Seismic Isolation Systems
Seismology and Seismic Energy
Seismic activity relates to earthquakes or other vibrations of the earth and its crust. The scientific study of earthquakes, often known as quakes, and the transmission of elastic waves through the Earth or other planetary bodies is known as seismology. It also covers research on many seismic causes, including volcanic, tectonic, glacial, fluvial, oceanic, atmospheric, and manmade processes like explosions, as well as earthquake environmental impacts like tsunamis. When an earthquake happens, the fault movement creates seismic energy (also referred to as seismic waves) that simultaneously go forth through the ground in all directions. Seismic waves can also be generated by volcanoes erupting, explosions, landslides, avalanches, and even flowing rivers.
Waves produced by earthquakes come in a variety of kinds and propagate at various speeds. Their various travel periods, which may be recorded at seismic observatories, enable researchers to pinpoint the earthquake's origin. A seismologist (a scientist who studies earthquakes and seismic energy) uses a seismograph to capture the Earth's seismic waves as they pass through and around it.
Types of Seismic Waves
In the form of waves, earthquakes emit seismic energy. Seismic waves can be divided into various categories, and each one moves in a unique fashion.
However, the two main type of seismic waves are the body waves and surface waves. In contrast to surface waves, which can only move along the Earth's surface like water ripples, body waves pass through the Earth's core layers. Body waves are characterized by travel speed and higher frequency. Hence, body waves from an earthquake arrive before their corresponding surface waves.
Primary Waves
Primary or P waves are the first type of body wave. This seismic wave arrives at a seismic station first and moves the fastest. Fluids like water, and the liquid layers of the Earth's crust and solid rock can both be traversed by P waves. Just like sound waves compress and enlarge the air as they travel through it, it squeezes and stretches the rock it passes through. Because they push and pull, P waves are often referred to as compressional waves. Particles subjected to P waves tend to move in the same direction as the wave and this path of energy transport is frequently referred to as the "direction of wave propagation".
Secondary Waves
S waves or secondary waves, are the second kind of body waves and are characterized by being larger, more noticeable and later-arriving waves in comparison to P waves. They are the waves that follow an earthquake. Compared to a P wave, a S wave moves around 1.7 times more slowly. Also, S waves can't pass through liquid. They travel perpendicular to the wave's direction, S waves always move rock particles up and down or side to side (the direction of wave propagation).
Seismic Isolation
A significant number of people are found to reside in seismically active areas, where they are vulnerable to earthquakes of different intensities and frequencies. In the field of earthquake engineering, it is now widely accepted that tolerating a certain amount of damage is necessary for the right design of a structure to withstand seismic forces in a seismically active region. That stated, it is crucial to investigate and create a solid foundation for determining the permissible level of damage as well as the potential locations for its emergence and growth. .
Seismic isolators are designed to reduce the seismic forces transmitted to a structure during an earthquake. They are typically made of a combination of rubber and steel, and their design involves selecting the appropriate materials and dimensions to provide the desired level of seismic isolation.
The design of seismic isolators involves several key considerations, including:
Load capacity: The isolators must be designed to support the weight of the structure and its contents, as well as any dynamic loads that may be applied during an earthquake.Stiffness: The stiffness of the isolators is an important factor in determining the level of seismic isolation provided. The isolators must be stiff enough to support the structure, but also flexible enough to deform and absorb seismic energy during an earthquake.
Damping: The damping characteristics of the isolators determine how quickly they can dissipate energy during an earthquake. The damping must be carefully designed to provide the desired level of energy dissipation without causing excessive deformation or damage to the isolators.
Durability: Seismic isolators must be designed to withstand multiple seismic events over the lifetime of the structure. This requires selecting materials that are durable and resistant to fatigue and aging.Installation and maintenance: The design of the isolators must also consider the ease of installation and maintenance, as well as any potential challenges that may arise during construction or operation of the structure.
The design of seismic isolators is typically based on a combination of analytical modeling, numerical simulations, and physical testing. This allows designers to optimize the performance of the isolators and ensure that they provide the desired level of seismic isolation for the structure.
Response Regularization
In seismic isolation, response regularization refers to the process of designing the seismic isolation system in such a way that it provides a predictable and consistent response to seismic loads.
This is important because seismic loads can cause unpredictable and complex dynamic responses in structures, which can lead to significant damage or failure.Response regularization can be achieved by carefully selecting and designing the seismic isolation components, such as bearings, dampers, and isolation units, to ensure that they provide a consistent and predictable response to seismic loads. This can involve optimizing the stiffness and damping characteristics of the components, as well as carefully considering the geometry and arrangement of the components within the isolation system.
In addition to designing the seismic isolation components for response regularization, it is also important to consider the overall system behavior and response to seismic loads. This can involve performing detailed modeling and analysis of the structure and isolation system, as well as conducting physical testing to validate the performance of the system under various seismic loading conditions. Overall, response regularization is an important aspect of seismic isolation design, as it helps to ensure that the isolation system provides a predictable and consistent response to seismic loads, which can reduce the risk of damage or failure to the structure.
