Viscous dampers are velocity dependent. The forces exerted by the damper are, therefore, different for different earthquake records. Friction dampers are independent of velocity, therefore, unlike viscous dampers ,exert constant force for all future earthquakes (DBE / MCE). A friction-damped structure is an engineered structure in which forces exerted are predetermined. This is a great technical and economic advantage over viscous dampers in which forces are much higher at MCE level.
The hysteresis loop of viscous damper is elliptical compared to rectangular for friction damper. For a given maximum force, the area of hysteresis loop (energy dissipation or damping) of viscous damper is about 70% of that for friction damper i.e. 70 friction dampers will achieve the same damping as 100 viscous dampers of a given force. Conversely, for a given number and damping value, the forces exerted by friction dampers are only 70% of those for viscous dampers. This leads to significant savings in cost of dampers, bracing, connections, columns and foundations.
While supplemental damping is beneficial in reducing the earthquake forces and amplitudes of vibration, added stiffness is beneficial for stability. A deflected building is similar to a stooped person. Similar to a cane support for stooped person, additional stiffness helps the deflected building against overturning. Pall Friction Dampers provide both added damping and added stiffness for stability - a complete structural solution.
Buckling Restrained Brace
Buckling-Restrained Brace (BRB) is often called by other names like 'Yielding brace'or‘Unbondedbrace’. It consists of a slender steel brace (core steel), surrounded by a steel tube or pipe, which is then filled with concrete or grout. The core steel is wrapped with plastic like material to separate it from the concrete around. The concrete in steel tube prevents buckling of the steel brace.
BRB dissipates energy through the process of yielding. Yielding involves damage. No damage, no energy dissipation or damping. After an earthquake, the brace may be damaged and needs to be replaced. Replacing a brace after an earthquake is expensive and time consuming. It is too difficult to replace before imminent aftershocks. Another problem is that the brace is enclosed in concrete and is not visible for inspection to verify if it is broken or otherwise.
The cost of supply of core steel and wrapping material may appear to be small, but the overall cost of an installed BRB, including filling of outer tube with concrete and connections, is comparable to the installed friction damper. As friction dampers dissipate a large amount of energy mechanically, the forces exerted are far less than those exerted by the BRB. Besides, the structure with friction damper is economical to design and always ready to resist earthquakes one after another without replacement.
Rotational Friction Dampers
Pall Friction Dampers® were Patented in 1982. Of the several types ofconfigurations, one type of our friction damper for tension-only cross bracing uses both rotational friction joints as well as slotted friction joints. Since 1987, such dampers have been used in many projects. Knowing fully well that rotational friction joints are much less efficient than slotted friction joints, these were purposely used becausethe change in geometry caused by therotation of this unique device was needed to keep the long slender braces from buckling. All other types of Pall friction dampers have only slotted friction joints.
Although the output of a friction damper can be maximized by applying higher clamping force, higher coeff, of friction, multiple friction layers and using loose friction pads to develop two-sided friction; but these efforts adversely affect the long term performance of damper, namely; freezing or jamming of friction damper after long inactivity,higher creep or relaxation in bolt, erratic behavior due to stick-slip phenomenon in loose friction pads as at times only one side andsometimes two sides participate. Such temptations on cost savings should be avoided for friction dampers to be used in buildingstructures.
Pall Friction Dampers® are responsibly manufactured with utmost importance to quality and long term high performance. Installed Pall friction dampers have been load tested after 10 years of installation with excellent results.
In 1988, Anagnostides, Hargreaves and Watt of Imperial College of Science and Technology (U.K), had developed a friction damper usingrotating joints (ESEE Research Report No. 88-1, February 1988, Earthquake Engineering). Their application for patent was denied on the basis that prior art existed. On the same analogy, the legality of patent for Rotational friction damper (2002) is questionable.