## 3 Experimental Analysis
### Symmetric Monotonic Loading Static Test  
This test was carried out in the Jiangsu Key Laboratory of Environmental Impact and Structural Safety in Engineering. The specimen and test loading device are shown in Figure 10. To simulate the actual condition of the connection under the action of vertical gravity load, the loading specimen was placed upside-down before loading, and then the constant axial force value was preset for the top of the column by a multifunctional long-axis testing machine. A hydraulic jack was used to add symmetrical force on both ends of the beam in a vertical direction. Before loading, the pressure plate of the long-axis testing machine and the column-top force transferring plate were centered geometrically and held closely together to ensure that both the column ends did not slide in the opposite horizontal direction, and that the loading of both ends proceeded synchronously. There were two security bolts at the bottom end of the column, which were not intended to keep the components in place but to prevent their accidental collapse. Preloading before the formal test was carried out to ensure that all parts of the joints were in full contact with each other. At the same time, the field test system and the loading system were adjusted. Before the formal loading, a constant axial force was applied on the square steel tube column with a nominal ratio of axial compression stress of 0.25. The axial load \(N\) at the end of the column is calculated as follows:  
\[N=0.25f_{yc}A_{s}=0.25\times 235\times 4544\times 2=533920\text{ N}\approx 534\text{ kN} \tag{1}\]  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Number** & **f\({}_{y}\) (Mpa)** & **f\({}_{u}\) (Mpa)** & **f\({}_{u}\)/f\({}_{y}\)** & **E (\textless{}10^{5}\) Mpa)** \\ \hline Beam, column & 3 & 309.87 & 415.73 & 1.34 & 1.77 \\ Bolt & 3 & 352.13 & 643.33 & 1.82 & 1.95 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Results of the material test.  
Figure 8: Object picture and dimension drawing of components. (**a**) Special bolt (**b**) base plate (**c**) twisting tool (**d**) upper-end plate (**e**) bottom-end plate (**f**) diagonal stiffener of S1 and S2.  
Figure 9: Object picture of column with three upper-end plates.  
where \(f_{yc}\) is the nominal yield strength of the square steel, and \(A_{s}\) is the cross-sectional area of the square steel tube column.  
For the sake of safety, the axial force was maintained at the rate of 20 kN/min to achieve 534 kN. After the axial force achieved the predetermined value, synchronous monotonous vertical loads were applied at both ends of the beam. When loading began at the end of the beam, the loading value was increased by 10 kN per level. When the nonlinear evolution of the load-displacement curve occurred, the loading value was reduced to 5 kN per level. The test site is shown in Figure 11.  
Figure 11: Testing site. (**a**) Front view of S1 (**b**) side view of S1 (**c**) front view of S2.  
Figure 10: Loading devices.  
The test mainly measured the force and displacement of the control point by the pressure sensor and the electronic displacement meter. The measuring point layout is shown in Figure 10. The displacement meters W1 and W2 were used to measure the deformation of the beam end. W3 and W4 were used to measure the rotation of the joints. W5 and W6 were used to measure the horizontal displacement of the column.  
The strain of the control points was collected by a DH3816N static strain tester. The strain gages were laid out as shown in Figure 12. The strain gauges of the front joint were laid out in areas A, B, C and D. Area X shows the strain gauge arrangement from a vertical view; that is, the layout of the strain gauge on the upper part of the upper beam. Area Y shows the arrangement of strain gauges at the left end of the column. Area Z shows the layout of the strain gauges on the lower part of the bottom beam.