# Experimental Study on Vibration Control of
## 6 Conclusions and Future Work  
In this study, two bounding tuned mass dampers (PTMDs) were designed to mitigate the vibration of a suspended piping system. A tuned mass damper (TMD) and the two designed PTMDs were tested under free vibration and forced vibration to compare their effectiveness regarding vibration control. Both spring steel-type PTMD and simple pendulum-type PTMD were investigated to verify their vibration suppression performance. In the free vibration experiment, the TMD decreased the vibration attenuation capability of the piping system, and the time for reducing displacement from 20 mm to two mm increased from 26.5 s to 48.8 s. Both kinds of PTMDs added considerable damping (spring steel-type with 10.4% and simple pendulum-type with 5.6%) to the piping system; thus, the free vibration of the piping system was rapidly attenuated. The time taken to reduce the vibration displacement to two mm was reduced from 26.5 s (without control) to 1.6 s (with spring steel-type control) and 2.85 s (with simple pendulum-type control), respectively. The comparison shows that for this experiment, the spring steel-type PTMD is more effective. In the forced vibration experiment, TMD can reduce the displacement of the pipe to almost zero at the tuned frequency. However, it also leads to new resonant regions on both sides of the original frequency spectra (2.12 Hz and 3.02 Hz) with displacement ratios of 90% and 68%. In the frequency domain, the amplitude frequency responses controlled by PTMDs have only one peak, which is 2.46 Hz. Both PTMDs provide considerable damping and reduce the displacement response significantly. The vibration mitigation performance of simple pendulum-type PTMD is slightly better  
Figure 12: Comparison of resonant vibration: (**a**) With simple pendulum-type PTMD; (**b**) With spring steel-type PTMD; (**c**) With TMD. than spring steel-type PTMD, and the displacements are reduced by 51% (simple pendulum type) and 49% (spring steel type), respectively. In practice, there are wires, liquids, etc. in the piping system, which can affect the effectiveness of the PTMD. According to [67,70], the PTMD is robust for a moderate range of frequencies (e.g., if the PTMD is detuned or an unexpected mass is added to the pipe). If the operator can anticipate the presence of additional mass, such as liquid, the PTMD can be designed beforehand to target the expected mass of the pipe during operation. Furthermore, the larger the amplitude of vibration, the stronger the impact between the mass and the delimiter becomes. Thus, there is an inherent adaptive behavior of the PTMD to both weak and strong vibrations. However, the limit to this adaptation has not yet been explored fully, and to keep the paper concise, only a certain level of amplitude has been investigated. The effects of the nonlinearity in the PTMD on how it can adapt to different amplitudes will be our future work. Another subject for future work is the design considerations for vertically placed pipes. For vertical pipes, gravity will need to be taken into account, which will require modifications to the fixture design. Also, as can be seen in Figure 4b, multiple, smaller peaks are present alongside the peak of the structural response. These peaks, while having a negligible impact on the overall results, were neither present in the FE analysis nor in the subsequent experimental results, and are generated by a currently unknown source. Thus, another facet of future work will include a more in-depth investigation of these additional peaks. As an additional limitation of this study, the vibration excitation was generated by a harmonic load. However, the response of the piping system under earthquake load will be different from a harmonic load. Future work will investigate the vibration performance of the piping system with PTMD under an earthquake load to further verify its effectiveness. All authors discussed and agreed upon the idea, and made scientific contributions. [PERSON] and [PERSON] designed the experiments and wrote the paper.