Behavioral model for a Contractable Viscous Dashpot

Damping is one of many different methods that have been proposed for allowing a structure to achieve optimal performance when it is subjected to seismic, wind, storm, blast or other types of transient shock and vibration disturbances. Conventional approach would dictate that the structure must inherently attenuate or dissipate the effects of transient inputs through a combination of strength, flexibility, and deformability. The level of damping in a conventional elastic structure is very low, and hence the amount of energy dissipated during transient disturbances is also very low. During strong motions, such as earthquakes conventional structures usually deform well beyond their elastic limits, and eventually fail or collapse. Therefore, most of the energy dissipated is absorbed by the structure itself through localized damage as it fails. The concept of supplemental dampers added to a structure assumes that much of the energy input to the structure from a transient will be absorbed, not by the structure itself, but rather by supplemental damping elements.
Fluid viscous dampers are known as energy dissipating devices with high capacity in reducing seismic effect on buildings Fluid dampers which operate on the principle of fluid flow through orifices have found numerous applications in the shock and vibration isolation of military and aerospace hardware and in wind vibration suppression of missile launching platforms. fluid Viscous damping reduces stress and deflection because the force from the damping is completely out of phase with stresses due to flexing of the columns. This is only true with fluid viscous damping, where damping force varies with stroking velocity. Other types of damping such as yielding elements, friction devices, plastic hinges, and viscoelastic elastomers do not vary their output with velocity; hence they can and usually do, increase column stress while reducing deflection.
Determination of mechanical characteristics of these devices is usually based on experimental studies using cyclic tests with different amplitudes and frequencies.
In this work, a new type of viscous damper is chosen for experimental studies in which the main body of the devices has been made of contractible steel bellows (developed in IIEES).
The nominal force capacity of dashpot is about 500kN and its maximum stroke is around 150 millimeters. Maximum axial force in damper device will be reached about 300kN during the test. A model for representative the above viscous device should also include axial flexibility for device in the form of Kelvin or Maxwell models. Finally this model represents the general behavior of the damper based on various factors that effects its performance. In this study, a nonlinear viscous behavior has been shown in the device with respect to velocity. During Cyclic test the average of initial frictional force is about 10kN, which can be used as a functional fuse for damper. In addition, the damping force, a second friction force is about 50kN that depends on oil pressure, which increases the capacity of damper and improve its behavior. Results show that the friction force can be considered as an effective factor involved in energy dissipation of dampers.