Revisiting Seismic Design After Türkiye/Syria Quake Part 2 - Wave and Structure

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3. The correlation between seismic waves and structures

The core of seismic design, similar to the causes of earthquakes, still lies in the correlation between the frequency characteristics of seismic waves and the resonance of structures.

Relying solely on the drawings of the designed structure to assess the success of seismic design is akin to unrequited love.

Even if the problem of resonance seems straightforward, when it comes to multi-degree-of-freedom systems, one must have a solid understanding of the mathematical significance of natural frequencies and mode shapes, which are characteristic features of structures in the frequency domain referred to as amplification factors.

Eigenvalue problems are not only confined to vibrational mechanics but also constitute a fundamental aspect in physics. A comprehensive understanding of Fourier series, spectra, and their implications is essential in addressing these issues.

​Seismic design involves creating structures capable of withstanding the upcoming seismic activity, preparing for potential earthquakes.

Just as we cannot predict the future, we cannot precisely determine the characteristics of the impending earthquakes. However, analyzing various seismic waves observed on Earth has allowed us to discern general trends.

Nevertheless, much like the diversity in human faces, the characteristics of seismic waves vary significantly depending on the location of the earthquake or the observed area. In other words, the features of seismic waves change based on factors such as the size or shape of faults at the epicenter, the nature of rock destruction, the influence of the propagation path from the epicenter to the observation point, and the amplification characteristics in the subsurface layers at the observation point.

Among these factors, the size of faults, particularly the dimensions of fault planes, influences the overall magnitude of seismic waves. Additionally, the impact of the near-surface layers on the characteristics of vibration frequencies is substantial. Understanding the resonance phenomenon provides an intuitive grasp of how these frequency characteristics of seismic waves affect structures.

We commonly think that tall buildings, easily swayed by everyday vibrations like wind, might be vulnerable to earthquakes, while low-rise buildings, being more stable, could be safer. However, in earthquake-prone regions like Tokyo, Japan, or California in the United States, many skyscrapers have been constructed. How is it possible to build tall buildings in earthquake-prone areas? The answer lies in the explanation of the response spectrum, which describes the frequency characteristics of seismic waves, and the resonance phenomenon between objects and vibrations induced by earthquakes.

In other words, earthquakes have the characteristic of gently shaking flexible objects with weak forces and forcefully striking rigid objects with strong forces. Utilizing this property, structures are designed to be flexible, allowing them to yield to seismic forces rather than resisting them. This concept enables the construction of tall buildings and large bridges, such as suspension bridges, in earthquake-prone areas.

3.1 What is Response Spectrum?

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Generally, when we think of the term "spectrum," the first concept that may come to mind is the beautiful rainbow of seven colors revealed through a prism by sunlight. Sunlight, when perceived with the naked eye, appears bright without any discernible colors. However, when passed through a prism, it decomposes into seven different colors based on their frequencies.

When examining the seismic waves recorded in the time history of seismic waves, relatively simple characteristics such as the duration of the earthquake or the maximum acceleration can be easily determined. However, it is challenging to discern the frequency components included in the seismic waves. Therefore, just like using a prism to identify the frequency components of light, mathematical tools are employed to decompose the characteristics of seismic waves into frequency components.

One of these analytical methods is the response spectrum, and it is developed through the following process.

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After applying a forced displacement to an object and releasing it, the object vibrates because the restoring force, attempting to return it to its original position, and the inertia force, trying to stay in place, come into play. The period of vibration of such an object is referred to as its natural frequency, and it varies depending on the material and shape of the object.

Moreover, the phenomenon of consuming energy through heat or sound while the object is vibrating is known as damping.

The analysis of the frequency components of seismic waves using an oscillating object with these characteristics, including both the period and damping, results in the response spectrum. A schematic representation of the construction method of the response spectrum is depicted in Figure 1.5.

Let's consider a scenario where numerous masses (particle system) with different natural frequencies are placed on a single plate and connected by various types of springs. If we imagine the plate as the ground and shake it, similar to seismic motion, these masses will vibrate according to their individual natural frequencies. By measuring the response of each mass over time and identifying the maximum values, a graph is created with the natural frequencies on the X-axis and the corresponding maximum response on the Y-axis, forming the response spectrum.

The shape of the spectrum has many undulations based on the periodic characteristics of seismic waves, and particularly, the period with the significant response is called the predominant period.

Furthermore, as damping increases, the characteristics of the period diminish, and the undulations become more gradual. Although the shape of the spectrum varies with different seismic waves, an analysis of spectra from earthquakes worldwide has revealed that spectra generally exhibit similar properties.

A simplified representation of the typical trend of the spectrum is shown in Figure 1.6.

