Viscous Damper

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, 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. Viscous dampers provide only damping and no stiffness - the structure is on its own to struggle for stability.


Unbonded Brace

Unbonded brace is often called by other names like 'Yielding brace' or 'Buckling-Restrained brace'. 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.

Unbonded brace 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. Replacement of 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 unbonded brace, including filling of outer tube with concrete and connections, is higher than 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 unbonded braces. Besides, the structure with friction damper is economical to design and always ready to resist earthquakes one after another without replacement.