Friction dampers are designed to have moving parts that will slide over each other during a strong earthquake. When the parts slide over each other, they create friction which uses some of the energy from the earthquake that goes into the building.
· Pall Dampers
It has been a common practice among mechanical engineers to apply friction-based brakes to absorb kinetic energy in machines and devices. This has encouraged the development of Pall friction based damper. Pall friction dampers have successfully gone through sophisticated experimental studies on shake tables in Canada and the United States. Pall friction dampers have been very attractive due to their simplicity and low cost of construction. High seismic performance of Pall’s damper has been a great motivation to extend its application out of its origin to other countries, specially to United states. General structure of Pall type dampers is made up of some steel plates layed on each other with high strength bolts pressing them together, generating friction between them. Contrary to Viscoelastic systems, Pall system is not sensitive to environmental temperature and state of loading. Pall system’s hysteretic behaviors are almost rectangular and completely similar to ideal elastoplastic behavior. Due to high dissipation energy capacity and stability of hyteresis loops, Pall system seems to show higher seismic performance, than other damping systems.
· Pall Friction Dampers Mechanism
As mentioned before, a Pall damper is very popular due to its low cost of construction and its simplicity. Furthermore, considering architectural restrictions, such elements are very easy to be hidden in internal partitions. Friction based dampers are formed of steel plates tightened together by means of high strength bolts with either axial or rotational deformation mechanism leading to transformation of kinetic energy to thermal one. Pall friction based bracing has utilized the lever mechanism wisely, to transform relatively low global displacements of braced frames to local high deformations of dampers. A transformation ratio in order of 10 may be easily reached, leading to high efficiency of the system. Potential of energy dissipation is affected by damper aspect ratio and its relation with braced frame aspect ratio, so that a maximum potential of energy dissipation may be reached in the case of aspect ratio of damper be equal to the frame one. Formation of 2 instantaneous centers of rotation in the case of different aspect ratios is the source of temporary instability of the system (figure 1-b) and reduction of energy dissipation potential.
Tests1 have been performed on 28 samples under 50 loading cycles. 3 types of contact surface, as smooth surface, rough surface and pad sandwiched system have been assessed through displacement controlled cyclic tests with amplitudes of 10 and 20 mm. All of 3 types of contact surface have shown complete elasto – plastic hysteretic behavior, but it is obvious that the added pad has significant influence on stability of hysteretic loops (figure 2-c). The pad sandwiched system has been chosen to be studied further in this program.
Source: SEISMIC DESIGN OF STRUCTURES USING FRICTION DAMPER BRACINGS
· Rotational Friction Damping
The novel friction damper device consists of several steel plates rotating against each other in opposite directions. The steel plates are separated by several shims of friction pad material producing friction with the steel plates.
The damper can upgrade structures in an economical way because it :
HOW IT WORKS
When an external force excites a frame structure the girder starts to displace horizontally due to this force. The damper will follow the motion and the central plate will rotate around the hinge. The horizontal plates will rotate in opposite direction to the central plate because of the tensile forces in the bracing elements. When the applied forces are reversed, the plates will rotate in opposite way. During this process the damper is dissipating mechanical energy by means of friction between the sliding surfaces in to thermal energy (heat) and thus minimizing the vibration of the frame structure.