The Tacoma Narrows Bridge

This suspension bridge, nicknamed “Galloping Gertie”, was known for its vertical oscillations in windy conditions. It collapsed four months after it opened, on November 7, 1940, a day with winds of 64 km/h.

Video

In this video you can observe two different types of bridge oscillation that happened in the collapse: vertical and torsional oscillation.https://www.youtube.com/watch?x-yt-cl=84503534&x-yt-ts=1421914688&feature=player_detailpage&v=j-

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I will focus on the vertical oscillation of the Tacoma Narrows Bridge.

Vertical oscillation in the bridgeThis kind of oscillation is an example of forced harmonic oscillations, where the sinusoidal motion of the deck is due to an external sinusoidal force (wind).

The vertical oscillations of the bridge displayed two defining characteristics of forced harmonic motion:

1. The vertical oscillations’ frequency varies linearly with the wind velocity.

2. The amplitude of the oscillation has a maximum when the angular frequency of the external sinusoidal force is equal to the natural angular frequency of the bridge deck

When the nature of the motion changed suddenly from vertical to torsional, the result was a violent twisting motion that persisted during 45 minutes. Then the bridge collapsed.