You can find information about seismic isolation devices made of rubber in the previous post. (Link: Types and Characteristics of Seismic Isolation Device - Rubber Based)
3. Metal-based Seismic Isolation Device
3.1 Principles of operation of the Pendulum Seismic Isolation Device
In the evolutionary process of bridge bearings, the development has progressed through pin bearings, spherical bearings, pot bearings, and elastomeric bearings, with the current trend being the adoption of laminated bearings known as pendulum bearings.
Behind this trend lies not only functional advantages but also economic considerations.
Examining the evolution of bridge bearings, the pivotal aspect has always been the incorporation of rotational functionality.
In the context of bridge structures, rotational functionality is essential for accommodating the slight rotations that occur at the supports due to live loads and the self-weight of the superstructure. Without the ability to accommodate these minute rotations, there is a risk of inducing shear cracks in the superstructure, hence necessitating the requirement for rotational functionality in bridge bearings.
Figure 3.1 illustrates the evolutionary process of bridge bearings in developing rotational functionality.
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(1) Pin-roller Bearing |
(2) Spherical Bearing |
(3) Pot Bearing |
(4) Elastomeric bearing |
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To fulfill the rotational functionality, pin bearings allow large rotations with a central hinge, spherical bearings enable pivotal rotation in a semi-spherical shape, pot bearings achieve slight rotations due to rubber sealed within a pot, and elastomeric bearings consist of subtle rotations of thin rubber layers laminated between steel plates.
In addressing the thermal expansion behavior of bridge structures, pin bearings use a metal hoop that moves, spherical bearings introduce a horizontal movement with a lubricant-free sliding surface made of high-strength bronze and stainless steel plates.
Pot bearings utilize a new petroleum-chemical product called fluororesin in addition to stainless steel for its slippery surface.
On the other hand, elastomeric bearings, while belonging to a different category that accommodates deformations through shear in rubber without using slippery materials, are considered innovative enough to be compared to a Nobel Prize-worthy idea in the construction field, according to the author's perspective.
Shape-wise, pin and spherical bearings have a 3-dimensional form, while pot and elastomeric bearings have a 2-dimensional appearance, yet the 3-dimensional rotational functionality is embedded in the rubber component.
With the global introduction of seismic design, laminated bearings that allow for the continuous design of multiple spans without concentrating seismic loads on fixed piers and demanding an increase in foundation construction costs began to emerge.
In the case of elastomeric bearings, as they inherently possessed laminated functionality, their appearance did not undergo significant changes. By simply inserting a lead core in the center, they successfully adapted to the changes.
Throughout the developmental process of bridge bearings, the functionality of horizontal movement is mostly two-dimensional. However, pendulum bearings uniquely exhibit a three-dimensional form.
We often use the expression "different dimensions." Being in different dimensions implies something entirely new.
Life forms living in 1-dimensional space cannot compete with those in 2-dimensional space, and 2-dimensional life forms cannot compete with those in 3-dimensional space. Similarly, if there were 4-dimensional life forms, they would also be impossible to compete with the 3-dimensional life forms that we inhabit.
What the author wants to convey here is not that the pendulum has excellent functionality due to its three-dimensional form, but rather that, for those of us accustomed to 2-dimensional products, there are many aspects that we are not familiar with.
While there are many unknown aspects in terms of functionality, the reason it is widely used in the market is due to its ease of installation and cost-effectiveness resulting from miniaturization.
The spherical bearing introduced earlier used high-cost high-strength bronze in the part responsible for rotation to prevent corrosion. However, recent developments in chemical materials have introduced engineering plastics capable of withstanding high contact stress without the corrosion issues associated with traditional materials. This innovation has significantly reduced the size of the bearing surface, ensuring cost-effectiveness.
The essential function of laminated bearings, which elongates the natural period to reduce the magnitude of seismic forces imposed on a structure, must simultaneously possess both the restoring force and the damping function to absorb vibrational energy.
