# Mechanical Performance of Built-Up Columns
## 3 Test Results  
### Failure Mode  
The ultimate loads of the four specimens are listed in Table 3. The deformed shapes at failure of the four specimens were shown in Figure 5. Only one mode of failure was observed in the tests, which was flexural buckling. For the four specimens, lateral deformations at the middle length of the column were visible when the loads were close to the peak. After the loads crossed the peak, the bending deformations increased rapidly. It is worth noting that all the specimens generated lateral displacement on both the x-axis and z-axis. However, the displacement on the x-axis was more pronounced. In addition, after the tubes bended, on the one hand, the end of the tubes was detached from the web of track (Figure 6a), and on the other hand, the angle between C-shaped connector and tube was no longer perpendicular (Figure 6b). The connectors rotated as the tubes bended, which led to the joint of the connector and tube being similar to a hinge point. The rotation of connectors was due to the non-negligible extrusion deformation of the tube plate caused by screws. This indicates that the bending deformation of tubes cannot effectively be avoided by the small web height of connectors and only two screws adopted for connecting connector and tube. [PERSON] and [PERSON] [11] once concluded that the type of connection is more important than the size of connectors. Therefore, the influence of screws connection should be investigated in future research. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Specimen** & \multicolumn{2}{c}{**Yield Stress \(\sigma_{y}\) (MPa)**} & \multicolumn{2}{c}{**Ultimate Stress \(\sigma_{u}\) (MPa)**} & \multicolumn{2}{c}{\(\sigma_{u}\)/\(\sigma_{y}\)} & \multicolumn{2}{c}{**Young’s Odulus \(E\) (GPa)**} \\ \hline \multirow{3}{*}{tubes} & 1 & 378 & 450 & 1.191 & 193 \\  & 2 & 372 & 447 & 1.202 & 203 \\  & 3 & 375 & 451 & 1.204 & 216 \\ \hline \multirow{3}{*}{Average values} & 375 & 450 & 1.199 & 204 \\ \cline{2-6}  & 1 & 352 & 360 & 1.022 & 180 \\ \cline{1-1} \cline{2-6} connectors and & 2 & 343 & 349 & 1.017 & 165 \\ \cline{1-1} \cline{2-6}  & 3 & 355 & 363 & 1.023 & 174 \\ \hline \multicolumn{6}{c}{Average values} & 350 & 357 & 1.021 & 173 \\ \hline \end{tabular}
\end{table} 
Table 2: Material properties. Figure 5: Deformed shapes at failure: (**a**) S1; (**b**) S2; (**c**) S3; (**d**) S4. Figure 7 shows the load-strain curves of connectors. It can be seen from Figure 7 that the strain of connectors for S1, S2 and S4 are small and most of them are lower than 25 \(\upmu\epsilon\). This finding shows that the deformation of connectors are mainly in the form of rigid motion, which indirectly validates the hinge points observed in Figure 6b. As shown in Figure 6c, it can be seen that a fracture occurred at the weld, which connected the X-type transverse diaphragm and tubes (S4). This shows that the X-type transverse diaphragm does play a role in resisting the relative deformation between tubes. However, the transverse diaphragm does not play a role in improving the carrying capacity, for the bearing capacity of S4 is 12.3% lower than that of S1. Figure 6d shows the distortion of the steel strip and pullout of the screws. The steel strip has undergone a large deformation, which can be verified by the large strain values as shown in Figure 7(S3). Although the loop formed by steel strips does not have rigid motion like C-shaped connectors, the steel strip cannot restrain the flexural deformation of tubes well because of the poor bending resistance. Therefore, the bearing capacity of specimen S3 is 15.85% lower than that of S1. The spacing between tubes of S1 and S2 are 200 mm and 110 mm, respectively, with a difference of 2.65% in ultimate bearing capacity, which illustrates that the factor of spacing between tubes has no obvious effect on improving the ultimate load of novel built-up columns.