Moreover, when representing the three components of acceleration, velocity, and displacement as a single spectrum, for the widely known earthquake that occurred on May 18, 1940, in El Centro, which is renowned for being the first recorded strong-motion earthquake, the resulting spectrum is depicted in Figure 1.6.

When examining the acceleration response spectrum, which is directly related to the magnitude of the force in seismic forces, it generally maintains a relatively constant value up to a certain short period. However, as the period lengthens, there is a tendency for the response to rapidly decrease.

A significant response at a specific period implies that the seismic waves contain a substantial amount of components with that period. Conversely, a smaller response indicates that there are fewer components with that period in the seismic waves.

3.2 Resonance phenomenon

Why is the decomposition of frequency components important among the properties of seismic waves, not only the magnitude of maximum acceleration?

This is because the damage to structures caused by earthquakes is not only related to the size of the earthquake but also to the resonance that occurs when the periods of the earthquake and the structure match.

Let's consider some examples from our everyday life regarding this resonance phenomenon.

During elementary school, you may have learned about the phenomenon of placing two tuning forks of the same size and shape apart and causing one to vibrate. In this scenario, the other tuning fork starts vibrating synchronously. This is because the vibrational energy generated by the vibrating tuning fork is transmitted through the air to the other tuning fork. Tuning forks of the same size and shape that can absorb the vibrational energy of the air vibrate, while those with different sizes and shapes cannot absorb the vibrational energy, preventing the occurrence of resonance.

This phenomenon of easily exchanging vibrations occurs only when the natural frequencies between two objects are similar.

As another example, let's imagine a scene from the timeless Korean classic, "Chunhyangjeon," where Chunhyang meets the young lord in Namwon.

In the moment when the young lord is passing through the forest, let's picture the scene reminiscent of Chunhyang pushing the swing from behind. Thanks to this assistance (or interference?), Chunhyang was able to swing on the swing with her skirt fluttering, catching the attention of the young lord. This event leads to the unfolding of Chunhyangjeon. Let's attempt to somewhat forcibly explain the scene where Hyangdan helps Chunhyang swing as a resonance phenomenon.

In this context, we can liken Hyangdan's skillful (or not-so-skilled) pushing of the swing to the concept of resonance.

• Like the pendulum of a clock that consistently swings with a regular rhythm, Chunhyang's swing, tied to a tree, also moves with a constant beat determined by the length of the swing rope and the weight of the person riding it.

• When pushing the swing in harmony with this regular beat, the swing moves smoothly. However, if the push doesn't synchronize with the swing's rhythm and is done arbitrarily, the swing will never move smoothly.

Therefore, Chunhyang was able to swing so gracefully because Hyangdan understood the resonance phenomenon well and applied the external force appropriately. In the tale of Chunhyang, the unsung hero is undoubtedly Hyangdan, who skillfully manipulated the swing to contribute to the unfolding of Chunhyang's story.

In a similar manner, when the vibrating period of an object applying an external force and the vibrating period of the object receiving the external force are close, resonance occurs. Considering this resonance relationship between the seismic force acting as an external force and the structure receiving the force is crucial in seismic design, a significant challenge we face.

For instance, if two buildings with significantly different numbers of stories are constructed in adjacent locations, the lower-rise building, due to its structural characteristics, will likely have a shorter natural period, while the higher-rise building will likely have a longer natural period.

If we assume that an earthquake with strong short-period components and weak long-period components occurs in this region, the lower-rise building is more susceptible to destruction due to the resonance phenomenon between the earthquake and the structure, while the higher-rise building has a higher likelihood of remaining relatively safe. Conversely, if an earthquake with strong long-period components and weak short-period components occurs, the situation could be reversed, with the higher-rise building being more vulnerable to damage, and the lower-rise building having a greater chance of remaining safe.

In essence, the safety of a structure during an earthquake hinges on whether the "predominant period of the earthquake matches the natural period of the building, causing resonance, or if the predominant period of the earthquake and the natural period of the building are mismatched, preventing resonance." This becomes a more significant concern than the question of whether the building was constructed robustly or poorly.

Therefore, seismic design for structures must consider the frequency characteristics of seismic motion, and the importance lies in response spectra that represent the frequency characteristics of seismic motion.

Predicting the occurrence of earthquakes with specific predominant periods at the construction site is a crucial issue. Unfortunately, as of now, we have not obtained precise conclusions on this matter. Instead, the application of foreign design standards or an understanding of the general characteristics of the ground is commonly used in design. However, for seismic design tailored to South Korea, it is essential to create design response spectra that incorporate the unique geological features of the Korean Peninsula. Simply adopting design standards from the United States or Japan may lead to results that significantly differ from the actual seismic damage and predictions in our country.