Lead laminated bearings augment the inherent restoring force of elastomeric bearings by inserting lead to provide additional damping functionality.
On the other hand, pendulum laminated bearings achieve restoring force by machining the plane of the upper and lower plates into a 3-dimensional curved surface, while also incorporating a damping function through the use of friction materials.
3.2 The Form of a Pendulum Bearing
(1) The early Pendulum
Figure 3.2 is the initial version of a pendulum product developed to address the limitations of elastomeric bearings, where structures requiring sizes exceeding 1,500 tons are demanded. It represents an enormous and large-scale product developed to meet the needs of such structures.
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At that time, friction materials used a friction lining with fiber texture, and due to the relatively high friction coefficient, the radius of curvature had to be small to exceed the restoring force resistance to frictional forces.
A small radius of curvature implies a concave shape, while a large radius of curvature implies a flat shape.
(2) Single Type Pendulum
The inverted pendulum, specialized for use in bridges, originated in Europe, including Germany and Italy.
For bridge structures, it is advantageous and convenient to have a small size for the lower plate, favoring the provision of clearance for installing on the coping of the piers.
The inverted form emulates the shape of a spherical bearing, where the lower part with a small radius of curvature handles rotation, while the upper part with a large radius of curvature accommodates temperature and seismic displacements of the bridge, providing restoring force and damping through friction.
To better understand the unique behavior of the special pendulum introduced later, let's briefly study the dynamic characteristics of the pendulum, comparing it to the behavior of a lead laminated bearing.
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In Figure 3.3, considering a mass m suspended on an additional string, the gravitational force results in a horizontal restoring force transforming potential energy into kinetic energy. The horizontal force is given by mg·sinθ, and since sinθ=d/R, the magnitude of the horizontal force is W/R·d. The slope of the hysteresis curve representing the second stiffness is expressed as Kh=W/R.
For lead laminated bearings, the slope of the second stiffness is given by Kh=G·h/A (G: shear modulus of rubber, h: pure height of rubber, A: area of the rubber bearing). The horizontal stiffness of rubber bearings is determined by the material and shape of the rubber bearing, independent of gravity. In the case of pendulum bearings, it is determined by the radius of curvature and the self-weight.
The horizontal stiffness of rubber bearings being independent of gravity has significant implications, and the characteristic strength Qd determined by the area of lead corresponds to the friction in the case of pendulum bearings.
Here, the friction material plays a crucial role, serving as the lifeblood of pendulum products.
In traditional products, obtaining restoring forces exceeding friction forces in concave cases with a small radius of curvature leads to inconveniences such as increased thickness of the upper plate and variations in the bearing height during horizontal movement. Compared to 2D products, these phenomena can be quite inconvenient.
Therefore, to increase the radius of curvature of the upper plate, materials with a low friction coefficient are advantageous. Particularly, materials with lower friction coefficients under high contact pressure provide the benefit of reducing the product size. However, using materials with excessively high strength increases the likelihood of encountering the stick-slip phenomenon, where static and dynamic coefficients of friction cross over.
The stick-slip phenomenon is a crucial concern as it can lead to the destruction of the friction material and must be strictly avoided.
While deferring the discussion of friction material characteristics to the next section, here, the focus is on mentioning the behavioral characteristics.
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Let's examine the behavioral pattern of horizontal movement for a single-type pendulum, as depicted in Figure-3.4.
Once again, it is emphasized that, unlike products with a 2D configuration represented by elastic and port bearings, 3D products like pendulums exhibit behavior during horizontal movement that is not inherently simple.
While the author previously mentioned that the lower curvature is responsible for rotation and the upper curvature is responsible for horizontal movement in the case of a single-type pendulum, it should be clarified that these actions are not entirely independent, as mentioned.
As the upper plate undergoes horizontal displacement, the thickness of the upper plate meeting the intermediate plate becomes thicker, causing an increase in the bearing height. To compensate for this, the intermediate plate rotates, addressing the effect of the vertical load due to the self-weight of the bridge.
