## 3 Test Results and Analysis  
### Test Observations  
Quasi-static tests are performed for six circular CFST column specimens. The observed failure mode of each specimen is as follows:  
(1) All six specimens demonstrate a compressive bending failure mode. The failure process of specimen CFST-N is elaborated here. When the lateral drift is smaller than 1/250, the circular steel tube exterior wall has no change. When the lateral drift is 1/200, at 50 mm above the column base in the North, a slight bulge of the steel tube can be felt by hand. When a reverse load is applied, the bulge is flattened, and there is no residual deformation. When the lateral drift is 1/100, bulge deformation is observed at both the South and North ends, and it gradually expands upward to 100 mm above the column base. When the lateral drift is 1/75, bulge deformation at the South and North ends grows continuously; at 100 mm above the column base in the North, paint starts to peel off, and there is residual deformation when a reverse load is applied. When the lateral drift is 1/50, the south-north bulge deformation expands toward the East and West and gradually penetrates, the paint layer peels-off in the North and becomes more severe, and concrete in the steel tube produces a sound. When the lateral drift is 1/33, the paint layers peel off at the East and West ends. When the lateral drift is 1/25, all paint layers within 170 mm of the column base peel off, bulge deformation around the column base completely penetrates, a semi-waveform bulge is formed, and local buckling of the steel tube is severe.  
(2) Specimens CFST-HS, CFST-VS and CFST-SR demonstrate similar failure modes. When the lateral drift reaches 1/25, unlike specimen CFST-N, the 3 specimens do not form a connected bulge  
Figure 5: CFST column test. (**a**) Test site; and (**b**) typical cyclic loading protocol.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Specimens No.** & **CFST-N** & **CFST-HS** & **CFST-VS** & **CFST-SR** & **CFST-DT** & **CFST-MC** \\ \hline
**steel ratio \(\rho\) (\%)** & 7.48 & 8.10 & 8.63 & 10.14 & 10.11 & 11.90 \\
**axial load ratio \(n\)** & 0.59 & 0.57 & 0.56 & 0.54 & 0.46 & 0.43 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Steel and axial load ratios.  
deformation around the column base. Bulge deformation occurs at the South and North ends, but the East and West ends show almost no deformation. Bulge deformation at the South and North ends ranges from 70-90 mm above the column base. Peeling of paint layers from the columns of the three specimens is not as severe as for specimen CFST-N, and it occurs primarily at the South and North ends.  
(3) Specimens CFST-DT and CFST-MC show bulge deformation at higher heights than the other four specimens. When the lateral drift reaches 1/25, the bulge deformation of specimen CFST-DT ranges from 120-140 mm above the column base; the East and West ends only show slight bulge deformation. Peeling of paint occurs within 140 mm above the column base; the East and West ends show more severe peeling than the South and north. Bulge deformation of specimen CFST-MC occurs in the range of 125-175 mm above the column base; the East and West ends show almost no deformation. Peeling of paint layers is concentrated at the South and North ends; the East and West ends are nearly unaffected. Failure images of typical specimens CFST-N, CFST-VS and CFST-MC are shown in Figure 6.  
Figure 6: _Cont._