## 2 Experimental Study
### Test Program  
A schematic sketch of a Warren CHS tubular truss under total vertical loading is shown in Figure 2. The test setup includes the reaction frame and supports, hydraulic jack (JSYZ-200, Xi'an Oriental Metal Structure, Xi'an, China), and load measuring system (DH3861N, DONGHUA, Jiangsu, China). The test setup was firmly connected to the strong floor. The specimens were simply supported at their extremities to allow both free in-plane rotation and in-plane longitudinal displacement. Compression force was applied at steel blocks welded at one-fourth and three-fourths of the top chord members with 1000 mm in distance as the loading points using two layers of spreader beams. A photo and a schematic of the test setup are shown in Figure 3.  
An initial load was applied to the specimens to eliminate any possible gaps between the specimens and the loading machine. All specimens were subjected to incremental monotonic static loading, which was equally divided into different load levels. During the tests, the applied loads in each load level were gradually increased to 10 kN when the materials were within the elastic range, while the applied loads in each load level were gradually increased to 2 kN when material plasticity was reached. The readings of the data acquisition system in each load level were recorded with a pause at the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Steel Tube** & **Elastic Modulus \(E\) (N/mm\({}^{2}\))** & **Tensile Yield Stress\(f_{y}\) (MPa)** & **Ultimate Tensile Stress\(f_{u}\) (MPa)** & **\(f_{y}\)/\(f_{u}\)** & **Elongation \(\delta\) (\%)** \\ \hline \(\Phi\phi 0\times 2.5\) & \(2.071\times 105\) & 268.57 & 430.14 & 0.62 & 17.87 \\ \(\Phi 33\times 2\) & \(1.957\times 105\) & 306.21 & 446.04 & 0.69 & 19.29 \\ \(\Phi 22\times 2\) & \(2.123\times 105\) & 324.36 & 462.35 & 0.70 & 15.62 \\ \hline \multirow{2}{*}{concrete type} & \multicolumn{3}{c}{elastic modulus} & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} & \\  & \multicolumn{1}{c}{\(E_{\text{c}}\) (N/mm\({}^{2}\))} & \multicolumn{3}{c}{concrete cube strength (MPa)} & \multicolumn{1}{c}{} \\ \hline Self-compacting & \multicolumn{3}{c}{\(2.77\times 104\)} & \multicolumn{3}{c}{18.3} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Material properties. Figure 3: Test setup. Figure 2: Layout of the Warren CHS tubular truss. applied loading level for 2 min. The applied loads were then increased to the next loading level and held in place for another 2 min and the readings of the data acquisition system were taken again. This test procedure was repeated until failure of the specimens. Five displacement transducers D1-D5 (D050, Jing Ming Technology, Yangzhou, China) were used to record the deflections of each specimen during the test. The midspan deflection of the bottom chord members was monitored by displacement transducer D1, and the vertical deflections of the end tubular joints at the bottom chord members were monitored by displacement transducers D2 and D3. In addition, the out-of-plane distortions of specimens were monitored by displacement transducers D4 and D5, which were arranged on the both sides of the midspan of the top chord members. The arrangement of the displacement transducers is shown in Figure 4a. Two single element strain gauges were attached at the middle length of the brace and chord members at half circumference interval for right half part of the specimens by taking advantage of the symmetry in geometry, loading application, and boundary conditions to obtain the strain intensity of each specimen under different load levels. The arrangement of single element strain gauges is shown in Figure 4b-d for the front view, top view, and bottom view, respectively. The strain along the welded connection of chord and brace members were measured by four multi-axial strain gauges (strain gauge rosettes) attached around the weld toe of connection joints.