How building shock absorbers work

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Earthquake tremors occur every once in a while all over the world. Ideally, when a building or other structure is struck by an earthquake, seismic waves penetrate the structure safely (inducing vibrations). Several structures, especially buildings, get shaken, weakened and knocked down as a result of the seismic forces exerted, posing a major threat to the lives of people residing in and around them.

Building shock absorbers, commonly referred to as Seismic dampers in construction, are used mostly in buildings and bridges to control vibrations on building structures caused by earthquakes. Whenever seismic waves begin to penetrate from the base of a building, seismic dampers come into play by allowing the building to either sway with the motion or safely redirect the seismic waves. This reduces their damaging effect and improves the seismic performance of the building upon occurrence of an earthquake, giving it a much higher chance of survival.

Image from https://www.quora.com/Is-there-such-a-thing-as-an-earthquake-proof-building

In order to reduce the swaying of a building structure during an earthquake, seismic dampers work by dissipating the energy of seismic waves. It has the waves moving through the building structure then converting the kinetic energy of the swinging into heat energy, which is then dissipated into hydraulic fluid.

A damping system like this works on the principle that seismic forces are dissipated and energy is absorbed, resulting in the motions of the structure reduced. These building shock absorbers are used instead of structural elements such as diagonal bracing to hold buildings from seismic damage in structures. In turn, this produces substantial savings, seeing as the elements can now be fully optimized to save cost.

Purposes of Seismic Dampers  

Seismic dampers have various purposes which include:

  • Boost the structural integrity.
  • Regulate structural damage.
  • Protect the structure against earthquakes hence providing safety.
  • Minimize the structural damages.
  • Increase strength of buildings and bridges.
  • Minimizes the seismic force exerted on buildings and reduces distortion of the structure.
  • Increases the structure’s life span.

What are the Types of Seismic Dampers?

Seismic dampers come in various types which vary based on their efficiency and effectiveness

Viscous Dampers  

Viscous dampers are designed to dissipate energy through the motion of a piston. Here silicon based gel or viscous fluid inside the cylinder is pushed. The seismic energy is absorbed as the fluid passes through the piston. Here the damper is quite similar to a car shock absorber.

Here are some of the positive attributes of Viscous dampers;

  • They adapt to a building’s structure and are easy to install.
  •  Viscous dampers are commonly used in high rise buildings as they are very efficient in absorption of minor earthquakes, strong earthquakes.
  • They can function at ambient temperatures ranging between 40 degrees to 70 degrees Celsius.
  • Viscous dampers work well in conjunction with other earthquake protection measures.
  • Viscous dampers are suitable not only for new buildings but also in retrofits of pre-existing buildings as they come in different sizes and strengths.

Figure 1: Viscous damper

Viscoelastic Dampers

Viscoelastic dampers work majorly on the principle of converting mechanical energy into heat. They consist of an arrangement of steel plates where viscoelastic materials are placed in between. Their performance mainly depends on the ambient temperature and the frequency of loading. Viscoelastic dampers are commonly used in buildings.

Figure 2: Viscoelastic damper

Friction Dampers

Friction dampers consist of a steel plate arrangement sliding against each other in an inclined position. A friction pad is placed in between the steel plates where seismic energy is dissipated by the friction action between the surfaces rubbing against each other. Some of its various advantages are:

  • One of the commonly used types of dampers being easy to install.
  • The performance of frictional dampers is very minimal as it is affected by changes in temperature and velocity.
  • The use of Friction dampers is the most economical means to dissipate energy since it does not require to be replaced after an earthquake.
  • The use of friction dampers is more effective and reliable.
  • The friction produced when the steel plates are sliding against each other disposes a much greater quantity of energy compared to other methods.

 Figure 3: Friction dampers

Tuned Mass Dampers (Vibration Dampers)

These type of dampers are made up of a half tunnel-like passive regulation device mounted on the building structure itself. It works based on the principle of frequency, that is, it decreases the amount of mechanical vibrations by resisting resonance frequency. The components are not heavy and therefore acts as a spring and largely reduces the amplitude of vibrations. It comprises of:

  • A large oscillating mass
  • A visco-damper
  • A spring

The manner in which the dampers are arranged facilitates the reduction of the vibrations of lateral forces such as wind and earthquake. These dampers are majorly used in very tall buildings, automobiles, and in transmission lines as well.

Figure 4: Tuned Mass Dampers, Taipei 101

Yielding Dampers

Yielding dampers are the type of dampers that consist of a metal alloy or “yielded metal.” Yielding dampers transfer the seismic energy by yielding, or plastic deformation. In relation to braces, it is more common to use submission metallic dampers. These dampers are seldom created by parallel steel plates.

In conjunction with a bracing system, they play the role of absorption and seismic energy transfer acting as a fuse in the structure. The energy dissipated to the structure is spent on submission in the element used.

When it comes to these dampers, metal inelastic deformation is used for the purpose of formation of metals like steel and lead for energy transfer. Yielding dampers are relatively efficient and quite economical as well.

Figure 5: Yielding Dampers

Magnetic Dampers

Magnetic dampers consist of two racks, two pinions, a copper disk and rare earth magnets.

They are also known as dynamic vibration absorbers as they can efficiently damp such vibrations.

They have the following advantages:

  • Magnetic dampers do not depend on temperature changes.
  • They are quite economical.
  • Magnetic dampers are clean.
  • They are very simple to adjust and they are effective.
  • Electromagnetic dumping with quad magnets is relatively cheap.

However, the magnetic damping is so weak, such that it is only effective in dynamic vibration absorbers which require less damping.

What are some of the disadvantages of Seismic Dampers? 

As much as seismic dampers come with the advantages of improving the structural performance of a building, they also comprise of a few disadvantages:

1. Costly installation and maintenance.

2. They are less affordable in residential buildings as a result of the high costs incurred.

3. Need to be integrated into the building design early on, as will dictate a large portion of the building configuration.

Summary

Building shock absorbers are usually meant for high rise buildings. Rightfully so as they take an enormous amount of money, maintenance and occupied space. Residential low-rise structures typically only need good structural practice and thoroughly consulted by your civil engineer.

If you are putting up a skyscraper, first consult a structural/civil engineer. They should advise based on your technical site findings (soil bearing test, fault line map, etc.) and building configuration on how to go about selecting the right kind of seismic damper.

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References

  1. T A Belash and M S Belashov 2020 IOP Conf. Ser.: Mater. Sci. Eng. 962 022068
  2. A.K. Agrawal, M. Amjadian, in Innovative Bridge Design Handbook, 2016. Digital Image https://www.sciencedirect.com/topics/engineering/viscous-dampers
  3. Victoria K. (2019, Aug 30). More ways to keep buildings safe in earthquakes. Earthquake science. https://blog.jumpstartinsurance.com/more-ways-to-keep-buildings-safe-in-earthquakes/
  4. Harris W. Technologies that help building resist earthquakes. https://science.howstuffworks.com/innovation/science-questions/10-technologies-that-help-buildings-resist-earthquakes.htm
  5. http://ijcsn.org/IJCSN-2016/5-2/Characteristics-and-Applications-of-Different-types-of-Dampers-as-Seismic-Energy-Dissipater.pdf