Report done by: Nicolás Ronchel Díaz-Leante, Manuel Torija Ayala, Konstantinos Ntoumos and Diego Alonso Bobillo.
1. Introduction
Modern structures need to stand for long periods of time. A proper analysis is required to guarantee the correct response of the building to any external force it will be subjected to, such as wind, vibrations caused by vehicles (with especial significance in the case of trains), as well as in the unfortunate event of an earthquake. This study aims to understand and improve the seismic performance of buildings. We will use SeismoStruct, a program for predicting how space structures move significantly under steady or changing forces, following the video tutorials and pdf's uploaded to TUM webpage.
As in the example from the teacher, we will focus on a 3D model of a multi level structure, specifically a two-storey steel building consisting in nine columns and nine beams, examining it under three different scenarios:
- Base Isolation: Initially, we will consider the building with a base isolation system. Base isolation systems, often made of rubber, are designed to decouple the structure from the ground, reducing the amount of seismic energy transferred to the building during an earthquake. The construction is directly separated from the ground by introducing this kind of suspension system between the base and the main structure.
By increasing the ductility of the building, the response of the building to the inertial forces caused by an earthquake (proportional to the mass of the structure, as well as to the ground acceleration) can be appropiately handled, guaranteeing that the capacity is larger than the demand, decreasing the latter instead of increasing the former.
The main advantages of using this kind of device as a passive control system are:
- Flexible design: according to the specific weight, size, and seismic hazard level of a structure, there will be a specific base isolation device that will satisfy the requirements of the structure, offering flexibility in terms of load capacity, period lengthening, and energy dissipation requirements.
- Easy to install: they are modular and relatively simple to install compared to some other seismic protection systems, like tuned mass dampers (involving large pendulum-like masses or tuned sloshing tanks that need precise placement and integration within the structure), and active control systems (relying on sensors, actuators, and control algorithms to actively counteract seismic forces), helping to reduce construction time and costs.
- No Base Isolation: In the second scenario, we will analyze the same building but without any base isolation system. This will provide us with a baseline to understand the effectiveness of the base isolation system.
From the picture above, it can be seen that the left structure experiences minimal displacements (shown in centimeters at the left side of the structure) due to the base isolation device that is installed at the bottom of the building (connecting the ground to the structure above), experiencing very low deformation along the entire height of the building, barely independ of the ground acceleration (measured in cm/s^2), whereas the right structure, that neglects the use of a base isolation system, suffers massive deformation, significantly increasing the ground acceleration levels as we move towards the top of the structure.
- Incorrect Base Isolation Parameters: In the third scenario, we will intentionally use incorrect values in the base isolation system analysis, still using rubber as the material for the base isolation. This will help us understand the importance of accurate parameters in the design and analysis of base isolation systems.
By comparing these scenarios, we aim to gain a comprehensive understanding of the role and effectiveness of base isolation systems in enhancing the seismic performance of buildings.
2. Types of based isolation systems
2.1. Elastomeric bearings
The most common kind of base isolation are elastomeric bearings, which are a single unit devices made of two mild steel plates (on top and the bottom, connecting this bearing to the foundation of the ground, as well as to the structure above) with a layer in between of natural or synthetic rubber (whose movement is not restrained by those steel plates), providing load capacity and stiffness to the structure.
Since the rubber layer can be deformed in the shear direction, when the body is subjected to vibrations resulting in vertical deformation, this rubber layer will move in the horizontal one, dissipating the energy and transmitting minimal or no motion to the structure above.
Based on this elastomeric concept, there are three main types of bearing:
- Natural Rubber Bearing (NRB)
- Synthetic Rubber Bearings (SRB)
- Lead Rubber Bearing (LRB)
Their main advantage is their excellent durability, allowing them to be virtually maintenance-free: since these systems are made of high-quality steel and elastomeric bearings, they are resistant to corrosion and degradation, requiring minimal maintenance compared to other seismic protection systems, such as viscous fluid dampers (which rely on fluid passing through orifices to dissipate energy, degrading over time), and friction dampers (wearing at the sliding surfaces, and requiring inspection of friction elements).
2.2. Friction pendulum bearings
The second most important type of base isolation devices are friction pendulum bearings (FPB), consisting of a globe or a cylinder (as long as it has a global contact surface) made of special metals (capable of withstanding very low and high temperatures without deteriorating their physical properties, being its fire resistance its main advantage with respect to the previous base isolation devices), slightly elevates the builiding druing an earthquake, reducing its effect on the structure.
Besides, because of the curve surface, once they are in action the period of the movement is considerably high, dissipating energy by friction and allowing large sudden movements.
Furthermore, they present a stable bilinear hysteresis loop, meaning that their behavior during an earthquake is predictable and repeatable.
2.3. Spring isolators
And finally spring isolators, they are particularly suited to damping the vibrations of machines. They consist of springs to isolate vibrating machinery and equipment from the supporting structure and surrounding ground. The main type is steel screw compression springs: which are block resistant, so that overloads (during installation, for example) can be accommodated without any problems. They generally use a bonding plate at the top and bottom, and can be easily fixed in place. These plates can also compensate for any unevenness in the mounting surface on site. The high coefficient of friction of the bonding plates/structure-borne sound insulation plates secures the units in place in most cases. Besides, no additional bolts are necessary, reducing the installation complexity.
