1. Introduction
Since transitioning from the Korea Electric Power Corporation Research Institute to the bridge equipment industry, the author believes that they have made a substantial contribution to seismic base isolation design by promoting seismic isolation device design. Even during their studies in Japan, where they pursued further education, they learned about surface mount design from textbooks and encountered elastomeric bearings. However, their confidence in discussing the practical advantages and disadvantages of surface mount bearings comes from practical experience acquired in manufacturing companies and theoretical background learned from interacting with senior professionals worldwide involved in the production of seismic isolation device.
Historically, seismic isolation devices are broadly categorized into metal-based pendulum types and rubber-based elastomeric bearing types. The pioneers who envisioned that rubber, a pliable material, could support structures such as buildings and bridges with rubber-based seismic isolation devices made truly remarkable innovations.
Many civil and architectural engineers tend to regard lead-rubber bearings, which involve the insertion of lead into elastomeric bearings, as an advanced technology capable of providing seismic isolation functionality. There is a tendency not to recognize elastomeric bearings themselves as a significant technology. However, the reality is that the ingenious concept of stacking thin layers of rubber and thin steel plates to maintain high rigidity due to the steel plates vertically while simultaneously preserving the softness of rubber horizontally is a magnificent idea.
The fundamental characteristics that seismic isolation devices should possess include low horizontal stiffness and, crucially, the ability to restore to the original position with restoration force.
For rubber bearings, the restoration force can be implemented in a two-dimensional form. However, metal bearings like pendulums face challenges in precision machining due to the three-dimensional curved surfaces, where the lower surface responds to rotation and the upper surface implements restoration force. Simultaneously solving the issues of precision machining for the 3-dimensional curved surfaces and addressing the conflicting problems of friction and restoration force poses a difficulty in the case of metal bearings like pendulums.
The difference between seismic isolation devices for bridge structures and those for architectural structures lies mainly in the accommodation of rotation.
In the case of rubber-based rotation applied to bridge structures, it is adjusted by the thickness of a single rubber layer and the total height of the rubber.
For architectural structures where rotation is not required, minimizing the thickness of a single rubber layer is necessary to minimize vertical displacement. However, maintaining a consistent thickness of the rubber layer becomes challenging when the size of the support is large, requiring a complex manufacturing process.
Moreover, the selection of friction material, along with spherical processing for pendulum bearings, becomes the most challenging issue.
While the friction coefficient of friction materials is generally considered constant, when subjected to high pressure over an extended period, as in the case of seismic bearings, the friction characteristics become sensitive to changes due to the development of affinity forces with moisture.
It seems that there is no product among the globally used seismic isolation devices that has been proven to be perfect.
In the case of rubber products, the flatness of rubber and steel plates often poses a problem, and there can be issues with the degradation of rubber coating due to ultraviolet radiation.
For metal products, the challenges of spherical processing and metal galvanization, along with the selection of friction materials, lead to various issues.
From the perspective of the crucial function of seismic isolation devices, ensuring safe horizontal displacement, the author believes that rubber products generally offer a slightly higher level of safety compared to metal products.
2. Rubber-base Seismic Isolation Device
2.1 Lead Rubber Bearing (LRB)
If we were to point out a drawback of elastomeric bearings, which excel as bridge or seismic isolators, it would be the significant displacement that occurs during vibrations. To mitigate this displacement, widely adopted methods include the use of separate viscous dampers or leveraging the nonlinear behavior of metals to absorb vibration energy.
Lead isolators, for example, elongate the natural period of the upper structure by utilizing the elastic bearing's function. Simultaneously, they feature a core-shaped lead insert within the elastic bearing to reduce the seismic force induced in the upper structure. This design incorporates an extended natural period and damping function, effectively serving as an energy-absorbing mechanism. (Refer to Figure 2.1)
In the past, the application of lead isolators faced challenges due to concerns that the shear resistance of lead might transmit excessive temperature loads to the bridge deck, potentially causing thermal stress.
However, the material characteristics of lead concerning horizontal loads, such as temperature loads acting gradually over an extended period, are easily yielded due to the creep properties of lead. This implies that lead efficiently mitigates the transmission of temperature loads to the bridge deck. Furthermore, for wind loads and braking loads from vehicles, lead acts with significant stiffness, enhancing stability.
Additionally, when subjected to substantial loads like seismic forces, lead undergoes complete yielding, achieving the desired seismic isolation effect by reducing induced seismic forces through the nonlinear behavior of lead. This leads to the absorption of vibrational energy in the bridge deck and the suppression of displacement.
