In this blogpost we will try and answer some questions in relation to earthquake proofing buildings using different materials and tools.
Earthquakes in short
Earthquakes are disastrous events influencing people, infrastructure and houses. The most prone areas to earthquakes are lie at fault lines where epicenters of earthquakes usually happen, see Figure 1 and 2 for epicenters and tectonic plates respectively.
The exact locations of when and where earthquakes will happen varies as the nature of these events is chaotic. However they usually follow patterns as outlined by the fault lines where tectonic plates meet and exchange information. In addition, the frequency of occurrence alongside with magnitude of earthquakes are completely unpredictable in nature.
This fundamental description of earthquakes causes problems for engineers and scientists alike. Engineers are faced with the challenge of accurately creating safe and sound buildings able to withstand the next earthquake, while scientists are faced with the problems of understanding the different phenomena which are chaotic in nature.
Related read: Top ten misconceptions about earthquakes, effective investment strategies for climate adaptation.
When engineers are facing the problems of designing buildings in earthquake prone areas It is often necessary to find solutions for the moving and differential settlement for which buildings are subjected to.
Earthquake proofing buildings – blocks
One solution to earthquake proofing buildings is for the building to move together with the earth during the earthquake. In order for the building to easily move together with the earthquake the foundation of the building needs to be placed on elastic blocks. See Figure 3 for an illustration.
The movement blocks allow the foundation to absorb some of the shearing movements caused by the earthquakes surface waves, the so-called Rayleigh and Love waves. This allows them to withstand stronger more devastating movements when compared to houses without mats.
Related read: earthquake epicenters
Earthquake proofing buildings – tuned mass damper
Another example of a way to earthquake proof buildings and larger structures such as bridges, towers and walkways is through the use of a tuned mass damper.
The tuned mass damper is a special type of damper designed specifically to the building in question. It works by adding a suspended mass into towers or structures. The suspended mass works by shifting the buildings eigenfrequency sufficiently higher than before such that earthquake vibrations are damped out before destroying the building. The dampening works by absorbing the deformation inside the tuned mass damper. For an illustration of the concept of a tuned mass damper see Figure 4 and Gallery 1 for real life counterparts.
As we can see the different types of tuned mass dampers varies in shape, form and size from the relatively small discrete ones underneath walkways till the large heavy ones inside the Taipei 101 and Shanghai towers.
Earthquake destruction in Turkey-Syria
Recently the Turkish earthquake Kahramanmaras lead to a massive destruction of buildings, infrastructure and railroads alike. The destruction of buildings was massive as can be seen when comparing the a before and after image from Kahramanmaras, see Gallery 2.
The destruction led to massive amounts of death and homelessness in the thousands of numbers, see source. This destruction is terrible for the population of Turkey and Syria where the death tool is immense alongside with the cold winter coming showing degrees below freezing during the cold nights.
What are the best materials for earthquake-proof buildings
The best materials for earthquake proofing buildings are steel and elastic materials. The strength of steel allow it to absorb stresses from the earth foundation strains.
Steel as a building material
Furthermore the elasticity of steel is sufficient for deformations to remain elastic as compared to plastic. Elastic deformations are important as the destructive forces usually occur due to permanent shifting of loads during the plastic deformation of bearing members of the structure.
Since steel is an exceptionally strong material with large elastic and plastic bearing capacities it makes it ideal as a construction material for earthquake prone buildings. For an example curve of the strength of steel see Figure 5 for an example of a steel loading curve.
We can see that the strength of steel during the elastic part of the curve is linearly proportional to the displacement, meaning that we can load up and down the displacement curve without loss of strength.
This curve is representative of all axial strains meaning that steel is isotopic and thus able to withstand forces with equal strength in all directions.
Wood as a building material
Unlike steel there is wood. Wood is a natural material whose properties depend on the type of wood, the age and location to name a few. In fact, wood is not isotopic since fibers are growing upwards from the stem.
The mechanical properties of wood therefore varies from tree to tree and depending on the force direction of loading including axial and torsional directions. An example of a typical axial stress-strain loading curve is shown in Figure 6.
As we can see when comparing the curve from wood with the curve from steel we can see that the elastic region is followed by a small plastic period with increased strength for steel while the wood curve is flattening out directly after yield stresses are observed.
In both cases the axial strength increases in a short period before failure for steel, which is the hardening period, while tree follows densification before failure.
Comparing wood vs steel
For steel the hardening period is followed by increasingly plastic deformation and loss of material strength throughout the necking period. This period allows buildings to deform providing a warning sign before failure. The behavior of deformation before failure is known as ductility and is a particular sought after characteristic for building materials.
For wood the linear elastic period of deformation in the axial direction is followed by plastic continual deformation. This means that the wood will compress without gaining strength up until the point of densification. Densification is the happening where tree fibers are compressing increasing the capacity of the wood before failure occurs due to cracking fibers.
With these considerations when trying to earthquake proof buildings it is important to make the bearing elements strong and elastic. For this purpose steel is an excellent material.
Building earthquake-proof buildings
In conclusion utilizing different measures such as tuned mass dampers or spring mats increases the foundation bearing capacity in regards of vibrational stressors from earthquakes.
For the building materials it should be performed in steel for the load bearing parts of the structure ensuring that maximum elasticity and strength against materials is achieved.
Steel unlike wood, is isotopic meaning that shear forces from earthquakes are easily absorbed in a manner similar to axial compression while wooden structures will act differently depending on the mode of deformation and earthquake surface waves.
References
Stress-strain curve for wood https://doi.org/10.1016/j.jobab.2020.07.004
Number of casualties in earthquake Turkey-Syria – https://www.theguardian.com/commentisfree/2023/feb/15/earthquake-turkey-corruption-buildings-collapse
