it seems that nearly everyone has seen the short home movie of the vibration and collapse of the nearly new Tacoma Narrows Bridge in 1940, usually in a high school science class. It is a popular video, as kids really pay attention for a minute! But few teachers are then able to give any description as to what happened or why. It is usually used as an introduction to the subject of vibration in machinery or in Church pipe organs or in a pop bottle with some liquid in it.
This presentation is meant as a relatively non-technical discussion of what happened, and why it hasn't happened again, and other applications of the concepts involved.
This last statement seems to have been altered recently! In May 2010, a videotape of a brand-new bridge built in Russia shows the same amazing flexing of the road surfaces. It is also a very narrow bridge, suggesting that maybe we don't learn very well!
The Tacoma Narrows Bridge happened to be made rather narrow for how long it was. At the time (1940) no one realized there was any disadvantage in that! That disaster caused extensive research to be done on vibrations, resonances and oscillations, and their relationships with physical forces. Specifically, a relationship was found between the speed of a constant wind and various "natural frequencies" of structures or flows. That analysis involved a new parameter called the Strouhal number.
Let's consider the situation of that day. There was a relatively constant strong wind flowing crossways to the bridge, at around 40 mph. That is also around 59 ft/sec. The relationship mentioned above involves a Strouhal number. I'm not sure if anyone has ever discovered why, but the Strouhal number is consistently around 0.2 for many situations, and so that is the normal "design value". Keep in mind that no one knew about the Strouhal number in 1940!
The constant windspeed is multiplied by the Strouhal number (59 * 0.2) to get 11.8 ft/sec, a resonance speed that must be avoided.
From the movie, it appears that the rather narrow, two-lane bridge was around 25 feet wide. Across and back is therefore 50 feet. And it was oscillating at maybe once every 4 seconds, or 0.25/second. We can multiply these two values to get 12.5 ft/second as a transverse speed of the resonance of the bridge.
These values are also actually dependent on the length of the bridge (as to the natural frequency of the structure lengthwise and in torsion). The fact that the bridge was so very narrow allowed it to be very flexible in being able to resonate (twist) at the natural frequencies of the structure of the bridge.