One of the crucial functions demanded of seismic isolation devices is related to post-seismic maintenance, particularly in terms of residual displacements that may occur after the earthquake has ended. Structures equipped with seismic isolation devices featuring restoring forces, such as elastomeric bearings, utilize the elastic force of rubber to exert a force aiming to return the structure to its original position in the face of any residual displacement post-earthquake. During the seismic event, the restoration force of rubber acts in the opposite direction to the displacement, resulting in a characteristic of minimizing residual displacements.
The author explains to numerous engineers that the restoring force of elastomeric bearings can return the structure to its original position after the earthquake. However, many people do not readily accept this, as there is confusion between the restoring force of rubber and friction.
In other words, some individuals may think there is a force resisting the horizontal displacement of the heavy bridge deck (often interpreting it as friction), and internally judge that the small restoring force of rubber cannot overcome this resistance.
However, rubber possesses an intriguing property of behaving like a solid while also having fluid-like qualities. This allows it to restore itself without depending on the magnitude of the vertical load, which is an interesting characteristic of elastomeric bearings.
For example, if we imagine the object supporting the bridge deck as a soft membrane containing non-bursting water, no matter how much vertical load is applied, as long as the membrane doesn't burst, the water, with its constant volume, can be pushed horizontally with even a minimal force.
Additionally, elastomeric bearings, being elastic, should ideally immediately restore to their original position when the bridge deck is lifted during usage. However, in reality, elastomeric bearings do not restore immediately after lifting the bridge deck. This is because, over an extended period, the rubber undergoes creep when kept in a deformed state.
To naturally restore deformed elastomeric bearings to their original position, exposure to sunlight or soaking them in warm water can be effective. This property is akin to the process of recrystallization in metals, which is temperature-dependent.
Next is the important consideration of the reuse of seismic isolation devices.
Metal dampers that absorb energy using the nonlinearity of metals lose some of their molecular structure through partial yielding when metal molecules undergo plastic deformation. This results in the loss of their original properties. Regarding lead embedded inside rubber, the uncertainty lies in the inability to confirm its condition, necessitating the replacement of the entire unit.
However, all metals have a characteristic of returning to their original molecular structure by applying heat to deformed metal molecules. The temperature at which 50% of deformed molecules return to their original structure within one hour is referred to as the recrystallization temperature of the metal. (Refer to Figure 2.2 for the recrystallization process of lead molecules.)
These recrystallization temperatures for different metals are as follows:
- Iron: 450°C
- Copper: 250°C
- Aluminum: 150°C
- Lead: 20°C
The slightly heated lead, which absorbed the vibrational energy of the structure, has the property of returning to its original molecular structure when cooled back to room temperature. This characteristic eliminates the need for post-treatment after the earthquake.
Another advantage of lead isolators is the ability to adjust the size of lead in specific piers. This feature allows for a load control function (Force Control System), reducing seismic loads on certain piers, especially in cases where the subsoil supporting each pier does not have equal supporting capacity and some piers have particularly weak support conditions.
2.2 High Damping Rubber Bearing (HDRB)
A high-damping rubber bearing is an elastomeric bearing with a similar external shape to that of elastomeric bearings, but it is manufactured by adding a special mixture that can absorb vibrational energy when rubber is compounded.
Therefore, unlike lead isolators, it has a mechanism within the rubber itself to absorb energy. Thus, it has superior restoration characteristics after seismic events compared to lead isolators. However, its energy absorption capacity is considered lower than that of metal-based surface mount devices.
Nevertheless, with ongoing advancements in the performance of high-damping rubber, there is a possibility that it could emerge as a replacement for elastomeric bearings and lead isolators, particularly in regions like South Korea characterized by moderate to low seismic activity, where achieving damping capabilities of approximately 15% to 20% equivalent viscous damping is possible.
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Natural rubber has less energy absorption, so it is highly repulsive. |
High-damped rubber has a high energy absorption capacity, so it is less repulsive. |
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High-damping rubber, initially developed for tires in Formula 1 racing cars, has gained expectations for use in structural surface mount products. Although it has been applied domestically, particularly in cases such as F1 racing vehicles where braking performance is crucial, there is a trend of decreasing application over time. This is due to the emergence of issues such as significant cracking on the surface and pronounced creep phenomena as time passes.
To enhance the damping capability of rubber, it is essential to incorporate solid materials such as carbon and ceramics. However, adding solid materials seems to increase the occurrence of creep while not necessarily improving the durability against ultraviolet radiation. As a result, the application of high-damping rubber in practical cases appears to be decreasing.
