Hospitals with the seismically isolated structure that prevented the loss of their functions even in the Great East Japan Earthquake
The mechanism of an earthquake is explained by plate tectonics. There are multiple solid plates covering the Earth’s surface, not one. The plates move and overlap each other to cause an earthquake on the boundary or inside of the plate.
Since Japan is located where several plates overlap, it is a region of the world that experiences frequent earthquakes.
Actually, the 2011 Pacific Coast of Tohoku Earthquake, which caused the Great East Japan Earthquake, had a magnitude of over 9. It was one of the world’s largest earthquakes. Since then, large shakes with a seismic intensity of lower 6 or greater have occurred almost every year.
The mechanism of an earthquake is on global scale, such that we humans cannot prevent it from occurring. To minimize the damage caused by an earthquake, researchers and engineers involved in seismic engineering have developed various technologies.
For example, a construction structure that can protect a building and the people inside it. This structure is referred to simply as an earthquake-resistant building; however, it is roughly classified into three types, each with a different structure and advantages: earthquake-resistance, the vibration control, and seismically isolated structure.
The earthquake-resistant structure is a structure that resists the shaking of an earthquake by increased strength and deformability with columns, beams, walls, or braces.
The vibration control structure is a structure designed to minimize damage by suppressing the vibration of a building efficiently through the installation of a device that absorbs the vibration energy, a damper, inside the building or on the roof.
The seismically isolated structure is a structure that prevents the shaking of an earthquake from propagating to a building directly through the installation of a device, an isolator represented by laminated rubber, on the foundation. A building with this structure shakes slowly owing to the effect of the isolator even if the ground shakes violently. Since this structure prevents damage to the building by absorbing the energy of an earthquake at the foundations, it can be regarded as a structure that protects lives and things inside the building.
When the 2011 Pacific Coast of Tohoku Earthquake occurred, a hospital building with a seismically isolated structure in Ishinomaki city was hardly damaged, so was its equipment; therefore, it maintained its function as a hospital.
Furthermore, this hospital provided the function of a disaster relief base because the surrounding social infrastructure was severely damaged and lost its functions.
Also in the 2016 Kumamoto Earthquakes, a hospital with a seismically isolated structure in Aso City experienced a similar situation.
For example, it maintained its function as a hospital without any problems right up to when the power was restored 40 hours later because the private power generation system for emergencies inside the hospital was not damaged at all even though the entire area was out of power.
This shows the major advantages of a seismically isolated structure. This structure protects not only lives but also the functions of a facility from an earthquake, so we can continue to use the facility even after the earthquake.
These functions are quite important for social infrastructure other than hospitals, such as the facilities of government offices and broadcasters, data centers, and large distribution warehouses. Therefore, the number of such facilities that are built with a seismically isolated structure has been increasing in recent years.
What are the problems of a seismically isolated structure?
Is the seismically isolated structure useful for everything? Actually, there are problems with this structure. To solve these problems and make it more reliable technology, numerous researchers and engineers continue research and development.
For example, the laminated rubber is flexible in the lateral direction to absorb the lateral vibration of an earthquake; however, it cannot absorb the vertical vibration.
This is because that the laminated rubber needs to be hard and strong in the vertical direction to support the weight of a building. To solve this problem, studies on seismic isolators that absorb the vertical vibration as well continue, e.g., three-dimensional seismic isolation technologies.
In the seismically isolated structure, seismic isolators are installed in a basement, called base isolation pits, constructed by digging down around a building.
To ensure safety when the building moves horizontally during an earthquake, a gap of about 60 cm, a horizontal clearance, is typically made between the building and the moat wall with the base isolation pit. However, if earthquake ground motions beyond the expected level occur, the building can collide with the moat wall.
Therefore, the estimation of the level of earthquake ground motion in designing is important for engineers; engineers need to explain it clearly to the owners so that they can understand.
If the measurement systems for the movement of a building during an earthquake, like seismometers and orbiters, are installed in a building, the measured data is useful for performance checks and maintenance.
This storage of data improves the technical level of the seismically isolated structure; thus, engineers should explain these things clearly so that the owners understand and introduce the measurement systems.
The other problem is that base isolation pits may be flooded by a tsunami caused by an earthquake. This is because the pits, which are the spaces unique to the seismically isolated structure, are in the basement. The pits may be flooded also by typhoons and heavy rain.
As a countermeasure, in some cases, the foundation of a building was raised by land elevation works. This countermeasure also requires the understanding of the owners to carry out.
In recent years, falsification of the performance data of seismic isolators has been revealed and become a social problem. The understanding and trust from the owners as well as society is essential to the dissemination of a seismically isolated structure. That kind of injustice should never occur.
In Japan, there are no permanent facilities for testing full-scale seismic isolators under the maximum levels of weight, displacement, and velocity estimated for earthquakes; thus, these tests are performed abroad as needed. We researchers and engineers play a role in changing these situations and introducing more reliable technologies into society.
Spreading the proper understanding of seismically isolated structures is essential
In recent years, SDGs have been gaining attention. Its 11th goal is to make “sustainable cities and communities.” To achieve this goal, making disaster-resistant cities is necessary.
Even though we humans cannot prevent an earthquake from occurring, we should develop a scheme of society to sustain city functions without interruption even if earthquakes occur. Seismically isolated structure is a technology to meet such needs of society.
Not only seismically isolated technologies but also combining them with earthquake-resistant and vibration control technologies is necessary to achieve this goal.
This requires the researchers and engineers to make further efforts and increase social understanding.
Seismically isolated structures have the characteristic of moving slowly and largely during an earthquake due to their mechanism. Sometimes, residents do not sufficiently understand this, complaining that the expected performance was not achieved.
Also, the above-mentioned clearance is a ditch surrounding a building when viewed from the ground. The part that covers this ditch and connects the building and the ground is called the expansion joint, a moving part for the seismic base isolation. You can see it if you watch carefully when you enter a building.
Many cases have been reported where this part was damaged or fell out because the part did not follow the shake of building during an earthquake. The following cases have been reported: consideration at the design phase was insufficient, and the structure was equipped after the completion without taking into account that the building with its seismic isolation can move largely.
Spreading the misunderstandings that the seismically isolated structure is not useful owing to these accounts is a social loss. It is necessary to make the public understand the characteristics of this structure: it moves slowly and largely during an earthquake.
Spreading the proper understanding of seismically isolated structures is valuable to society, which in turn will lead to the achievement of the 11th goal of the SDGs. Working on this is the responsibility of us, the researchers and engineers.
* The information contained herein is current as of November 2021.
* The contents of articles on Meiji.net are based on the personal ideas and opinions of the author and do not indicate the official opinion of Meiji University.
* I work to achieve SDGs related to the educational and research themes that I am currently engaged in.
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