4. The Age of Artificial Intelligence in the Fourth Industrial Revolution

​The illustration depicting the causes of earthquakes and a structural model of earthquake occurrence was created by the author in 1992 upon returning from studying abroad at the Earthquake Research Institute of the University of Tokyo. It was designed to introduce the mechanism of intra-plate earthquakes while working at the Korea Institute of Energy Research.

Although Japan is recognized as a country prone to frequent earthquakes due to its location along tectonic plate boundaries, the illustration explains that even in Korea, which is not situated at plate boundaries, it does not belong to a seismic-safe zone. This was presented with the intention of describing the mechanism of intra-plate earthquakes.

The author returned from studying abroad in their 30s and is now approaching their 70s.

In the meantime, the world has entered the era of the true Fourth Industrial Revolution, where advancements like the defeat of Lee Sedol by AlphaGo through deep learning are considered natural and past events. OpenAI's chatbot GPT engages in conversations like a human, sings songs of lesser-known famous singers, draws cartoons and pictures of famous artists, creates lifelike faces, generates video content effortlessly, and witnesses the emergence of tumbling robots and self-driving cars.

However, the mechanism of earthquake occurrence has remained entirely unchanged.

So, before starting the article, I asked OpenAI's chatbot GPT and Microsoft's BING about seismic design and inquired about the physics and general scientific knowledge I have studied.

1. What are the causes of earthquakes?

2. Can you explain seismic design?

3. How about the seismic performance of pile structures?

4. What is the concept of response modification coefficient in seismic design?

5. What are the characteristics of seismic wave response spectra?

6. Is there potential for using deep learning AI in predicting earthquake occurrences?

7. Which is more effective in seismic retrofitting, stiffening or damping methods?

8. Is there potential for applying deep learning to ensure seismic safety in building structures?

9. Explain the principles of quantum computers using superposition and entanglement.

10. What is the age and temperature of the universe?

11. Can you show an example of a deep learning program written in Python?

Here is the English translation of the provided text:

The impressions of the author who has used them are as follows:

ChatGPT tends to respond as if it knows even unfamiliar topics, and BING seems to investigate and respond based on data from sources such as blogs and Wikipedia.

Regarding the concept of the response modification coefficient, BING answered:

"The response modification coefficient was first introduced in 1978 by the U.S. Applied Technology Council as a design factor to consider the economy, risk, and nonlinear behavior in seismic design. The response modification coefficient is a value that allows the structural review to be conducted with reduced seismic forces when the structure exhibits inelastic behavior. It is composed of material ductility, energy dissipation laws, and structural ductility."

I was somewhat surprised by this response.

Because it is a point that the author always emphasizes in lectures targeting engineers. The author often stresses that without understanding the concept of the response modification coefficient, one cannot comprehend how seismic design is being conducted.

The author seems to have a good understanding of widely known scientific common knowledge and appears capable of providing detailed explanations when prompted.

The advice the author would like to recommend to future generations is not just to approach artificial intelligence as consumers but to grasp the concept of studying AI itself. The author suggests delving into coding to gain a precise understanding of AI and finding ways to participate in the contemporary demand for applying AI to the design and seismic reinforcement methods of civil and architectural structures.

5. What is the conclusion of seismic design?

​Experiencing the earthquake in Turkey, we became acutely aware of the human vulnerability to the forces of nature through the damage caused by Japan's Kobe earthquake in 1995. Additionally, the Pohang earthquake in 2017 prompted national attention to whether such major earthquakes could occur on the Korean Peninsula, which is adjacent to Japan. Despite initial interest, most people have now shifted their focus away from these concerns.

While predicting earthquakes remains an impossible task even for modern science, devising coping strategies is relatively straightforward.

In modern society, poverty itself is an unavoidable natural disaster. No one desires to live in a mud-brick house just because they want to. The first solution to escape from the natural disaster of earthquakes is to avoid using mud-brick structures for residential purposes. Advocating against using mud-brick houses for residences is an obvious statement from the perspective of an engineer.

During my time studying earthquake engineering abroad, the discussions involved not only advanced seismic design research but also research on improving the strength of mud-brick structures when indigenous people in developing countries create them using locally available materials such as straw and wood. However, there was a memory of a lack of funding sources to support such research.

Looking at earthquake damage photos, not only from Turkey but also from developing countries, it is common to find instances where reinforcing steel cannot be identified in shattered concrete fragments. This is often not due to people being unaware of the necessity of reinforcing steel in buildings but rather because of cost-cutting measures in construction expenses.

The seismic design poses a challenge of how to appropriately use insufficient reinforcing steel. Civil and architectural engineers are generally aware of the importance of the main reinforcing bars in resisting bending moments. However, surprisingly, many individuals are unaware of the significance of stirrups. Especially, those involved in daily labor, who are at the forefront of seismic design implementation, tend to be indifferent and often lack knowledge about this.