Seismic Isolation Systems
A structure’s reaction to seismic shaking is decreased by the flexibility between the structure and the ground. Seismic Isolation systems involve Installing isolators and damping devices beneath the superstructure in order to provide structural safety and security for occupants and property in the building against earthquakes. The seismic isolators dampen the seismic energy under the ground of the building, thereby minimizing the effects of lateral loads on top floors.
Three categories of isolation systems exist, namely the Passive system, Active system, and Semi-Active system.
Active Control Systems
Active control systems rely on external energy sources to provide constant power during operation. This explains why installing these systems is expensive. The structure's acceleration, displacement, or velocity can all be controlled by the system. The electronic components of active control systems include things such as starters, actuators, and computers. The design of active control systems is unrelated to how strong the ground motion is. Depending on the strength of the ground motion, the system modifies its rigidity or amount of motion. Therefore, in designs that use active control systems, factors like the uncertainty or unpredictability of future ground motions are not significant.
Passive Control Systems
On the other hand passive control systems are systems which run Without using any external energy sources. As a result, compared to active systems, the setup cost for these systems is lower. These systems have a limited capacity for controlling displacement. The safety features are built into the passive control systems based on the level of protection required for earthquakes of a particular magnitude. These systems consist of readily accessible devices such as dampers and isolators.
Semi-Active Control Systems
In recent years, semi-active structural control systems have been created that combine the use of active and passive systems.
Base Isolation Systems
Seismic isolation systems and base isolation systems are terms that are often used interchangeably to describe a method of protecting structures from earthquake damage. However, there is a subtle difference between these two terms.
Seismic isolation systems typically refer to a system of devices that are installed between the foundation of a structure and the ground to reduce the seismic forces transmitted to the structure during an earthquake.
These devices, which are commonly referred to as isolators, may be made of various materials such as rubber, steel, or lead. Seismic isolation systems can be designed to provide different levels of isolation, depending on the seismic hazard and performance goals of the structure.
On the other hand, base isolation systems refer specifically to a type of seismic isolation system that involves installing a flexible bearing system between the foundation of a structure and the ground. This system allows the structure to move independently of the ground during an earthquake, which reduces the seismic forces transmitted to the structure. Base isolation systems typically use a combination of rubber and steel bearings to provide the necessary flexibility and strength.
So, while seismic isolation systems and base isolation systems are similar in that they both involve isolating a structure from seismic forces, base isolation systems are a specific type of seismic isolation system that uses a flexible bearing system. Currently, flat plate sliding bearings, friction pendulum sliding bearings, and elastomeric bearings are the three basic types of base isolation systems.
Energy Dissipation Systems
A type of passive structural control includes the energy dissipation approach. The fundamental function of passive energy dissipation equipment is to absorb or consume some of the energy input from an earthquake or wind, to lessen the structural response, and to safeguard structural members.
After a significant earthquake, some of the input seismic energy may be transferred into energy dissipation devices installed inside a structure, enabling the simple replacement or retrofitting of damaged components. Buildings, both new and retrofit, are increasingly using energy dissipation or damping systems to disperse earthquake-induced energy in materials.
These substances are either viscoelastic, viscous, or hysteretic (yielding steel devices). Despite the fact that these elements have different behaviors, they are all employed to reduce or even avoid structural frame damage. The most popular devices for seismic protection of structures include yielding steel devices, friction devices, viscoelastic devices, and most recently, fluid viscous devices.
Types of Energy Dissipation Systems
Viscous Fluid Dampers
In order to safeguard structures from earthquakes, viscous fluid dampers are frequently utilized as passive energy dissipation devices. These dampers consist of a hollow cylinder that is usually filled with silicone-based fluid. Fluid is compelled to flow through orifices either around or through the piston head of the damper piston while the piston rod and piston head are stroked. Very strong forces that oppose the relative motion of the damper can be produced by the pressure differential that results, which is very high pressure upstream and very low pressure downstream.
Viscoelastic Solid Dampers
Solid elastomeric pads made of viscoelastic material are typically bonded to steel plates to create viscoelastic solid dampers. Chevron or diagonal bracing is used to secure the steel plates to the building. The viscoelastic material is sheared as one end of the damper moves in relation to the other, creating heat that is released into the surrounding air. Viscoelastic substances naturally display both elasticity and viscosity.
Metallic Dampers
Buckling-restrained brace(BRB) dampers and additional damping and stiffness(ADAS) dampers are the two main categories of metallic dampers. A metallic damper is made up of a rigid steel tube around a steel brace with a cruciform cross section and often poor yield strength. A concrete-like substance is used to fill the space between the tube and the brace, and the brace is given a specific coating to prevent bonding to the concrete.
As a result, the brace might move relative to the tube filled with concrete. The concrete-filled tube’s confinement allows the brace to be subjected to compressive loads without buckling, allowing the damper to yield in either tension or compression with the steel brace carrying all of the tensile and compressive pressures.
Friction Dampers
The sliding friction that occurs across the interface of two solid bodies is how friction dampers dissipate energy. A slotted-bolted damper is one type of such damper, in which a number of steel plates are fastened together using a specific clamping force. The clamping force is set up so that slide starts to happen at a certain friction force. Special materials may be used to promote steady coefficients of friction at the sliding interface between the steel plates.
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