In this blog post, we will look at the misunderstandings and the truth surrounding the cause of the Tacoma Narrows Bridge collapse, and the difference between resonance and flutter.
If you are interested in science, you have probably heard of the scientific term “resonance” at least once. Resonance is a phenomenon in which the amplitude and energy increase when an external force with a similar frequency to the characteristic natural frequency of an object is applied. Since resonance is a phenomenon commonly seen in radio and television in real life, many students may have had many opportunities to come across it in science books, articles, and classes. However, resonance is not just an everyday example. There have been cases where a certain event has been distorted for a long time due to misinformation, and a typical example is the Tacoma Narrows Bridge collapse in 1940.
I have a question for you. Have you ever seen a video of a bridge that sways and then collapses in that article or class? If so, most people would have learned that the bridge collapsed due to a resonance caused by the wind. This was once accepted as the official theory, and many physics textbooks and related books have described it that way. But the truth is different. The Tacoma Narrows Bridge did not collapse due to resonance.
The Tacoma Narrows Bridge is an 850-meter-long suspension bridge that spans the Tacoma Narrows in Washington, USA, and was completed on July 1, 1940. The bridge was said to have been designed and constructed to the highest standards of the time and was designed to withstand strong winds of 53 meters per second. However, four months later, in November, the bridge collapsed due to a wind speed of 19 m/s that had been blowing since the morning. At the time, many people were shocked that the bridge collapsed at a wind speed that was less than one-third of the designed limit, and the cause was pointed to be a resonance phenomenon that occurred when the wind and the bridge’s natural frequency coincided. However, this was a scientifically incorrect interpretation.
At the time of the incident, there was no resonance at the Tacoma Narrows Bridge. Resonance occurs when an external force of a certain frequency is continuously applied to an object. For example, when you push a swing in time with the force you apply when you are on it, the swing goes higher. Resonance only occurs when an external force is applied at a certain frequency. However, the wind on the day of the Tacoma Narrows Bridge collapse blew at a constant speed, so there was no possibility of resonance.
The cause of the collapse of the Tacoma Narrows Bridge was not resonance, but the aerodynamic flutter phenomenon. Flutter is a phenomenon in which unstable vibrations occur in a structure due to air flow, which can be a dangerous phenomenon, especially in long structures such as bridges. At the time, the Tacoma Narrows Bridge had a length of 850 meters, but the width of the two-lane road was narrow and structurally thin, making it susceptible to swaying in the wind. On the day of the collapse, there was enough wind to sway the bridge in the Strait of Tacoma, and the bridge began to bend upward. The combined force of the wind, the tension of the bridge itself, and gravity caused the bridge to swing from side to side with increasing vibration. Eventually, the bridge collapsed under the weight of this vibration.
After the Tacoma Narrows Bridge collapse, the incident caused a major shift in the scientific and engineering communities. Aerodynamic research was conducted to prevent similar accidents, and this led to the establishment of new standards for bridge design. Ten years later, the Tacoma Narrows 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, which made the bridge much more resistant to the vibrations caused by the wind. This incident also had a major impact on the design of other suspension bridges, and the bridges that were subsequently opened were designed to be safer, taking the failure of the Tacoma Narrows Bridge as a lesson.
The Tacoma Narrows Bridge collapse affected not only bridge engineering. Research on flutter has also been active in the aviation sector. When flutter occurs in a high-speed airplane, the wings vibrate violently due to the air flow, which can put the plane in serious danger. To prevent this, structural research was conducted to limit the speed of airplanes and reduce flutter. After the Tacoma Narrows Bridge incident, many engineering technologies developed and had a significant impact on the design of modern bridges and aircraft.
By 1999, scientists had properly identified the cause of the bridge collapse, clearing up the misconception that the Tacoma Narrows Bridge collapsed due to resonance. However, many physics textbooks and related materials still explain this incident in connection with resonance. The Tacoma Narrows Bridge collapse remains an important historical case, but the cause of the collapse is often misrepresented. If someone is under the mistaken impression that the Tacoma Narrows Bridge collapsed due to resonance, why not set them free from 60 years of false accusations and tell them the truth?
