What Is Resonance? How Is It Created? How Does It Affect Structures?

” Resonance, The word “is one of the most used concepts in physics, so” What is resonance? ” The question is asked very often. To explain this concept, we must first have a thorough understanding of the following terms:

Period and Frequency Concepts

  • Period

– the time for the formation of a full-wave period is called. The period is denoted by the letter T.

  • Frequency

– The number of waves formed per unit of time is also called frequency. The frequency is indicated by the letter f.

What is Resonance?

Resonance is the tendency of a system to oscillate at some frequencies at greater amplitudes than others. Resonance occurs when a material oscillates at a high amplitude at a certain frequency.

Rezonans in Dictionary;

  • “The state of a system in which an abnormally large vibration is produced in response to an external stimulus that occurs when the stimulus frequency is the same or almost the same as the natural vibration frequency of the system.”

Resonance in Physics;

  • “Resonance is the tendency of a system to oscillate at some frequencies at greater amplitudes than others. These are called the resonance frequencies of that system. At these frequencies, even small periodic forces can produce very large amplitudes. “

Resonance in Engineering;

  • Technically in resonance engineering; It is explained as “amplitude going to infinity”.

What is Resonance? – Why Does Resonance Occur?

We know that there are oscillations in the system that is under a periodic effect. The amount of displacement the system makes relative to its normal state during oscillations is called amplitude. If these oscillations are equal to the natural frequency of the system, the amplitude of the system tends to increase infinitely; this event is called resonance.

The effects that can cause oscillation can be very diverse. For example; A bridge under the effect of intermittent wind, a building under the influence of oscillations caused by earthquake waves, or an electrical system under the effect of alternating voltage may resonate.

For linear systems to resonate, the oscillation amplitude must be directly proportional to the applied force. If the frequency of the applied force is equal to the natural frequency of the system, resonance occurs. If we consider a bridge under the effect of intermittent wind, as a result of vibrations and oscillations caused by sudden and variable wind blowing, the natural frequency of the bridge and the periodic wind frequency to which the bridge is exposed can be equal.

As a result, the oscillation amplitude will begin to go to infinity, and the bridge will resonate and collapse after a while. A true example of this happened at the Tacoma bridge built in Washington in 1940. This bridge was destroyed by resonating with the effect of wind.

Why Do Some Buildings Crumble While Others Survive During an Earthquake?

All buildings have a natural period or resonance; This is the number of seconds it takes for the building to naturally vibrate back and forth.

Also, the ground has a certain resonance frequency. Hard rocky floors have higher frequencies and softer deposits.

If the period of ground motion is the same as the natural resonance of a building, the structure will be exposed to the greatest possible oscillations and suffer the greatest damage.

Let’s explain the situation with an example as follows:

While swinging on a swing, we accelerate by following the oscillation movement and positioning our feet and body back and forth at the right moment. We can compare the concept of building resonance to this. When the direction of the earthquake is the same as the direction of the oscillation of the building, the building will be exposed to more and faster movement and possibly take a significant amount of damage.

For example; Let a building be designed to withstand the effects of 5 units of acceleration. Let the frequencies of the ground and the building be 3 units of force. Since the frequencies of the ground and the building will be in different directions at different times or at the same time, they will damp each other or the building will encounter a maximum of 3 units of acceleration and will remain standing.

However, if the frequencies of the building and the ground are the same (resonance) since two acceleration forces will combine at the same time, a force of 6 units of acceleration will affect the building and cause the building to collapse.

How to Prevent Resonance in Buildings? What Kind of Precautions Can Be Taken?

Resonance is one of the most important events that cause buildings to collapse in earthquakes. When the oscillations are equal to the natural frequency of the building, the building collapses, unable to withstand the increased amplitude and the stress this causes. It is not possible for a resonant object not to be damaged. However, the resonance of the object can be prevented by taking precautions.

This precaution is that the structure can damp the vibration with its oscillations. Earthquake-resistant flexibility tolerance of up to the size of 9 can be applied in architectural structures built today.

What is Resonance? What are the effects?

In this way, the buildings dampen oscillations caused by earthquakes up to 9 magnitudes, and the natural frequency of the building is prevented from equalizing the vibration frequency. Especially in Japan, buildings are built with a size of 9 because there are very large and active fault lines in this area, so earthquakes are often experienced.

Earthquakes can also resonate not only with buildings but also the ground. For example, on wet soil, under the periodic forces caused by the earthquake, the sand particles can slide over each other and settle in the interstitial spaces.

In this case, the ground turns into a sand pile with fewer interstices and some of the water filling the gaps goes up and covers the sand pile. If there are buildings built on such ground, they can lean back and forth or slide completely. In some cases, even if the building is earthquake resistant, it may collapse as a result of resonance and slipping of the ground.

Why Is Building Resonance Important?

The response of a building to shaking at its base due to seismic waves depends on several factors related to its design and construction. However, one of the most important factors is simply the height of the building, as this determines the building resonance frequency. Short buildings have a high resonant frequency (short wavelength), while tall buildings have a low resonant frequency (long wavelength). In terms of seismic hazard, therefore, short buildings are susceptible to damage from high-frequency seismic waves from relatively imminent earthquakes. On the other hand, tall buildings are at risk due to low-frequency seismic waves that may have originated from much greater distances.

Building Resonance and Engineering Connection

Structural engineers calculate the natural frequency of buildings to design buildings and other man-made structures to withstand earthquake and storm forces. For example, if the frequency of seismic waves matches the natural frequency of a building, resonance occurs and the structure is distorted. Engineers conduct research and field studies to learn how various structural designs and materials perform under expected hazardous conditions so they can design the safest structures possible. They also design real-time wind and earthquake monitoring systems that include wind, vibration, and ground motion sensors to provide early failure warnings.

High or Low-rise Buildings Have More Resonance?

While tall buildings shake loud and fast during earthquakes, the “soft” shaking is a result of the building resonance response to the low-frequency waves emitted by large earthquakes. Imagine a particularly tall building being pushed from below to the side. The gigantic building has a lot of inertia and it takes time for the underlying force to be transmitted upwards through the beams. When the forces reach the top (possibly less than a second) the entire building will be in motion and move sideways. If you suddenly stop the bottom, the top will retain momentum and the structure will overcome this position until its stiffness stops it, and the elastic property that allows it to stretch forces it to recover this deformation and turn in the other direction. This single push (pushing the building a certain distance) results in an oscillation at the unfixed end of the building (top) while the force applied to its base is gradually reduced or absorbed by stretching, heating, and squeaking.

Now, instead of stopping the bottom of the building, if you reverse it and push it back in the other direction, this will increase the distance the hill must travel when it returns, accelerate the swing oscillation, and increase the swing. The same goes when you push someone on a swing: Push the rocking person as they swing “forward” so that the energy you enter the system is added to the energy already pulled by gravity. If you push them as they come towards you, all your energy is wasted resisting the power of the “back” oscillations and the system loses all its energy.

To make a tall building truly shake, seismic waves must drag its base back and forth at a frequency that matches the building’s natural oscillation, that is, its resonant frequency. In other words, if you “tear off a building from the ground” and let it sway, it will do so at a frequency determined by its material properties, geometry, and weight, among other things. If you then continue to shake at the same frequency, you will “resonate” by emphasizing the motion like the same set of oscillations.

Taller buildings have lower natural resonance frequencies than shorter buildings; This means that when a wide range of seismic wave frequencies is released, buildings with different resonant frequencies will sway differently. Small 2-story houses have extremely high resonance frequencies and are therefore more susceptible to very sharp seismic waves most strongly experienced near the epicenter of the earthquake.

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