If the structure lacks a mechanism for the intermediate plate to rotate, eccentricity occurs, leading to damage to the friction material. Therefore, lubricant is applied to the friction material on the lower plate to facilitate rotation.
The left illustration in Figure-3.4 represents the state with vertical load, while the right illustration depicts the condition where the upper plate has moved as much as possible to the right.
During horizontal movement, the upper plate contacts the thicker side. However, due to the action of the vertical load, the lower plate rotates to compensate for the height. It is easily discernible from the color representation that the left lower plate is under significant stress.
Because of these specific behavioral characteristics of the pendulum, testing machines with fixed heights, commonly used for 2D-shaped products like elastic and port bearings, are not suitable. Instead, a dedicated testing machine with the ability to apply a constant vertical load while adjusting the height is required.
(3) Double type and triple type pendulum
Similar to the single type, which has one friction surface providing both restoring force and damping, there is also a double type of pendulum in which two identical curved surfaces exist instead of one.
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The double type, as shown on the left in Figure-3.5, has the advantage of absorbing more energy as the middle plate moves, and it can accommodate seismic displacement up to twice as much. In other words, it has the advantage of reducing the size of the upper and lower plates.
On the other hand, when rotation occurs in the upper structure, there is a disadvantage of possible deflection in the middle plate since there is no mechanism to allow rotation. It may be argued that the center axis of the middle plate shifts, causing eccentricity in the bearing. However, the issue is not deflection but rather the possibility of concentrated loads on one side of the friction surface, leading to the potential for friction material damage. Therefore, it is not suitable for bridge structures with rotational movement but is commonly used in architectural structures without rotational movement.
Though not explicitly mentioned here, there is also a triple pendulum with a new curvature on the middle plate to allow rotation.
As the saying goes, "Too many cooks spoil the broth." The more components there are, the more inconveniences there may be.
In the last part of the description about the double type pendulum in the previous section, there is a mention of the triple pendulum. However, the question is raised about the benefits of having a complex structure. Subsequently, a catalog of the triple pendulum from a renowned German manufacturer was obtained, and only the illustrations are introduced.
As the author writes about the operation principle of the pendulum, the realization is that the logic of the pendulum started with the consideration of a point mass with no rotational stiffness rolling on the surface of a sphere. However, as the size of the intermediate plate, corresponding to the weight, gradually increases, the rotational stiffness also increases. This leads to a geometric behavior of the intermediate plate in response to the rotation of the upper structure becoming less smooth. Due to this, there is a need to add a curved surface to reduce the rotational stiffness of the intermediate plate.
3.3 Importance of Friction Materials
The above discusses the radius of curvature for providing restitution in the pendulum surface support. However, the damping function is determined by the friction forces between the two surfaces.
Simple contact of metal surfaces cannot achieve a constant friction force over an extended period. Therefore, in pendulum supports, the selection of suitable friction materials is crucial for effective damping.
PTFE :
The material serving as the sliding function on the upper surface of the pot bearing has a maximum vertical stress of around 30 MPa and is evaluated to have a durability of approximately 20 years concerning the structural movements of the bridge.
Pot bearings have solid lubricant embedded in the dimples on the friction surface.
Recently, there have been attempts to conduct tests using a reinforced PTFE plate as the friction material, where pure PTFE is reinforced by adding fiber structure.
UHMWPE (Ultra High Molecular Weight Polyethylene) :
Germany has started using a high-density chemical material with a molecular structure of over 3.5 million as a replacement for PTFE in bridge bearings. It exhibits a maximum vertical stress of 60 MPa and boasts excellent wear resistance, allowing it to be used as a friction material without the need for lubricants.
Its superior wear properties compared to metals, coupled with an absorption rate close to zero, play a crucial role in maintaining a constant friction coefficient. This polymer product has evolved from LDPE (Low Density Polyethylene) and HDPE (High Density Polyethylene).