2.3 Steel Rubber Bearings (S.R.B)
Recently, there has been a global focus on the slogans "Reducing Environmental Burden" and "Reducing the Use of Hazardous Substances." In response to these concerns, lead has been replaced by tin in the development of surface mount devices for seismic isolation.
Tin is inherently a stiffer material than lead, and it has been evaluated to have approximately 1.7 times superior yield stiffness and damping values compared to lead isolators. However, while tin isolators offer advantages in terms of reduced displacement under braking and wind loads, the fact that they have higher yield stiffness and damping values than lead isolators also means that they transmit seismic forces more effectively to the piers.
Just like pure lead with 99.99% purity undergoes recrystallization at room temperature, the use of tin in seismic isolators should adhere to the principle that the tin used should undergo recrystallization at room temperature, similar to being inserted in a core form within elastomeric bearings.
The mechanical properties of tin developed in Japan are presented in Table-2.1.
Table-2.1 Characteristics of lead, tin, and aluminum
|
항 목 |
Lead |
Tin |
Aluminum |
|---|---|---|---|
|
Strength(MPa) |
12.9 |
19.2 |
61.3 |
|
Shear Stiffness(GPa) |
17.6 |
51.9 |
61.0 |
|
Elastic Strain(×10-4) |
7.33 |
3.70 |
10.0 |
Additionally, Figure-2.5 illustrates the relationship between stress and strain, stress variation due to cyclic deformation, and a comparison with other metals. Tin developed in Japan, like lead, has been confirmed to undergo recrystallization at room temperature.
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(a) Relationship between stress and strain |
(b)Characteristics of Stress Changes due to Repeated Deformation |
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 |
With these facts, tin exhibits properties such as ductility, cyclic characteristics, and recrystallization at room temperature that are equal to or superior to lead. Therefore, tin can be considered an excellent material for use in seismic isolation devices as an energy-absorbing material.
However, the use of tin as a material involves higher costs compared to other non-ferrous metals, posing an economic challenge.
To overcome the economic challenge, there have been attempts to use a mixture of tin and lead, but there are concerns about the difficulty in achieving a physical blend, making caution necessary in its use.
2.4 What is high-quality elastic support?
The author prefers laminated rubber bearings over metal-type bearings among seismic isolators.
Commonly known as elastomeric bearings, these consist of a simple structure composed of a thin steel plate and a thin rubber sheet stacked together, serving as a seismic isolator with a sophisticated algorithm. Although it is a human invention with a perfect algorithm, the exact origin of the idea is not precisely known.
Rubber was introduced to Europe by Columbus in 1492, and in 1839, Charles Goodyear in the United States accidentally developed vulcanization, turning rubber into an industrial material for tires, transportation, and military supplies. After its adoption in the 1960s for nuclear power plant bearings in France, rumors have it that this technology spread to general industrial structures such as bridges and buildings.
Since the introduction of elastomeric bearings as an IBRD (International Bank for Reconstruction and Development) deputy governor in the 1970s in Japan, there has been a significant difference in the production systems between Japan, where rubber bearings are widely used, and South Korea.
Japan's production system for laminated rubber bearings is centered around tire manufacturing companies such as Bridgestone, Toyo Tire, Yokohama Rubber, and Showa Denko. In contrast, South Korea has established a production system primarily focused on small and medium-sized enterprises.
The most fundamental reason for this difference is believed to be that Japan has a well-established market size for seismic isolation products, while South Korea's market size is considered to be smaller.
The production system determined by market size is causing incidental issues related to product quality.
To the general public, elastomeric bearings composed of a simple structure with a thin steel plate and a thin rubber sheet stacked together may seem straightforward, and anyone with even a basic understanding of rubber might believe that production is possible for anyone. In fact, this is true.
Rubber products are produced nationwide, ranging from tire companies to shoe factories and companies manufacturing rubber bands. There are countless companies producing rubber products across the country.
So, what are the differences between general rubber products and laminated rubber bearings?
The most crucial difference lies in size and volume.
The manufacturing process of rubber is very similar to the process of making fish-shaped buns. Depending on the intended use, properly mixed rubber is confined in a metal mold, and when heat and pressure are applied, the sulfur molecules form cross-links between rubber molecules, giving elasticity. This is the vulcanization mechanism invented by Charles Goodyear, the world's first.
Imagine asking a street vendor familiar with making 500-won fish-shaped buns to make a 5,000-won-sized fish-shaped bun. What might happen?
Even a skilled street vendor, a master of everyday life, may feel perplexed.
