The Tacoma Bridge collapse was long misunderstood as a resonance phenomenon, but the real cause was aerodynamic flutter. The event had a profound impact on the scientific and engineering communities, leading to significant changes in bridge and aircraft design.
If you’re interested in science, you’ve probably heard the scientific term ‘resonance’ before. Resonance is a phenomenon in which an object increases in amplitude and energy when an external force of a similar frequency to its natural frequency is applied. Resonance is a common phenomenon in real life, with applications in radio and television, so many students will have had many opportunities to encounter it in science books, articles, and classes. But resonance is more than just an everyday example. There are cases where misinformation has distorted the way certain events have been communicated over a long period of time, such as the 1940 Tacoma Bridge collapse.
I have to ask: Have you ever read about it or seen the video of the bridge collapsing in class? If so, most people have probably been taught that the bridge collapsed due to wind resonance. This was once accepted as orthodoxy, and many physics textbooks and books have written about it. But it’s not true. The Tacoma Bridge did not collapse due to resonance.
The Tacoma Narrows Bridge is an 850-metre-long suspension bridge that spans the Tacoma Narrows in Washington State, USA, and was completed on 1 July 1940. It was technically the best designed and constructed bridge of its time, and was said to be designed to withstand winds of up to 53 m/s. However, four months later, in November, the bridge collapsed after a morning of sustained 19 m/s winds. At the time, many people were shocked that the bridge collapsed at wind speeds less than a third of its designed limit, and the cause was blamed on a resonance phenomenon caused by the matching of the wind and the natural frequency of the bridge. However, this was scientifically incorrect.
Resonance did not actually occur on the Tacoma Bridge at the time of the collapse. Resonance occurs when an external force of a certain frequency is continuously applied to an object. For example, when you’re on a swing and you push in time with the force, the swing will go higher, but resonance only occurs when the external force is applied at regular intervals. However, on the day of the Tacoma Bridge collapse, the wind was blowing at a constant speed, so there was no chance of resonance occurring.
The Tacoma Bridge collapse was not caused by resonance, but by a phenomenon called aeroelastic flutter. Flutter is a phenomenon in which the flow of air causes unstable vibrations in a structure, which can be dangerous, especially in a long structure like a bridge. At the time, the Tacoma Bridge was 850 metres long, narrow for a two-lane roadway, and structurally flimsy, making it sway easily in the wind. On the day of the collapse, there was enough wind in the Strait of Tacoma to sway the bridge, causing it to begin to bend upwards. The combination of wind force, the tension in the bridge itself, and gravity caused the bridge to sway from side to side in increasingly large vibrations. Eventually, the bridge was unable to withstand the vibrations and collapsed.
After the Tacoma Bridge collapse, the event sparked a major shift in the scientific and engineering community. To prevent similar accidents, aerodynamic research was intensified and new standards were set for bridge design. Ten years later, the Tacoma Bridge reopened with an improved design and truss structure. The truss structure was designed to allow wind to pass through the structure of the bridge, making it much more resilient to wind-induced vibrations. It also had a major impact on the design of other suspension bridges, and subsequent bridges have been designed to be safer, having learnt from the Tacoma Bridge’s failure.
The Tacoma Bridge collapse didn’t just affect bridge engineering. It also spurred research into flutter in aviation. When flutter occurs on an aeroplane travelling at high speeds, the wings are violently vibrated by the flow of air, which can put the plane in serious danger. To prevent this from happening, aeroplane speeds have been limited, and structural research has been done to reduce flutter. Since the Tacoma Bridge incident, many engineering advances have been made that have greatly influenced the design of modern bridges and aircraft.
By 1999, scientists were able to properly identify the cause of the bridge’s collapse, dispelling the misconception that the Tacoma Bridge collapsed due to resonance. However, many physics textbooks and related materials still link the event to resonance. The bridge collapse remains a historically significant event, but its cause is often misrepresented. If someone is under the misconception that the Tacoma Bridge collapsed due to resonance, why not clear up 60 years of wrongful accusations and tell them the truth?
