In this blog post, we’ll explore what resonance is and examine the principles behind why high-rise buildings can be safer during earthquakes than low-rise buildings.
Major cities around the world are home to famous skyscrapers. When looking at skyscrapers like the Shanghai World Financial Center in China, everyone has probably wondered at least once, “What would happen to a skyscraper like that if an earthquake struck?” However, when designing skyscrapers, engineers comprehensively consider not only earthquakes but also strong winds, structural sway, and occupant comfort. Generally, high-rise buildings are often more earthquake-resistant than low-rise buildings. In other words, during certain earthquakes, low-rise buildings may actually be more severely affected. The reason for this lies in a phenomenon called resonance. So, let’s explore what resonance is and why, in some cases, low-rise buildings can be more vulnerable to earthquakes than high-rise buildings.
To understand the phenomenon of resonance, let’s recall our childhood memories of riding a swing. We might have sat still while someone pushed us, or, as we got a little older, moved the swing ourselves by bending and straightening our legs. Children with good motor skills could even stand on the swing and swing higher using only their body movements. No matter how you swing, the swing swings higher and higher with increasingly larger swings without requiring much effort. How can a swing reach such heights without a significant external force?
The principle behind the swing rising higher and higher is resonance. To understand resonance, we first need to understand the concept of vibration. In engineering, vibration refers to the phenomenon where an object moves back and forth with a constant period. Everything from vibrations that are large enough to be seen with the naked eye—like a pendulum—to the microscopic movements of molecules that are invisible to the eye falls under the category of vibration. The time taken for one round-trip motion is called the “period,” and the number of round-trip motions that occur in one second is called the “frequency.” The period and frequency are inversely proportional to each other.
For example, consider Chul-soo, who eats candy at a constant rate. Saying, “Chul-soo eats 10 pieces of candy per second,” expresses this in terms of frequency, while saying, “It takes Chul-soo 0.1 seconds to eat one piece of candy,” expresses it in terms of period. When understanding resonance, it is easier to think in terms of frequency rather than period.
There’s a reason I explained frequency first. When any object is struck, it vibrates for a brief moment, and each object has its own natural frequency. This natural frequency is determined by the object’s mass, stiffness, and shape, regardless of the magnitude of the impact. Let’s revisit the example of Chul-su and Young-hee from earlier. Even if you give them the same number of candies, Cheol-su eats 10 per second, while Young-hee eats 5 per second. Just as people eat at different speeds, every object has its own natural frequency. This is called the “natural frequency” or “natural vibration frequency,” and it is determined by the object’s mass and structural characteristics.
So, what happens if a force with the same frequency as the natural frequency is continuously applied to an object? When a force of a constant frequency is repeatedly applied and matches the object’s natural frequency, a very interesting phenomenon occurs. In this case, even a small, continuous force can cause the amplitude to increase dramatically. This phenomenon, in which the externally applied frequency matches the object’s natural frequency, leading to a significant amplification of the vibration, is called resonance. It is precisely this phenomenon of resonance that causes a swing to swing higher and higher.
Let’s imagine riding a swing. The swing has its own natural vibration, and the person creates additional vibrations by moving their legs. When the swing begins to move downward, you extend your legs, and when it moves backward, you bend your legs, causing it to swing higher and higher. This happens because the swing’s natural motion and the vibrations created by the person are in sync. In terms of the concept explained earlier, resonance occurs when the swing’s natural frequency and the frequency generated by the person are nearly identical, allowing a large amplitude to be produced even with a small force. Even young children who are unaware of this principle can naturally learn to swing well through experience by mastering the proper timing—and behind this lies the physical principle of resonance.
The reason low-rise buildings are more vulnerable to earthquakes than high-rise buildings is directly related to natural frequency. However, contrary to the explanation in the original text, the natural frequency of high-rise buildings is generally lower than that of low-rise buildings. This is because as a building gets taller, its oscillation period lengthens and its natural frequency decreases. Earthquakes generate seismic waves with a wide range of frequencies; low-rise buildings may be more severely affected by seismic waves with relatively high frequencies, while high-rise buildings may be more severely affected by those with relatively low frequencies. Therefore, high-rise buildings are not always safer in every earthquake; in certain earthquakes, low-rise buildings may sustain greater damage, while in other cases, high-rise buildings may be more severely impacted. The key point is that when a building’s natural frequency matches that of a seismic wave, resonance occurs, causing the shaking to be greatly amplified. Modern buildings are designed to avoid this resonance as much as possible, which significantly reduces the risk of collapse even during major earthquakes.
This resonance phenomenon is a factor that mechanical engineers must take into account when designing machinery. For example, let’s say you’re designing a fan. If the fan’s rotational speed is set to match the structure’s natural frequency without considering resonance, even a small vibration can gradually amplify until the fan eventually fails and breaks. Another example is an experiment shown on TV where a soprano shatters a wine glass using only her voice. If the soprano precisely matches the natural frequency of the wine glass, resonance occurs, causing the vibrations to amplify significantly and eventually shattering the glass. As this example shows, resonance is a critical factor even in small machines or objects, and its importance grows as the scale of the structure increases.
A prime example is the collapse of the Tacoma Narrows Bridge in the United States. Shortly after opening in 1940, the bridge began to sway violently due to strong winds and eventually collapsed. For a long time, this incident was cited as a classic example of resonance, but it is now understood to have resulted from a combination of factors, including not only resonance but also aeroelastic flutter and structural vibrations. However, this incident is still widely cited as a prime example demonstrating that failure to fully account for the interaction between a structure’s natural vibration characteristics and external forces can lead to catastrophic consequences.
Nevertheless, resonance is not necessarily something to be avoided at all costs. On the contrary, when properly harnessed, resonance can yield significant results with minimal energy. Tasks that are difficult to perform using conventional methods can be carried out more efficiently by utilizing resonance. For example, ultrasonic cleaners use resonance and vibration to remove microscopic contaminants, and this principle is also applied in medical ultrasound equipment and industrial ultrasonic technology. Furthermore, some wireless charging technologies use magnetic resonance to transfer power. As such, even resonance—which is generally considered something to be avoided—can become a highly useful technology when applied appropriately.
From a child’s swing to building design, various machines, and cutting-edge technologies, the phenomenon of resonance is a crucial principle. Whether to avoid resonance or to actively utilize it, the natural vibration characteristics of an object must be thoroughly considered in every design process. Understanding this principle makes it easy to grasp how the various machines and structures we encounter in daily life are made safer and more efficient.