If you heat it enough to cook the inside, the outside won't get cooked, and if you heat it to cook the outside, the inside will remain uncooked.
In other words, the manufacturing process and methods vary significantly depending on the product's size. This phenomenon is also applicable to rubber.
The fundamental temperature condition for sulfur molecules to form cross-links between rubber molecules actively is that it should be above 120 degrees Celsius. If it exceeds 150 degrees Celsius, a phenomenon called reverse vulcanization occurs, where the connection links of the bonded sulfur molecules are actually broken.
Therefore, to meet the temperature conditions for vulcanization, proper temperature transfer conditions need to be created. In the case of large-scale production, slow heating is required. Consequently, for rubber bearings, there are challenges such as the cost of large-scale production and the prolonged occupation of large press equipment.
Furthermore, consumers raise concerns about the reliability of rubber quality, as they cannot visually confirm the quality of rubber based solely on its black appearance.
Moreover, in the case of elastomeric bearings where rubber and steel plates are stacked, there is a more significant issue than the vulcanization mechanism—the quality known as the flatness of the steel plate.
Generally, steel plates are considered flat in the production stage, and this is universally true.
However, in the case of laminated rubber bearings, there are some different aspects. Typically, steel plates are stored with supports on both ends for an extended period, causing slight deformation. However, depending on the usage, a slight deformation may not be crucial.
Furthermore, rubber belongs to materials with high expansion rates with temperature, and applying heat and pressure in the mold is also done to expel air inevitably trapped during the rubber mixing process.
Mothers raising infants always burp them after feeding, and the process of expelling air swallowed with food is something we routinely do.
Therefore, during the vulcanization process, the expanded rubber, along with air, escapes from the mold's exterior. If the flatness of the internal steel plate is defective, the unevenness in the amount of rubber expelled between layers provides a cause for gradually making the flatness between layers uneven.
This flatness defect significantly increases the deviation in product characteristics, and this tendency becomes more prominent as the size of the product increases. Particularly, the often overlooked vertical stiffness is even more susceptible to this issue.
On the other hand, products where the internal steel plate is exposed use a mold with rings, similar to those used to secure internal steel plates. This mold is designed to fix the steel plate, maintaining a consistent gap between rubber layers.
Even if there is slight deformation in the steel plate, the mold restrains it, ensuring a uniform gap. This exposure of the internal steel plate also improves the heat transfer efficiency to the rubber, allowing for a shortened vulcanization time and guaranteeing product quality.
The role of molds with exposed steel plates is particularly effective in buildings where minimizing vertical deflection is crucial. Unlike bridge structures that require a thicker layer of rubber for rotation accommodation due to live loads on the top surface, buildings necessitate an extremely thin layer of rubber to minimize vertical deflection. In such cases, molds with exposed steel plates play a more dramatic role.
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(1) Iron plate exposed laminated support |
(2) Existing products with problems with the flatness of the iron plate |
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(1) Vertical shear deformation test of iron plate exposed type
Figure 2.7 illustrates the results of vertical shear deformation tests conducted by the Korea Institute of Construction and Living Environment Testing as part of the research project "Development of Circular Rubber Bearings with Individual Shear Performance Deviation Within 10%." This project was carried out in 2016 as part of a small and medium-sized enterprise support initiative.
The red line represents the graph of 400% shear deformation under a vertical load of 15 MPa, the blue line represents the graph of 400% shear deformation under the same vertical load of 15 MPa, and the green line represents the graph of 400% shear deformation under a vertical load of 26 MPa.
Although there is a slight difference in hardening beyond the 350% stage, the results align well with the theoretical outcomes for elastomeric bearings.
Comparing the blue and green lines with different vertical loads, it shows a unique theoretical result specific to rubber bearings, indicating that as the vertical load increases, the horizontal stiffness is actually reduced.
The manufacturing method that crucially maintains a consistent thickness of the rubber layer between steel plates in elastomeric bearings has been emphasized for rubber-based laminated bearings.
However, consumers face the challenge of being unable to inspect the internal structure of rubber bearings, leading to lingering doubts about the long-term durability of rubber.
3. Metal-based Seismic Isolation Device
The content regarding seismic isolation devices made of metal is continued in the following post. (Link: Types and Characteristics of Seismic Isolation Device - Rubber Based)
Topics
- Seismic Isolation Design
- Seismic Isolation Device
- Pendulum
- Elastomeric Bearing
- Lead Rubber Bearing
- High Damping Rubber Bearing
- Tin Seismic Isolation Bearing
- Friction Material
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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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