By Minseo Kim
One of the most fundamental jobs of an engineer is to construct or develop anything which is fully safe to be used. When we talk about this in terms of building bridges or buildings, this gets extremely vital as the lives of its users and inhabitants are decided upon. In the 21st Century, almost all the constructions are fully safe to be used as engineers spend a copious number of times to carry out simulations to withstand any possible damages occurring from earthquakes, winds, rains, and many other natural events. However, if we go back in time when all the sorts of simulation methods and physics knowledge were not adequately available, the people at that time confronted serious consequences from it.
All objects in this world contain something called a natural frequency of oscillation. This is nothing but the frequency at which objects naturally choose to oscillate when left alone. For example, if we place multiple stationary tuning forks on the wooden table with different frequencies, let’s say 200Hz, 220Hz, and 250Hz each, and we use another tuning fork which is vibrating at 220Hz to have it contacting the surface of the table.
Surprisingly, the results are that the tuning fork of the natural frequency of oscillation (220Hz) is the only one on the table that vibrates.
This is because it was left to vibrate at its natural frequency due to the vibrations from the vibrating tuning fork. However, in real life, something called resonance takes place on similar occasions and this could be extremely perilous, as it can destroy buildings and bridges very easily even if they were constructed reasonably well.
Resonance is a case when a system or an object is forced to vibrate at its natural frequencies. This will continue to absorb more and more energies from vibrations until there is a steep rise in the amplitude in the motion of its vibration. This generally happens when the driving frequency is equal to the natural frequency of an object.
And in 1940, at Tacoma narrows in Washington State, the tragedy occurred.
The suspension bridge, widely known as the Tacoma bridge, began to be built in 1937 purposed to connect Tacoma, Washington, and Kitsap Penisula.
In fact, this bridge was built extensively strong to withstand even a strong tornado from destroying this bridge; the third-longest suspension bridge in the world at the time.
However, all these efforts soon became meaningless just after 4 months when the bridge was built, large fluctuations, and twists throughout the bridge, just like a viper, and collapsed down along with vehicles on the bridge; it all occurred just because of an ordinary wind blow.
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On the day of the Tacoma Bridge collapse, it experienced winds of about 19ms^-1. It might sound bizarre from hearing that the bridge, which was supposed to resist even a tornado, collapsed due to a wind blow. However, it was not the intensity of the wind that led to the destruction of the bridge, but it was the frequency of the wind. It happened all because of resonance; the wind was vibrating at a frequency equivalent to the natural frequency of the bridge. As this continued over time, the bridge absorbed all these energies from vibrations and at one point, there was a steep rise in the amplitude of the vibration which consequently led to collapsing on the bridge. There were approximately 14 large vibrations/minute which significantly weakened the main body and connector of the bridge and soon led to this tragedy.
What did the engineers and scientists do to learn from this mistake?
After the Tacoma bridge incident, it took a long time for scientists and engineers to ponder and come up with a solution to prevent such catastrophe, resonance, from happening. And their effort sprouted into existence through the development of the new civil engineering mechanism, damping.
Damping is a material or a system that is purposed to take away the energy of the oscillating system as the work is done and the energy is dissipated on the damping system and hence, the amplitude of oscillations decreases. This is the direct challenge to prevent resonance from occurring, and it worked perfectly.
There are various forms of damping, including light, heavy critical, and overdamping. They are named differently as on how quickly the amplitude is reduced. Thanks to this, it has been applied in many forms around us. Ranging from washing machines all the way to bridges and buildings.
From this lesson, we learned that all of us must learn from mistakes and overcome them by directly confronting them. Despite the fact that the consequences and sacrifices from the Tacoma bridge were astronomical, it prevented further tragedies in the future through coming up with the solution, the damping method. Every building and bridges built on the 21st century hold this in mind and could be the reason why almost all of them are safe to use and live in. It is just very difficult to imagine building skyscrapers in the city without this damping method.
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