# Experimental Study on Vibration Control of
## 2 Pounding Tuned Mass Damper (PTMD)  
As reviewed in the last section, the tuned mass damper (TMD) is one of the most popular dampers because of the effectiveness in vibration suppression near the resonance frequency [76; 77]. It connects a mass to the superstructure by a spring, as shown in Figure 2a, and the frequency of the spring-mass is tuned to that of the superstructure. The TMD can absorb the kinetic energy; however, its own energy cannot be dissipated effectively. Therefore, the TMD is often used with other energy dissipating components, which may require continuous maintenance [78]. In addition, a TMD's performance is sensitive to the frequency variation of the superstructure. As shown in Figure 2b, an impact damper includes a single mass or multiple masses with motion delimiters. The impact/pounding between the mass and the delimiters dissipates energy. The impact damper has some advantages [79] such as simple construction, easy adjustment, high energy dissipation, low cost, and robustness to frequency variations.  
The bounding tuned mass damper (PTMD) was developed to combine the advantages of a TMD and an impact damper with the help of viscoelastic materials [63]. Similar to a TMD, a PTMD connects the mass to the structure by a spring, and the natural frequency of the spring-mass system is tuned close to that of the superstructure. However, unlike a TMD, the displacement of the mass in the PTMD is limited by two delimiters. The spring-mass system consumes energy similar to a TMD when the displacement is small, while the collision between the mass and the delimiter effectively dissipates energy when the displacement is large. In order to improve the vibration suppression capacity during the impact, an energy-dissipating material is bonded to the delimiters. Commonly used energy dissipating materials include viscoelastic materials [80; 81] and shape memory alloys [82]. Figure 2c shows the schematic of the PTMD. In addition, experimental studies have shown the good impact fatigue life of the viscoelastic materials [83]. Both analytical and experimental studies have shown that a PTMD is much more robust to frequency variation as compared to a TMD [67; 70].  
Based on the original double-sided PTMD proposed in [63], [PERSON] et al. [64] proposed a novel single-sided PTMD. In the single-side PTMD, the mass is in close contact with the only delimiter in the initial equilibrium state, and the mass can move only in one direction relative to the equilibrium state, as shown in Figure 2d. This proposed method has several significant advantages, including a simple structure and ease of design, since the design parameter of the gap between the mass and the delimiter is now eliminated. Specifically, for the single-sided PTMD, the moving mass is in simple contact with the viscoelastic delimiter during idle times. There are no preloads to press the mass against the delimiter. There is no need to design the size of the gap between the mass and the delimiter. Since there is no gap, the PTMD will not work similar to a TMD, even at low amplitudes. Additionally, the single-sided PTMD is more efficient than the double-sided PTMD regarding energy dissipation. During the working process of the single-sided PTMD, when the mass returns to the equilibrium position, the kinetic energy of the mass and the primary structure is at its maximum in the opposite direction; thus, the impact dissipates the largest amount of energy.  
In previous studies, the PTMD showed excellent performance regarding structural vibration control in various complex environments [84; 85; 86]. Thus, in this paper, the single-sided PTMD model is selected to accomplish the vibration control of the suspended piping system.