# Evaluation of Structural Behavior of Hysteretic Steel Dampers under Cyclic Loading
## 3 Experimental Results  
### Shear Force Versus Displacement Relationships  
The shear force versus displacement relationships of the specimens are shown in Figure 4, and the experimental results for the first yielding and maximum load of the specimens are shown in Table 2. As shown in Figure 4, the stiffness of all specimens decreased significantly after first yielding, and gradually increased until the target displacement. For all specimens during the repeated load cycles, the peak load slightly increased or almost remained steady, and no decrease in structural performance of the specimens was found. The experiment of Specim SD1 was terminated after performing only 2.5 repetitions due to an error with the test equipment, but it showed sufficient behavioral characteristics to allow for a comparison with the hysteresis behavior of the Specimens SD2 and SD3.  
Figure 4: Shear force versus displacement curve of tested specimens: (**a**) SD1; (**b**) SD2; (**c**) SD3.  
Figure 3: Loading history.  
As shown in Figure 4, SD1 with U-shaped damper and SD2 with the upper horizontal connecting member showed similar loading history curves. On the other hand, as shown in Tables 2 and 3, SD2 had a similar load at first yielding as SD1 but showed some differences in maximum load and elastic and plastic stiffness. This is due to the height of the vertical members and the shape of the connecting member. However, the increase in load-resisting capacity of SD2 was not higher than that of SD3. In other words, SD3 with additional horizontal connecting member between shear plates showed a greater load-resisting capacity than SD1 and SD2 did.  
As shown in Table 2, with the addition of the lower horizontal connecting member, the yield strength of SD3 was about 2.0 times higher than that of SD1. In addition, the maximum load of SD3 was up to 3.9 times higher in the first cycle and 3.5 times higher in the second cycle than that of SD1. Furthermore, as shown in Figure 4, the overall behavior of SD3 was similar to that of SD1, which means SD3 was ductile despite its high load-resisting capacity. Both SD2 and SD3 showed no decrease in strength at the target displacement during five repetitions.  
Table 3 indicates the experimental results for the elastic and plastic stiffness of the specimens. SD2 was 1.29 times and 1.97 times greater than SD1 in the elastic and plastic moduli, respectively, during the first positive loading phase. On the other hand, after showing the Bauschinger effect, the plastic modulus of SD2 was 1.13 times that of SD1, meaning there was no significant difference. SD3 showed a 5.9 times greater elastic modulus compared to U-shaped SD1. A high elastic modulus means the damper reacts more quickly to an external seismic load. In particular, as shown in Table 2, closed-type SD3 had a faster yielding point and higher initial rigidity than open-type SD1 and SD2, which indicates excellent energy dissipation capability. The plastic modulus of SD3 was 9.1 times and 2.4 times greater than that of SD1. In this experiment, no significant decrease in strength, stiffness, and energy dissipation capacity was observed in SD3, which is similar to the experimental results of SD1 and SD2.