How could a reinforced concrete bridge across the Taiwan Strait be easily destroyed by the wind?

Bridges made of reinforced concrete are very strong, especially those cross-sea bridges that integrate various "black technologies", which can withstand strong typhoons.

For example, our Hong Kong-Zhuhai-Macao Bridge is designed to withstand a maximum wind force of 16, which means that even a 16-force typhoon cannot do anything to it. So, if there is a Strait Bridge that collapses when blown by ordinary wind, what is the reason? The first thing we think of is that there are quality problems with the bridge. But in fact, there is such a bridge, which has no problems from design to construction, but it was easily destroyed by the wind four months after it was built. This bridge is the "Tacoma Narrows Bridge". In fact, this bridge has been shaking in the breeze since it was built, but people did not expect that such a bridge with no obvious quality problems would eventually collapse.

Why was the Tacoma Narrows Bridge destroyed by the wind? It is due to a fluid mechanics phenomenon called "Karman vortex street".

What is a Karman vortex street? Suppose there is a straight stream flowing smoothly forward. What will happen if we put a log in the center of the stream? We usually give this answer: the stream will fork due to the obstruction of the log, with one part flowing from the left side of the log and the other from the right side. After forking and flowing through the log, the two streams will reunite and continue to flow smoothly forward. This is a self-evident answer, but it is in line with our common sense and logic, so not only do we think so, but von Karman's mentor Ludwig Prandtl also thinks so.

In order to study the laminar flow phenomenon, Prandtl asked his students to make a device similar to a log inserted in the middle of a stream. However, no matter how he improved it, he could not produce the imagined situation of the water flow branching and then converging.

Prandtl believed that the experimental device was not made perfectly, but von Karman had his own opinion. Through observation, von Karman found that after the water flow was blocked by the log, it would form staggered vortices after flowing through the log. To put it simply, the water flow first passed quickly from the left side of the log and formed a downward vortex after passing through. Then the water flow would quickly pass through the right side of the log, pushing the first vortex formed away, and then forming an upward vortex. In this way, the water flow staggered through both sides of the log and formed vortices, which were pushed away by the newly formed vortices. Von Karman called this phenomenon "Karman vortex street."

The appearance of the Karman vortex street leads to another phenomenon, which is the vibration of the logs.

Why does the log vibrate? This is due to the "Bernoulli principle". The Bernoulli principle states that in a flowing liquid or gas, the faster the fluid flows, the smaller the pressure, and vice versa, the slower the flow, the greater the pressure. When the Karman vortex street phenomenon occurs, the water flows quickly from the left side of the log. At this time, the flow rate on the left side of the log is fast, so the pressure is small, while the flow rate on the right side is slow, so the pressure is high, so the log will be pushed to the left, and then the water flows quickly through the right side of the log, so the pressure on the right side is small, while the pressure on the left side is high, so the log will be pushed to the right again. Under the interaction of the left and right forces, the log vibrates.

The Tacoma Bridge is a very spectacular cross-strait bridge. Its length reaches 1,524 meters, but the width of the bridge deck is only 11.9 meters. It can be said that the entire bridge is thin and long, which laid the first hidden danger of it being destroyed by the wind in the future.

Suspension bridges usually have truss beams installed on both sides of the bridge, which is a basic configuration. The Tacoma Narrows Bridge is no exception. In its original design, there was indeed a 7.6-meter truss beam, but due to tight budget, the famous suspension bridge designer Moisev changed it to a 2.4-meter steel plate. Moisev believed that such a change would not only greatly reduce the construction cost of the bridge, but also increase the rigidity of the bridge. There is no doubt that Moisev's view is completely correct, but he overlooked the fact that the characteristic of the truss beam is "air-permeable", while the steel plate is not air-permeable.

Not long after, the bridge was built. The steel plates on both sides of the bridge blocked the passage of airflow, and the Karman vortex street phenomenon appeared. The bridge has been shaking constantly since its completion.

Although the bridge was shaking, people did not think it was dangerous and it became a tourist attraction. It was not until four months later that a steel cable broke because the bridge had been shaking for too long. The shaking of the bridge turned into twisting and finally collapsed. After the collapse of the bridge, an investigation was conducted on the entire bridge collapse accident. It was found that there were no problems from the design to the construction, so it was decided to rebuild it according to the original plan. At this time, von Karman also noticed this incident and made calculations. The calculation results showed that the collapse of the bridge was inevitable, so he hurriedly wrote a letter to prevent the same mistake from happening again.

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