## 4 Results and Discussions
### Skeleton Curves  
For the convenience of comparison, for specimens with different parameters, the strength and energy dissipation were selected when the applied displacement was 8.4 mm. The main test results are shown in Table 2, in which \(F_{\rm m}\) represents the mean value of strength for a specimen. The skeleton curves of specimens are illustrated in Figure 11.  
Figure 11a shows the force-displacement responses for specimens without buckling-restrained devices. In the negative direction, the strengths of specimens W30-O15\({}^{\circ}\) and W30-O30\({}^{\circ}\) increased with the increased displacement before the buckling behavior was noted. After that, the strengths decreased rapidly and kept a lower value. The buckling behavior of specimen W30-O30\({}^{\circ}\) occurred earlier than that of specimen W30-O15\({}^{\circ}\) due to the larger oblique angle. However, the strength of specimen W30-O30\({}^{\circ}\) was close to that of \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Specimen** & **Direction** & \(\Delta\)**/mm** & **F/kN** & **F\({}_{\text{m}}\)/kN** & **E\({}_{\text{d}}\)**/(kN\(\cdot\)mm)** & **_h_e** \\ \hline W30-O15\({}^{\circ}\) & Positive & 8.4 & 28.33 & & & & \\  & Negative & \(-\)8.4 & \(-\)15.27 & 21.8 & 387.44 & 0.33 \\ W30-O30\({}^{\circ}\) & Positive & 8.4 & 31.35 & & & & \\ W30-O30\({}^{\circ}\) & Negative & \(-\)8.4 & \(-\)6.36 & 18.86 & 250.52 & 0.23 \\  & Positive & 8.4 & 23.95 & & & & \\ W20-O0\({}^{\circ}\)-LS & Negative & \(-\)8.4 & \(-\)22.56 & 23.25 & 405.12 & 0.33 \\  & Positive & 8.4 & 26.17 & & & & \\ W30-O0\({}^{\circ}\)-LS & Negative & \(-\)8.4 & \(-\)24.69 & 25.43 & 448.94 & 0.33 \\  & Positive & 8.4 & 36.58 & & & & \\ W30-O15\({}^{\circ}\)-LS & Negative & \(-\)8.4 & \(-\)30.65 & 33.62 & 601.35 & 0.34 \\  & Positive & 8.4 & 51.37 & 45.97 & 810.68 & 0.33 \\  & Negative & \(-\)8.4 & \(-\)40.57 & 24.56 & 503.78 & 0.39 \\  & Positive & 8.4 & 29.88 & & & \\ W30-O15\({}^{\circ}\)-LP & Negative & \(-\)8.4 & \(-\)26.62 & 28.25 & 575.71 & 0.39 \\  & Positive & 8.4 & 46.4 & & & & \\ W30-O30\({}^{\circ}\)-LP & Negative & \(-\)8.4 & \(-\)33.99 & 40.19 & 708.3 & 0.33 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The characteristic test results of the specimens. Figure 11: The skeleton curves of specimens: (**a**) specimens without buckling-restrained devices; (**b**) specimens with cover plates; (**c**) specimens with stiffening ribs; (**d**) W20-O0\({}^{\circ}\)-LS and W30-O0\({}^{\circ}\)-LS. specimen W30-O15\({}^{\circ}\) in the case of buckling failure. In the positive direction, specimens W30-O15\({}^{\circ}\) and W30-O30\({}^{\circ}\) showed obvious elastic, elastic-plastic, and plastic behaviors of steel plates. At the applied displacement of 17.4 mm, the strength of specimen W30-O15\({}^{\circ}\) was approximately 32.95 kN, which was 16.83% smaller than that of specimen W30-O30\({}^{\circ}\). Additionally, the max strength of specimen W30-O30\({}^{\circ}\) reached 47.95 kN. Figure 11b,c show the force-displacement responses for specimens with different oblique angles. It can be seen that the strength increased with the increase in oblique angles. At the applied displacement of 8.4 mm, the mean strengths of specimens W30-O15\({}^{\circ}\)-LS and W30-O30\({}^{\circ}\)-LS were approximately 33.62 kN and 45.97 kN, which were 32.21% and 80.77% higher than that of specimen W30-O0\({}^{\circ}\)-LS, respectively. For specimens with cover plates, the mean strengths of specimens W30-O15\({}^{\circ}\)-LP and W30-O30\({}^{\circ}\)-LP were 15.02% and 63.64% higher than that of specimen W30-O0\({}^{\circ}\)-LP, respectively. In addition, the mean strengths of specimens W30-O0\({}^{\circ}\)-LS, W30-O15\({}^{\circ}\)-LS, and W30-O30\({}^{\circ}\)-LS were a little higher than those of specimens W30-O0\({}^{\circ}\)-LP, W30-O15\({}^{\circ}\)-LP, and W30-O30\({}^{\circ}\)-LP, respectively. This indicated that stiffening ribs could not only restrict the buckling behavior of X-shaped metal plates but also slightly increase the strengths.