## 2 Experimental Study
### Loading Scheme and Test Program  
The loading devices used in the test included the reaction frame, upright column, cross beam, transfer beam, test system (DH3861N, DONGHUA, Jiang'su, China), and loading system, as shown in \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Specimens** & \(L\) **(mm)** & \begin{tabular}{c} \(L_{\text{b}}\) \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} \(E\) \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} \(D\) (B) \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} \(T\) \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} \(D\) \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} \(T\) \\ **(mm)** \\ \end{tabular} & \
\begin{tabular}{c} \(D\) \\ **(mm)** \\ \end{tabular} &
\begin{tabular}{c} \(T\) \\ **(mm)** \\ \end{tabular} \\ \hline K-CC & 1200 & 450 & 35 & 159 & 6 & 89 & 5 & 45\({}^{\circ}\) & unstrengthened \\ K-CCC & 1200 & 450 & 35 & 159 & 6 & 89 & 5 & 45\({}^{\circ}\) & ordinary concrete \\ K-CCC2 & 1200 & 450 & 35 & 159 & 6 & 89 & 5 & 45\({}^{\circ}\) & fly ash concrete \\ K-RC & 1200 & 450 & 55 & 200 & 8 & 95 & 5 & 45\({}^{\circ}\) & unstrengthened \\ K-RCC1 & 1200 & 450 & 55 & 200 & 8 & 95 & 5 & 45\({}^{\circ}\) & ordinary concrete \\ K-RCC2 & 1200 & 450 & 55 & 200 & 8 & 95 & 5 & 45\({}^{\circ}\) & fly ash concrete \\ \hline \hline \end{tabular}  
* K-CC: The K-joint with circular chord and brace; K-CCC: The joint strengthened on the basis of K-CC by grouting ordinary concrete in chord; K-CCC2: The joint strengthened on the basis of K-CC by grouting fly ash concrete in chord; K-RC: The K-joint with a square chord and circular braces; K-RCC1: The joint strengthened on the basis of K-RC by grouting ordinary concrete in chord; K-RCC2: The joint strengthened on the basis of K-RC by grouting fly ash concrete in chord.  
\end{table} 
Table 1: Geometric parameters of specimens. Figure 2: Samples of experimental materials. **(a)** The standard specimens of Q345; **(b)** The standard specimens of concrete. Figure 3. The K-joints were placed in a common plane with the reaction frame, and the left end-plate of the chord was fixed with the transfer beam and the right end-plate was used as a sliding support. Two braces were articulated along their axis with two hydraulic jacks (JSYZ-500, Xi'an Oriental Metal Structure, Xi'an, China) which could provide tension and compression for 500 kN, respectively. The reciprocating load was exerted on the end-plate of the braces by hydraulic jacks. That is, when the through brace was subjected to a compressive load, the overlap brace was controlled to experience an equal magnitude tensile force. In order to eliminate the installation gap and check whether the instrument was normal, 25% yield load was applied to the joint before formal loading. The force-displacement hybrid loading mode was adopted, and the loading scheme is shown in Figure 4. The formal test loading was divided into two stages: The first stage was controlled by the load, which was divided into three levels, and the increment of each stage was 25% of the yield load and each level load circulated two times. The second stage was controlled by displacement. When the first stage was completed, the displacement \(\triangle_{\text{y}}\) corresponding to the yield load of the brace was taken as the increment. Each level load circulated three times, until specimen failure. The measurement contents of the test included the reaction force, displacement, local deformation of the joint, stress distribution of the brace and strain distribution of the joint. Each experimental joint setup incorporated five displacement transducers (D050, Jing Ming Technology, Yang'zhou, China), 16 strain gauges and 16 strain rosettes. The layout of the strain gauge and displacement transducer is shown in Figure 5. Each joint was equipped with a strain gauge and T1-T16 strain rosettes to monitor  
Figure 4: Loading device for cyclic loading. Figure 3: Loading device. (**a**) Design drawing of test loading device; (**b**) The test site.