In this blog post, we will focus on “The Principle of Resonance and Examples in Architecture and Daily Life.”

Introduction

Do you remember kicking your feet to swing higher on a swing when you were a child? Can you believe that the principle behind this simple action—one we perform naturally without any instruction—could bring down even the sturdiest structures we can imagine in an instant?

The Principle of Resonance

Resonance is generally a phenomenon where the amplitude increases and energy builds up when an external force of the same frequency is applied to an object that already has a specific vibration frequency. A frequently cited example is the collapse of the Tacoma Narrows Bridge in Washington State, USA. The Tacoma Narrows Bridge, built across a strait near the coast, was a suspension bridge with the deck secured by cables connected to piers on both sides; it was narrow and approximately 853 meters long. Although it was originally designed to withstand winds of up to 53 meters per second, the wind speed at the time of the collapse was only 19 meters per second. When this wind struck the thin deck, it caused vibrations whose frequency matched the bridge’s natural frequency, triggering a resonance phenomenon that amplified the vibrations. Ultimately, the bridge could not withstand the vibrations and collapsed.
Let’s understand this through a simple experiment. When a weight suspended from a string is swinging back and forth, if you gently push it in the direction of its movement at the right moment, the swing becomes progressively larger. When the interval of the weight’s swing matches the interval of your pushes, the amplitude increases—this is precisely how resonance occurs.
Applying this process to buildings might raise the question: “Doesn’t resonance only begin if the building is already shaking?” However, the moment the wind impacts a building, it begins to vibrate. Since buildings above a certain height are always exposed to the wind, they can be viewed as systems that are constantly vibrating. Of course, they are optimized during design to minimize internal impact, so it is usually difficult to feel wind-induced vibrations inside the building.
It is important to note that each building has its own natural frequency. Just as tapping on several cups filled with different amounts of water produces different sounds, buildings also vibrate at their own unique natural frequencies. When a structure vibrates, it always does so at its own specific natural frequency.
Another question is how wind causes a building to vibrate when it acts upon it. Imagine waving your arm underwater. Even if you move it straight with force, you can feel it swaying from side to side due to water resistance. Since air is also a fluid, when wind blows against a building, a similar type of vibration is applied to the structure as when your hand sways underwater.
The problem arises when the frequency of the vibration caused by the wind matches the building’s natural frequency. If this situation persists, the building finds itself in the same predicament as the pendulum in the earlier example. When an external force maintains a constant amplitude and the frequency matches the building’s natural frequency, the amplitude gradually increases until it eventually exceeds the building’s structural limit, potentially leading to collapse. Just as David defeated the giant Goliath, even a breeze that feels gentle to us can bring down a building.

The Risks and Applications of Resonance

That said, there is no need to feel anxious every time you are inside a building. Learning from past disasters, the field of vibration engineering has advanced, and most buildings are designed during the planning phase to minimize wind-induced resonance. Additionally, regulations require the installation of dampers (vibration absorbers) in buildings above a certain height to reduce resonance, significantly lowering the actual risk.
On the other hand, resonance is not a phenomenon that must be avoided at all costs. There are many technologies around us that utilize resonance. A prime example is the magnetic resonance imaging (MRI) machine, which generates vibrations at a frequency matching the natural resonance frequency of the hydrogen nuclei in the human body to obtain internal images. It creates images by detecting the energy emitted as the hydrogen nuclei inside the body resonate, causing their energy to rise and then return to their original state.
In addition, the sounds produced by musical instruments, such as string and wind instruments, are excellent examples of resonance in action. String instruments produce sound through the resonance of their strings, while wind instruments utilize the resonance of air within their tubes; depending on where the player holds the instrument or how they manipulate it, resonance occurs at specific frequencies, producing the desired notes.
As such, while resonance can threaten human life, understanding its nature and utilizing it appropriately can also make it a useful tool for civilization. The next time you attend an orchestra performance or undergo an MRI scan, recall the story of David and Goliath. By reflecting on both the destructive power hidden in a gentle breeze and the benefits resonance brings to our lives, your daily routine will become a little more interesting.