Examining the failure mechanism of reinforced concrete,

• When a structure is subjected to vertical load and additional horizontal load is applied:

• The outer cover concrete around the stirrup may fail.

• Once the cover concrete fails, the stirrup's confinement is released.

• With the stirrup's confinement released, the simple lap-spliced main reinforcing bars may separate or bend, causing the core concrete to be displaced.

• Under the influence of vertical load, buckling occurs, leading to the collapse of the structure.

Therefore, to prevent the buckling of vertical members, it is crucial in seismic design to ensure that the cover concrete does not fail, and even in cases where the cover concrete does fail, the confinement of the stirrup should not be easily released. This is at the core of seismic design.

To prevent the easy release of stirrup confinement, it is necessary to wrap around and secure the end of the stirrup at the end of the main reinforcing bar. However, achieving this on-site, especially through manual labor, is extremely challenging.

In large construction sites, it might be possible to insert stirrups processed with closures from the bottom in a flat condition, but doing the same from the top of vertically standing main reinforcing bars is operationally difficult and rarely implemented.

As of now, the author has not seen any tools or equipment capable of creating closures for stirrups in place when vertical rebars are already installed.

Along with the spacing of stirrups, the closure condition is crucial, but preventing overlap of the main reinforcing bars is also very important.

If the stirrup closures are separated, and the overlapping main reinforcing bars are detached, the concrete cross-sectional area that supports vertical loads is lost, leading to the inevitable occurrence of buckling in the member.

If buckling occurs in one vertical member, the load is transferred to adjacent vertical members in a domino effect, resulting in the collapse of the entire structure. This phenomenon is a major cause of casualties in seismic damage sites, often referred to as the pancake collapse.

In the end, the core of seismic design for reinforced concrete structures is not an overstatement when described as maintaining the stirrup spacing narrow while ensuring that the core concrete does not separate, preserving the closure. This is not only the beginning but also the end of seismic performance. Particularly in small structures, failure to adhere to these fundamental construction practices can lead to severe damage.

Recently, the problem of overlapping main reinforcing bars has been mitigated with the widespread use of couplers, which connect the main reinforcing bars by screwing them together. This innovation seems to have alleviated many issues related to the main reinforcing bars.

No matter how much the author discusses the issue of inadequate closure of stirrups on small construction sites that are in the blind spot of supervision in our society, it cannot be a fundamental solution.

One representative site in the blind spot is the construction of pile structures by individual property owners. Pile structures have been widely adopted as an alternative for small buildings that cannot secure underground parking in urban areas.

Our society has already forgotten the issue of inadequate stirrups on the columns of pile structures exposed in the damage caused by the 2017 Pohang earthquake.

There should be a solution to address the stirrup issue on columns supporting already constructed and in-use pile structures. However, even if the author raises the concern, the voice is too weak, and there has been no significant societal impact.

The core of seismic design is to prevent buckling of the vertical members, allowing the concrete to be damaged as long as the main reinforcement does not detach externally. To achieve this goal, a common method is to wrap even the external part of a completed vertical member, and one representative way is using steel plates. However, steel plates are heavy, and on-site tasks like welding are required. As a more convenient alternative, continuous polyester fibers, similar to those used in car seatbelts or crane operations, can be wrapped around. Polyester fibers, widely used in firefighting hoses, are easy to roll up and have suitable tensile strength and elongation properties, making them suitable for wrapping around the column. Finishing with plaster or mortar completes the process.

In particular, the Pilotti structure has a small number of columns, and the load is concentrated, so it is easy to identify the member to be buckled. If the column is buckled, the structure itself must be demolished even if it was not severely damaged, so property damage is serious for the owner of the building.

On the other hand, even if the pillar is damaged, it is a very effective way to recover from earthquake damage not only at the owner but also at the national level because it can be lifted by the building if it is not completely collapsed by the buckling.

The reason I mentioned artificial intelligence systems using deep learning earlier is because most of the filotti structures have a constant shape, making it easy to model the collapse mechanism, and if the expected destruction modeling is secured through deep learning, it is possible to present a reliable collapse mechanism for the customer's request and to present an appropriate reinforcement method.

I thought about ways to utilize deep learning artificial intelligence, which is currently exciting in our society, in the field of civil engineering and architecture.

About the Editor

Gyusik Jeon

Doctor of Architectural Engineering Over 20 years of experience

Dr. Gyusik Jeon, with over 20 years of experience in Architectural Engineering, has a diverse career including roles at KEPCO's Power Research Institute (1980-1997), Technical Director at Unison Construction (1997-2000), and currently works with a seismic base isolators production company. He graduated from Busan National University in Civil Engineering and holds a Master's and Ph.D. from the University of Tokyo's Seismic Research Institute.

 

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