What is Resonance? How does Resonance Affect Structures?
The concept of “resonance” is one of the factors that play a key role in explaining the universe and earth physics on both the earth (macroscopic) and atoms (microscopic) scales. The resonance phenomenon is related to oscillating systems, earthquakes and the oscillation movement of the earth, the frequency of this oscillation movement and the internal oscillation frequencies of the buildings are directly related to the resonance event. From an oscillating system statement, we understand systems that start in motion from the static equilibrium position, move away from the starting point, and then return to the starting point, as in the pendulum example.
“What is Resonance?” in order to answer the question, we need to know some concepts. Let’s talk about these concepts first in order to eliminate misunderstandings and confusions on this subject. As it is known, “period” is the time when a repetitive motion of an object completes one complete turn.
“Building natural periods”; refers to the time it takes to complete one full tour during oscillation movements-vibrations occurring in buildings with the effect of horizontal forces. “Soil dominant period” is the oscillation period in the motion of earthquake waves that develops depending on the characteristics of the soils while spreading on the ground. “Frequency” is the reverse of the period (f = 1 / T) and indicates the motion of the object in one second.
What is Resonance in Buildings-Buildings?
If the “resonance” event occurring in the buildings is the same as the ground dominant period (hence the frequency) and the natural period (hence the frequency) of the structure, the addition of two forces in the same direction that causes oscillations to the building, and as a result of the oscillation (amplitude) of the building, thus the acceleration (hence the effective force). increase event. As a result of resonance, the structures are exposed to great forces and therefore great damage. Even collapses due to resonance are depicted as the explosion of the building.
Why Does Resonance Occur?
Let’s try to explain “resonance” roughly as follows; Let’s say a building is designed to withstand the force of 3 units of acceleration. Assume that the building and ground brakes generate 2 units of acceleration. When the ground frequency and the structure frequency are different, these 2 units of acceleration will affect the building at different times (or directions) and the maximum acceleration the building faces will be 2 units. Therefore, the building will not be damaged or will receive limited damage from this acceleration.
However, when the frequencies of the ground and the building are the same, these two accelerations will simultaneously affect the structure “in the same direction” and a total acceleration of 4 units will occur. Therefore, the strength of the structure will be exceeded and the building will receive great damage. The phenomenon of these forces occurring in the same direction and affecting the structure in the same way and causing great damage to the structure is called “resonance of the building”.
SEE ALSO: What is the Butterfly Effect?
How to Prevent Resonance in Buildings? What Kind of Precautions Can Be Taken?
As can be understood from above, the resonance does not arise solely from the ground factor or solely from the geometry of the structure. This phenomenon occurs as a common result of the ground-building interaction. The prevailing vibration periods of soils cannot be changed. However, during the projecting of the buildings, different characteristics of the grounds can be determined and the dominant vibration frequency or period of the ground during an earthquake can be determined. According to these determined frequencies, the characteristics of the building’s structure (height, number of floors, carrier system, rigidity, etc.) can be selected and avoided the resonance risk.
In this respect, some features of a building planned to be built should be determined by a civil engineer. Due to factors such as zoning plans, economic and architectural reasons, it is practically not possible to determine the number of floors and heights of the buildings by civil engineers. However, while the development plans and city planning studies are being carried out, the frequency-periods of the regional grounds can be determined by geological-geotechnical-geophysical studies and plans can be made according to these factors. This issue should definitely be taken into consideration when planning such as “Urban Transformation” or “Transformation of Areas Under Disaster Risk”.
One of the issues that should be taken into consideration while conducting ground investigations is to go down to sufficient depths and determine the ground dominance period in wider layers. As it is known, during the static analysis of the structures, the natural vibration period occurs according to the mass, tightness, hardness, strength and dimensions (height, width, length) of the structure. When this value falls between the spectrum curve corner coordinates (characteristic periods, Ta-Tb), earthquake regulations penalize the building and rightly require the structure to be resistant to greater earthquake forces. (dimensioning of the structure in a more rigid, less displacement-oscillating way).
This significantly affects the construction cost of the building. Or, the natural period of the building should be subtracted from these Ta-Tb values, which is the building geometry, building height, load-bearing system, etc. provided by changing factors such as. Although it is an option to compare the costs in these two situations in terms of optimum cost, we can say that it is more appropriate to consider life safety first from the cost factor and to deduct the building periods from these characteristic periods.
While dealing with these issues, let’s clarify the famous statements such as “low-rise buildings on rocky grounds” and “multi-storey buildings on soft grounds”. As the floor height increases, the vibration periods of the buildings “generally” increase, that is, their frequency decreases (as the height increases, the oscillation period increases, the movement-frequency decreases per unit time, the force decreases), as the floor height decreases, the period decreases, the frequency increases (oscillation time decreases, movement-frequency increases, strength increases).
The dominant vibration periods of rocky soils are relatively low (frequency is high), whereas for loose soils the dominant vibration periods are high (frequency is low). Therefore, these discourses are about the non-fabrication and resonance of the floors and structures in close periods. Of course, it should not be forgotten that the building period is not only dependent on the height of the building.