Viscous fluid dampers
are similar to shock absorbers in a car. They consist
of a closed cylinder containing a viscous
fluid like oil. A piston rod is connected to a piston
head with small holes in it. The piston can move in and out
of the cylinder. As it does this, the oil is forced to flow
through holes in the piston head causing friction.
When the damper is installed in a building, the friction
converts some of the earthquake energy going into the moving
building into heat energy.
The damper is usually installed as part of a building's bracing
system using single
diagonals. As the building sways to
and fro, the piston is forced in and out of the cylinder.