## 4 Conclusions and Future Work  
The following conclusions may be drawn within the limitations of the research:  
1. Failure mode of CFCSTCs under axial compression can be divided into two serials. Mode 1 is that local buckling occurs at the impact location; Mode 2 is that local buckling happens at the upper zone and underside zone of mid-span. Mode 1 most likely occurs on samples with bigger \(H\), while Mode 2 easily occurs on samples with lower \(H\) or with bigger \(D/t\) ratio under the same \(t\) or with lower \(D/t\) ratio under the same \(D\).
2. The transverse impact height of drop-hammer (\(H\)) and \(D/t\) ratio have noticeable effects on the axial compression performance of CFCSTCs. Bearing capacity (\(P_{\rm u}\)) decreases with the increasing \(H\). For samples with same \(D\) and \(t\), the highest \(P_{\rm u}\) is obtained by the undamaged sample, while the lowest \(P_{\rm u}\) is achieved with the sample with \(H\) of 4 m. The maximum reduction of \(P_{\rm u}\) reaches about 35%, compared with that of undamaged columns. The influence of \(H\) on \(P_{\rm u}\) is obviously smaller with increasing \(D\) or \(t\). Moreover, the residual mid-span lateral displacement (\(\Delta_{\rm r}\)) increases with increasing \(H\). In addition, under the same \(H\), the \(P_{\rm u}\) decreases with increasing \(D/t\) ratio under the same \(D\), while decreases with decreasing \(D/t\) ratio under the same \(t\); conversely, \(\Delta_{\rm r}\) increases with increasing \(D/t\) ratio under the same \(D\), but decreases with increasing \(D/t\) ratio under the same \(t\).
3. For \(P\)-\(\Delta_{\rm v}\) curves of CFCSTCs under axial compression, both the \(P_{\rm u}\) and the vertical displacement corresponding to \(P_{\rm u}\) increase with the increasing \(D\), whereas the influence of \(H\) on the \(P\)-\(\Delta_{\rm v}\) curves decrease significantly with the increasing \(D\). Furthermore, with the increasing \(t\), both the \(P_{\rm u}\) and the initial slope of the \(P\)-\(\Delta_{\rm v}\) curve increase, while the vertical displacement corresponding to \(P_{\rm u}\) decreases.
4. A three-dimensional FEA model is proposed to predict the response of CFCSTC under axial compression. A reasonably good agreement is achieved between the predicted and the experimental results in terms of deformation and stress distribution.  
Future research will involve conducting the experiment on the performance of CFCSTC with axial load under transverse impact, and then analyzing the residual bearing capacity. Lastly, through theoretical analysis and numerical simulation, simple design calculations can be proposed.  
Figure 15: Effect of \(D/t\) ratio on bearing capacity.  
**Author Contributions:** [PERSON] incepted the research; [PERSON] and [PERSON] designed the experiments; [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] conducted the experiments; [PERSON], [PERSON] and [PERSON] analyzed the data; [PERSON], [PERSON] and [PERSON] wrote the paper.  
**Acknowledgments:** The research presented in this paper is part of Project (51778064) supported by National Natural Science Foundation of China, the Project (2016D-5007-0605) supported by Petroleum Science and Technology Innovation Foundation of China, and the Project (2016 CFA022) supported by Natural Science Foundation of Hubei Province, China. The financial support is highly appreciated.  
**Conflicts of Interest:** The authors declare no conflict of interest.