## 3 Experiments
### Test Result  
In case of SP-0, the outer steel tube at the front side began to buckle when the drift ratio was 1.0% (=20 mm). The maximum moments were measured as 996.83 kN-m when the specimen was pushed and 996.98 kN-m when the actuator pulled the specimen so that the averaged maximum moment was 996.92 kN. This is 12% higher value than the expected maximum moment from the pre-analysis by using CoWiTA. After that, the load was decreased to 80% of the maximum load when the drift ratio was 4.0% (displacement: \(-\)82.215 mm), and then the experiment was completed. The maximum displacement was 82.195 mm by averaging the values from pushing and pulling cases. Figure 23 shows the load-displacement hysteresis curves of SP-0. The specimen finally fractured by buckling of the outer tube in the plastic hinge as shown in Figure 24.  
SP-S refers to the specimen using the socket type connection by friction coupling of bolts. After a 1.0% drift ratio, the laterally applied load at the top of the specimen was not completely transferred to the bottom of the specimen owing to the deformation of bolts; hence, the LVDT at the bottommost module detected only a slight displacement. The bolts lost their friction after being released at 1.0% drift ratio, and the module joints were torn when the drift ratio was 6% as shown in the right side of Figure 25; the experiment was  
Figure 23: Load–displacement hysteresis curve of SP-0. Figure 22: Definition of specimen direction. completed at that test level. It appears the connecting plate did not have enough thickness. There was a small space between the bolts and bolt-holes during the manufacturing process for easy assembly. The load-displacement hysteresis curve for SP-S is shown in Figure 26. The maximum moment was 583.25 kN-m (push: 498.72 kN-m and pull: 672.28 kN-m), which was only 58.6% of the maximum moment on SP-0 (996.92 kN-m). The maximum displacement was 123.275 mm. For the SP-H joint, the connector was welded to the upper module and was then bolted to the lower module. Similar to the SP-S case, the bolt connection lost its bearing stress by the excessive deformation of the bolt-holes. As Figure 27 shows, the connections began to lose friction at the space between the bolts and the bolt-holes when the drift ratio was 0.5%; then the joints failed by shear stress by the bolts when a 5% drift ratio was applied. In the right side of Figure 28, it is observed that the connecting plate welded to the upper module moved upward. It was supposed that the connecting plate should have larger thickness. In addition, it seems that the tolerance between the bolts and bolt holes contributed to the loss of bearing stress. By the excessive deformation of the bolt-holes, the lateral load was not completely transferred the bottom of the specimen. Finally, it caused the failure of the connecting plate. Figure 28 shows the load-displacement hysteresis curve for SP-H. The maximum moment was 541.04 kN-m (push: 503.03 kN-m, pull: 579.04 kN-m), which was only 54.3% of the maximum moment on SP-0. The maximum displacement was 102.885 mm. Figure 26: Load–displacement hysteresis curve of SP-S.  
Figure 25: Failure of connecting plate of SP-S.  
For the case of SP-W, the outer tube above the base plate started to locally buckle when the drift ratio was 1.0%. The maximum load was measured when the drift ratio was 1.5%. After that, the load decreased to below 80% of the maximum load when the lateral displacement reached a 4.0% drift ratio, and the experiment was terminated. As the loading steps went by from 1.0% to 4.0% drift ratio, the displacement of the outer tube by local buckling increased. When the drift ratio was 3.0%, the height of buckling deformation was about 20 mm; it extended up to 25 mm when the drift ratio was 4.0%. Figure 29 shows the states of the locally buckled shape of the outer tube when the drift ratio was 1.0%, 1.5%, 3.0%, and 4.0%, respectively.