Study on a Contractible Viscous Dashpot with Variable Damping Constant

One of the new challenges in structural engineering is the mitigation of seismic hazards from structures using flexibility and energy dissipation approaches. This is in contrast with the typical seismic design methodologies in which strength and ductility resources of structural members are tapped to tackle earthquake demands. In this approach, adding to the flexibility of the structure should be in accordance with the energy dissipation potential in the system. In this case, earthquake demands for lateral strength in structures reduces, but energy dissipation devices are needed to subside the lateral deformation of such flexible structural systems.
To meet the huge demand for energy dissipation potential in these structural systems, large scale damping devices are required. Such equipment, to mention a few, comes in the form of metallic, frictional, viscoelastic, memory shaped alloys and viscous dashpots. Among the others, viscous dashpots are considered the most favorite ones to use in large structural systems due to their sizable capacity and impartiality to ambient vibration and temperature loads. Moreover, since these devices are velocity dependent energy dissipaters, they are capable of reducing both deformation and acceleration responses of the structural systems more effectively.
Viscous dashpots are typically made from a metallic cylinder, a piston, a shaft, cylinder caps and elastomeric seals (to provide confinement on the liquid inside of the cylinder). The existence of elastomeric seals in configuration assembly of these devices is considered a weak point in mechanical design of such dashpots considering maintenance issues. To address this problem, a contractible viscous dashpot was introduced earlier in which there was no need for elastomeric seals. In this work, a new version of this dashpot with variable damping constant have been tested for determination of its functionality and characteristics.
Contractible viscous dashpots are made from two flexible chambers that axially contract or expand to accommodate liquid movement between the two. In this mechanism all parts of the device are made of steel and there is no relative movement between cylinder caps and the main shaft. Therefore, there is no need for elastomeric seals to confine the liquid inside of cylinders at cylinder caps.
The Dashpot was designed for load capacity of  and the maximum stroke of . In the test procedure, however, due to some limitations in the test setup, the attainable load was around 310 KN. The test results show stable hysteretic loops under sinusoidal excitations with the amplitude of  in the frequency range of 0.1-0.25 Hz. The hysteresis loops resemble a viscous device with viscoelastic behavior that can be roughly represented by Kelvin model. As expected, damping constant of the dashpot reduces by an increase in excitation frequency. The capability of change in damping characteristics of the dashpot was embedded in the device. This ability was shown in the experiments where damping constant of the device became almost tripled during the test process by adjusting the embedded mechanism in the device.
The contractible dashpot used in this study has shown an initial frictional behavior due to imperfection in its manufacturing process. Increase in the internal liquid pressure in the device expands this frictional behavior to about 10% of total capacity of the dashpot. The initial frictional force in the device can be easily improved to help the functionality of the dashpot.
This dashpot has shown acceptable performances in all the experimental investigations carried out in the course of this study. Considering its simplicity and practicality (low maintenance costs), there would be a good chance for such devices to be used in large structures in near future.