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| The Spire of Dublin. Image courtesy Vmenkov. CC BY-SA 3.0 Unported via Wikimedia Commons. |
Did
you know The Spire in Dublin has a shock absorber? Technically these
are called dampers and your car and even your washing machine has them
to reduce vibrations. Dampers are also used on doors, cupboard drawers
and even toilet seats to slow their movement. Many large structures such
as bridges need dampers to reduce oscillations caused by wind or
vehicular traffic.
Resonance
All
objects have a natural frequency that they tend to vibrate at. The
phenomenon is called resonance and that's how musical instruments work. A
column of air in an organ, string on a guitar or bell will vibrate or
oscillate at its natural frequency when struck, plucked or vibrated
using a reed. If you blow over the mouth of a bottle, the air inside
vibrates at a certain pitch. Resonance frequency depends on several
factors including physical dimensions, speed of sound in air and mass of
the object that's resonating. So in an organ or piano, the longest
pipes or strings respectively give the lowest notes. Large structures
can resonate due to strong winds and eventually the oscillations can
build, ultimately leading to destruction of the structure. (Think of
pushing a child on a swing and pushing at the right moment causes the
swing to extend further and further).
What's a damper?
Dampers are used to reduce oscillations so that they continue to decrease in size and die out.
A
mechanical damper is typically a cylinder with a piston and also an
external coil spring. The viscous damper is connected to the object
being damped. As the piston slides along the cylinder, filled with air
or oil, viscous friction slows it's movement, converting the energy of
the oscillating object into heat and reducing the amplitude of
oscillations and causing them to decrease in amplitude.
The Spire of Dublin
The
Spire in Dublin is a 120 m tall structure and to reduce potentially
damaging oscillations, a damper is built inside its structure. The
reciprocal of resonance frequency is the period. This is the length of
time it takes to do one complete oscillation. So if a guitar string
vibrates 100 times a second (100 Hz), the period of the oscillation is
1/100 = 10 mS. The period of The Spire has been analysed as 3.65
seconds. That means when it's oscillating, it sways backwards and
forwards once every 3.65 S. To absorb the oscillations and damp them
out, the damper was tuned to this frequency. It consists of two large
masses, 2 tonne weight in total, attached to the inside of the structure
by viscous dampers. You can read more in this article.