## 3 Experimental Results and Discussion  
### Experimental Observations  
It could be observed from the structural responses that the specimens vibrated most intensely under the input excitations of the Kobe wave, which possessed the larger vibration amplitude and stronger excitation energy, seen from Figure 15, compared to the El Centro and Taft waves when subjected to the same PGA during the shaking table tests. Before the loading case #10 with a maximum PGA of 0.4 g, no visual and acoustical evidence of all the steel components was demonstrated in the test model, and the structural performance remained in an elastic working condition during the frequent earthquake. When the PGA reached up to 0.6 g, a noticeable tremor and shaking sound could be observed from the upper additional mass block, and only a slight shaking response was found at the lower mass block. However, local or lateral buckling in the LYP steel energy absorbers or columns was not observable, thereby revealing that the test model remained elastic until such loading stage. When the PGA level of the input ground motions increased from 0.7 to 1.0 g, a significantly strong shaking response, with an extremely loud and continuous sound, occurred in the mass block. It was accompanied by a considerable amount of dust drifting down, and the overall model intensely vibrated with the increasingly visible displacement. Meanwhile, loosening of the high-strength bolts at the column bases incurred. Moreover, a small number of the displacement sensors mounted on the model broke and dropped down, and some connecting wires of strain gauges fell off (Figure 16a). When the PGA reached up to 1.3 g, the columns, beams, and lateral braces continued to work well. However, the dramatically visible and irreversible bending deformation close to the middle of the LYP steel plates, caused by the shear action, was observed (Figure 16b). This indicates that the ability to dissipate the input earthquake energy mainly relies on the out-of-plane bending mechanism of energy absorbers in any rare earthquake. When the PGA further increased from 1.4 to 1.6 g, the energy absorbers buckled severely, and the corresponding residual plastic deformation was more prominent.  
Figure 15: Corresponding frequency spectra of waves. (**a**) El Centro wave; (**b**) Kobe wave; (**c**) Taft wave.  
In addition, the welds located at the energy absorbers and beam-column joints, where the uneven tension texture appeared on the steel surface, were slightly fractured due to the recursive tensile and compressive forces, as shown in Figure 16c,d.  
Throughout the whole loading process of the tests, except for the LYP steel energy absorbers, no apparent failure occurred for the frame columns, frame beams, and lateral braces. The test model did not seriously damage and collapse until the end of the seismic loading cases. In summary, out-of-plane local buckling accompanied by significant bending deformation of LYP steel plates, and the slight fracture of fillet welds, were considered to be the governing failure modes for the test model in this research. These results clearly suggest that the seismic resisting performance of a steel structure retrofitted by additional LYP steel energy absorbers is superior. As the first line of defense in the lateral force resisting system, the energy absorbers, as seismic applications in a structure, contained adequate elastic stiffness to withstand a small earthquake or wind load. However, they performed really well in the elastic-plastic stage and exhibited excellent energy absorbability in a rare earthquake or severe wind, thereby reducing or avoiding the damage of the primary structures, such as frame columns and beams.