Furthermore, they allow to reduce the vibration transmission to the building structure, protecting it from damage; getting lower noise levels generated by vibrating machinery, as well as improved occupant comfort by minimizing vibration in floors and walls.
They are more suited for lower frequency vibrations compared to systems designed for earthquake protection (like friction pendulum bearings, they generally have a lower load capacity, and they often a more cost-effective solution for vibration control applications.
3. Most important environmental effects that can affect base isolation systems.
- Temperature: Extreme temperature variations can affect the material properties of components in base isolation systems, potentially leading to changes in stiffness and damping characteristics. This could impact the system's performance during seismic events.
- Aging: Aging can affect the surface properties and morphology of PTFE, potentially leading to changes in its frictional behaviour. For instance, oxidation or exposure to environmental contaminants over time can alter the surface chemistry or roughness of PTFE, which may impact its friction characteristics.
- Wear ( Travel ): Movement and travel of the isolation system components, especially in systems with sliding or rolling elements, can result in wear and tear. Excessive wear may lead to reduced effectiveness of the isolation system and increased vulnerability to seismic forces.
- Contamination: Environmental contaminants such as dust, debris, or pollutants can accumulate on the surfaces of isolation system components, potentially interfering with their operation or causing corrosion. Regular cleaning and inspection are essential to maintain the performance and longevity of the system.
- Scragging: Scragging refers to the phenomenon where the isolation system becomes stuck or jammed due to uneven settling or deformation of the structure. This can occur over time, particularly if the foundation or supporting structure experiences differential settlement or distortion, compromising the effectiveness of the isolation system.
3.1. Cities with the most important use of base isolation systems in buildings
According to the graph, the number of buildings with base isolation in Italy increased steadily over this 25 year period. There were very few buildings built with base isolation before 1981. By 2006, there were over 250 buildings that had been completed, were in progress, or had been designed with base isolation.
Both graphs highlight the increasing importance of earthquake-resistant construction techniques in Italy and Japan. However, Japan shows a much larger number of buildings constructed with seismic isolation, likely due to factors like higher seismic activity and stricter building codes.
3.2. Base isolation in bridges
- Deck: As mentioned before, this is the uppermost surface that carries the traffic on the bridge.
- Isolator: This is the heart of the base isolation system. These are bearings, typically made of lead rubber bearings (LRBs), placed between the deck and the pier. LRBs consist of alternating layers of rubber and steel with a lead core. During an earthquake, the ground shakes, but the isolators absorb the horizontal forces by deforming, essentially decoupling the bridge deck from the ground movement.
- Stopper System: This system prevents the excessive horizontal movement of the bridge deck in case of a very strong earthquake. It acts like a giant shock absorber, limiting the movement of the deck and preventing it from going off the pier.
- Dampers: These are energy dissipation devices, often placed near the stoppers, that further reduce the shaking of the bridge by absorbing some of the energy from the earthquake.
3.3. Examples of some important base isolated buildings.
- Apple Headquarters, California. Apple Park office is a landmark base-isolated building and is the largest single base isolated building in the world. The ring-shaped building has 4 above grade and 2 below grade levels combined create 4.9 million square feet of floor space.The isolation system consists of 692 large steel saucers located two stories underground. This system is a modified version used in Japan and protects the campus from all but the most severe earthquakes.The isolators were customized for low friction, according to the lead structural engineer, John Worley, of Arup. Construction of the entire Apple Campus 2, including the headquarters building as well as a 1,000-seat auditorium
2. Adana Integrated Health Campus, Adana, Turkey, 430,000 square meters. The campus was developed as a public-private partnership between ADN PPP Sağlık Yatırım A.Ş., a joint venture of four firms, and the Turkish Ministry of Health. The campus will have a total capacity of 1,550 beds housed in three hospitals: the 1,300-bed main hospital, a 150-bed physical-therapy and rehabilitation hospital and a 100-bed high-security criminal psychiatric hospital. The campus is supported by 1,512 base isolators. The complex was designed by HWP, and built by Rönesans Sağlık Yatırım. The structural engineer was Ulker Engineering Ltd. It was completed in May, 2017. Photo Courtesy Ronesans
3. Shinagawa Season Terrace, Tokyo, 205,786 square meters. An office building, it was designed by the NTT Facilities Design Office and built by Taisei Corp. It was completed in 2015. Photo Courtesy Tokyoing
4. SeismoStruct rubber model
The model constist of a 3 storie building with rubber isolation for Base Isolation system, Te next picture describes the model references for stiffnes and other variables in the Isolation System.Something different wiht non Base isolation models is that the Isolation parts have 2 nodes in each Base isolation System, this will add an other connection to the flor and an other row of nodes.
Also the comparisons and the modles will be based in the Tabas earthquake from 1978.
5. Comparison between models, one with Base Isolation and the other one without Base Isolation
With the model described before with rubber base isolation and other one without base isolation.