PA (Polyamide) :
Italy, in competition with Germany, became the first to use a high-strength and heat-resistant material for bridge bearings. The friction material in bridge bearings has faced challenges such as stick-slip phenomena, leading to its gradual disappearance from the market. Its absorbency, higher than that of PE (Polyethylene), poses issues when used as a friction material. DuPont refers to this material as Polynylon, using PA (Polyamide) as its designation.
PET (Polyethylene Terephthalate) :
The material has very high tensile strength, mechanical strength, and hardness. However, it has brittle fracture characteristics, making it unsuitable as a friction material for pendulum applications. This material, known by its official chemical name, is commonly used as a friction material in bridge bearings. There are many names derived from its official designation.
The table below represents the characteristics of the friction material defined in the EDA specification, which is an approval standard for pendulum products in the European EN regulations. The most important points can be summarized as follows:
The ratio of the yield strength to the ultimate tensile strength should be 2.5 or higher. Additionally, the ratio of the yield strain to the ultimate strain should be 13 or higher.
The physical significance of these regulations implies that the material, apart from its elastic properties, must have sufficient ductility. Materials with high strength but prone to brittle fractures are deemed unsuitable as friction materials for pendulum applications.
4. Closing Remarks
With the introduction of seismic design, bridge bearings, which were originally used to accommodate temperature-induced expansion and contraction in bridge structures, have become a crucial factor determining the economy of bridge structures. Recently, a wide variety of products, often beyond the comprehension of bridge engineering practitioners, have been introduced and applied for this purpose.
The most traditional approach involves the use of pot bearings, introduced in Europe, which consist of a sealed pot containing rubber for rotational accommodation and a sliding plate made of PTFE resin on the upper part to address temperature-induced expansion. In Japan, elastic bearings, composed of a rubber layer and a bonded steel plate, were widely adopted due to their simplicity and ease of manufacturing.
As the need for larger and more easily manufacturable bearings arose, lead-rubber bearings were introduced, incorporating lead in a core form. Simultaneously, various forms of sliding bearings were introduced. Despite potential unknown issues, there is a tendency for manufacturers to promote and distribute these bearings based on their marketing efforts, even if the actual seismic performance remains unverified. In many cases, operational problems are concealed, and unless there are casualties, they might not be exposed in the media.
Bridge components, like any products used over the lifespan of a bridge, face the challenging task of performing their functions flawlessly for an extended period.
Rubber products undergo aging phenomena, including surface cracking, when exposed to ultraviolet rays, and metal products inevitably encounter corrosion issues. Achieving trouble-free, long-term functionality in bridge components remains a complex problem.
In addition to the inevitable limitation of a lifespan common to all materials, the crucial factor for rubber bearings lies in the manufacturing technology of the flatness between the steel plate and rubber. This is especially critical for products applied to large-scale structures like bearing components in structures with minimized vertical deflection.
Pendulum bearing devices that primarily use metal as the main material provide restitution through three-dimensional surface processing. However, there are issues with vertical movement occurring during horizontal behavior depending on the number of surfaces forming the curved surface. Moreover, unexpected problems can arise due to the choice of friction material.
The difficulty in selecting friction materials stems from the fact that the coefficient of friction is not fixed by the material but varies depending on the surrounding environment. Friction between non-corroding materials is affected by the affinity due to environmental moisture, making the selection of friction materials that do not absorb moisture crucial.
Among the materials currently developed, UHMWPE (Ultra High Molecular Weight Polyethylene) is known to have the lowest moisture absorption properties.
In South Korea, there are also widely used disc bearings known as disc bearings. Discs made of urethane with higher hardness than regular rubber may appear rigid, making it easy to assume that there is no vertical displacement.
However, since the disc lacks internal reinforced steel plates, larger-sized products are more prone to vertical displacement due to creep. While the risk of slight vertical displacement in regular bridges may be challenging to anticipate, long-term occurrence of gaps in railway turnouts, such as those in high-speed railroads, cannot be ignored.
In some bearing products, issues have arisen due to the occurrence of rust on the sliding surface coated with metal.
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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