MA 325 Mechanical Vibrations

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MA 325Mechanical Vibrations
Hien T. Tran
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The Tacoma Bridge disaster

• July 1, 1940, the Tacoma Narrows Bridge at Puget
  Sound in the state of Washington was completed
  and opened to traffic
• From the day of its opening the bridge began
  undergoing vertical oscillations
• Nicknamed Galloping Gertie
• For four months, the bridge did a thriving
  business as people came from hundreds of miles in
  their cars to enjoy the thrill of riding over a
  galloping, rolling bridge
• The authorities become more and more confident of
  the safety of the bridge that they were planning
  to cancel the insurance policy on the bridge

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The Tacoma Bridge disaster (contd)

• Starting at about 700 on the morning of November
  7, 1940, the bridge began undulating persistently
  for three hours

Segments of the span were heaving periodically up
and down as much as three feet
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The Tacoma Bridge disaster (contd)

• At about 1000 a.m., something seemed to snap and
  the bridge began oscillating wildly

When the twisting motion was at the maximum,
elevation of the sidewalk at the right was 28
feet (8.5m) higher than the sidewalk at the left
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The Tacoma Bridge disaster (contd)

• At 1030 a.m. the bridge began cracking, and
  finally, at 1110 a.m. the entire bridge came
  crashing down

Notice the car in the top right corner. It
belongs to a newspaper reporter who had abandon
the car and its sole remaining occupant, a pet
dog (the only life lost in the disaster)
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The Tacoma Bridge disaster (contd)

• Humorous and ironic incidents
• Professor F.B. Farquharson (Univ. of Washington)
  had conducted numerous tests on a simulated model
  of the bridge and had assured everyone of its
  stability
• He was the last man on the bridge (even when the
  span was tilting more than twenty-eight feet up
  and down)
• He made it to safety by scientifically
  following the yellow line in the middle of the
  roadway.
• A local travel agent was imprisoned for pocketing
  the insurance policies on the bridge (800,000).
  He ironically pointed out that his embezzlement
  would never have been discovered if the bridge
  had only remained up for another week (at which
  time the bridge officials had planned to cancel
  all of the policies)
• A large sign near the bridge approach advertised
  a local bank with the logan as as the Tacoma
  Bridge. Immediately following the collapse of
  the bridge, several representatives of the bank
  rushed out to remove the billboard

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Epilog

• The original plans for the Bronx Whitestone
  Bridge in New York City were very similar to
  those of the Tacoma Bridge. These plans were
  hastily redrawn following the collapse of the
  Tacoma Bridge

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Active Noise and Vibration Control

• Lord Corporation
• In many aircraft, a major component of the
  interior sound pressure field is due to
  structure-borne noise from the engines
• For example, the rotating blades in turboprop
  engines generate low frequency, high displacement
  acoustic fields that couple with the fuselage
  dynamics
• These mechanical vibrations produce, through
  interactions with the air inside the cabin,
  interior sound pressure oscillations.

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Vibration Control

• Commercial products
• Tennis rackets

• Skis

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Beam Vibration