Take the case of the 1978 Tabas earthquake, which offers an actual model for comparing buildings with and without base isolation.
5.1.1. Displacement Comparison:
Without Base isolation Displacement :
In the case of a building that is not isolated, greater displacement would occur, and such a graph exhibits higher peak displacement, which refers to the fact that the building would sway from its position towards one side and then to another over a greater distance compared with the isolated building.
Enhanced Vibration at High Frequencies: The structure could be vulnerable to high-frequency shaking during an earthquake, which could lead to considerable destruction. The pattern might display quicker changes in the displacement curve.
An increased risk of damage includes fractures that penetrate through the entire structure, which can cause a collapse and the possibility of a calamity or loss of life.
With Base isolation Displacement:
There are several ways in which a base isolation system can reduce displacement: this would be an energy absorption system that will absorb the energy of the earthquake, and as a result, the amount of force transmitted to the building will decrease, which means less displacement.
So, based on the graph, the isolated building will show a smaller peak displacement compared to a non-isolated one. In terms of vibrational control, the isolation base would eliminate or dampen most of the high-frequency vibrations that would normally have an adverse effect on the building. These vibrations are mainly due to seismic motion, which can affect the structural integrity of a building if left uncontrolled.
The graph appears as a smoother displacement curve with fewer rapid fluctuations as shown in graph. A new level of protection from damage to structures arises from reduced forces that can prevent possible collapse and increase the safety level for the individuals inside the building. The basic isolation is actually a sort of shield that redirects most of the earthquake power and ensures that the building itself is not damaged. Bear in mind that these are generally expected. The actual performance of each building would depend on several factors, including the specific design of the base isolation system and the building itself.
Base Isolation Displacement Graph Without Base Isolation Displacement Graph
From a first look, the response of the building to the Tabas earthquake seem to be pretty similar, since the studied vibrations (and how they propagated in the ground and reached the structure) are very close. Thus, the pattern followed by the curves are alike for both cases. Nonetheless, the displacement observed in the left graph (corresponding to the response of the building including a base isolated system) is significantly lower than the one neglecting the use of one of these devices, reaching a displacement peak of 0.09 m instead of 0.26 m, respectively. Overall, the displacement experienced by the structure with no passive control system is considerably higher for the entire length of the earthquake.
5.1.2. Acceleration Comparision
We are taking for the comparison the node n122 ,this node is the one in the center and more comparative.
Without Base Isolation: Acceleration affects buildings in different ways. The building will shake violently, potentially causing extensive damage and posing greater safety risks.
With Base Isolation: Buildings move less, receive less damage, and provide better protection for residents.
By absorbing seismic energy, base isolation systems essentially decouple a building from violent ground movements, thereby significantly increasing its ability to withstand earthquakes.
Without Base Isolation Acceleration Graphs:
With Base Isolation Acceleration Graphs:
5.1.3. Base shear vs roof displacement (relative to the ground).
Without Base Isolation:
Base shear
roof displacement
Without Base isolation the graph shows a steeper upward slope. As the earthquake's ground motion intensifies (increasing base shear on the x-axis), the building would experience more significant swaying (increasing roof displacement on the y-axis).This is because the full force of the earthquake is directly transmitted to the structure, causing greater deformation.
With Base Isolation:
Base shear
Roof displacement
With Base isolationt the graph shows a flatter upward slope compared to the non-isolated building.
Even as the base shear increases, the base isolation system would absorb a portion of the energy, reducing the force reaching the structure and limiting its overall deformation.
The roof displacement would be noticeably lower for the same range of base shear values compared to the non-isolated building.
6. Conclusion
By comparing two models, a building with rubber base isolation system and a building without rubber base isolation system, we can clearly see the benefits of base isolation in earthquake-prone areas. Here is a summary of the most important points:
Reduce forces and deformations:
Buildings with base isolation experience less base shear (the total horizontal force acting on the foundation during an earthquake).This reduces roof movement, meaning the top of the building shakes less compared to the ground.Lower forces and deformations significantly reduce the risk of structural damage such as cracks, beam failure or even collapse.
Improved performance and security:
Base-isolated buildings are generally more resistant to earthquakes. They are more likely to remain functional after an earthquake, allowing residents to evacuate safely and speeding up recovery efforts.The reduced risk of damage also provides a safer environment for residents during earthquakes.
Practical example - Tabas earthquake:
The 1978 Tabas earthquake is a real-life example. We can assume that buildings in Tabas without base insulation will suffer severe damage due to high forces and deformations caused by ground movements caused by earthquakes.On the other hand, in buildings with base isolation, the damage that occurs may be significantly less due to the lower forces absorbed by the isolation system.
Graphs:
- A graph comparing base shear to roof displacement can visually illustrate the difference. Uninsolated buildings will have steeper slopes, indicating a greater increase in roof displacements as base shear increases.
- A base-isolated building plot will have a flatter slope, reflecting lower roof displacements for the same base shear.
In summary, Base isolation systems are a major benefit for earthquake engineering. Base isolation helps structures endure earthquakes with less damage by absorbing part of the energy and minimizing the forces imparted to the structure. This enhances overall safety and post-earthquake functionality.