end-plate in specimen ST-4. (**c**) The local buckling at the root of the steel beam in specimen ST-6. (**d**) The fractured bolt shank in specimen ST-6 (bending and shearing fracture).