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The Tacoma Bridge Incident: it's engineering flaws, environmental factors and the development based

On October 7th, 1940, a shocking event that forever marked worldwide physicists and engineers occurred; the Tacoma Narrows bridge, one of the largest suspension bridges in the United States, collapsed. Having been opened in July of the same year, the bridge had already been nicknamed "Galloping Gertie" by construction workers due to its abnormal behaviour of vertical motion under windy conditions. This behaviour was continuous even after the bridge opened to the public. Even though, during the collapse, no people were on the bridge, the accident was fatal to Tubby (R.I.P Tubby), a three-legged cocker spaniel inside a car while the bridge fluttered and then fell. For a long time, there was speculation that the bridge collapsed because of resonance, but it was later concluded that the origin of the tragedy instead led back to aeroelastic flutter.

An article published by the New York Times two days after the accident compared the bridge to a pendulum. It stated that "...Time successive taps correctly and soon the pendulum swings with its maximum amplitude. So with this bridge. What physicists call resonance was established." [Harvard] But before debunking this explanation, it is important to understand what resonance is. Resonance is defined as "A phenomenon in which an external force or a vibrating system forces another system around it to vibrate with greater amplitude at a specified frequency of operation." [Byju's]. A famous example of it is the constantly seen depiction of an opera singer breaking a wine glass. Check this video to observe the phenomena. In the case of the Tacoma bridge, NY times claimed that it occurred due to the natural frequency of the bridge having been matched by that of the wind. However, at the day of the collapse, the wind's frequency was said to not have been equal to that of the bridge, but instead, was constant, and not oscillating, meaning that resonance could not have been the true cause of the accident.

Decades of research later, it was then determined by physicists that the previously mentioned aeroelastic flutter was what actually led to the collapse. Aeronave elastic flutter is the same oscillating effect that could be observed if a piece of paper was held up, facing the ground, and then was blown from the side by a fan, which would cause it to oscillate [minute physics]. As seen in this video, it is the same effect that can be seen in oscillating aeroplane wings. Just like it had done for months, on October 7th, 1940, the bridge undulated in the wind, however, during the time of the accident, winds stronger than usual caused the bridge to twist slightly, but then gravity and tension twisted it back down more intensely, while the wind twisted it again but stronger and this cycle repeated itself until this higher-than-usual motion caused one of the cables to snap.

It can be said that the main engineering failure of the Tacoma bridge was the fact that it was a non-aerodynamic body, which can be noted through several different elements of it. Firstly, since the bridge had large, solid steel plates instead of a truss through which the wind could flow through, any magnitude of twist gave way to vortices in locations that enabled the expansion of the twisting motion. Apart from the large steel plates, a feature that is usually included in modern bridges to avoid the flutter effect, is a gap in the centre of the deck to make the pressure equal in both sides, and this was not done in the Tacoma bridge, even though it can be partially justified due to the limited knowledge in relation to flutter at the time of its development. Lastly, another notable feature was that the side spans were disproportionately long in relation to the centre span, along with the excessively large distance between the side span and the cable.

After the accident happened, the flaws in the tacoma bridge were brought to light and used to build on human knowledge in physics and engineering, even aiding the safer construction of other bridges. Because of the analysis of flutter models that were built to recreate the accident, a new field of engineering sparked, aerodynamics-aeroelastics, helping improve on the design of bridges decades and decades later. Therefore, it can be concluded that even catastrophic failure can provide us with an opportunity to seek improvement, especially in engineering, that evolves to find more cost effective and safer structural solutions.


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