What is a seismically isolated building?

Introduction
Japan is one of the most earthquake prone countries, from the 1923 Great Kanto Earthquake up to the 2005 Fukuoka West Offshore Earthquake, big quakes have occurred more than thirty times. Seismically isolated buildings have been developed on the basis of protection for buildings against big quake-vibrations. Recently 1500 buildings with seismic isolation excluding detached houses were designed and constructed in Japan.

So, what is the system of seismic isolation?
The following figures show the properties of buildings during quakes.

	Conventional buildings: These buildings are set on the ground, vibration of quake is brought directly to the superstructure. Building vibration is severe, there are possibilities that furniture will move and overturn sometimes, doors distort, and walls crack.
	Seismically isolated buildings: Devices are installed between building and the ground, and they absorb energy of vibration during the quake. They can reduce vibration of building. Building vibration is very slow, there are few possibilities of the foresaid situations.
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What kinds of Devices for seismic isolation used in buildings?
Generally, isolators and dampers are used. There are three types of isolators as shown below.

1. Elastomeric Isolator:

	It consists of steel and rubber, it made of sandwiches of soft rubber sheets and hard steel plates as shown in the Photograph. It works as a bearing to sustain the weight of the building and is able to move the building laterally. Soft rubber reduces the building vibration to slow shaking, and hard steel plate contributes to sustain the weight of building.
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2. Slider:

	The slider has a coating of PTFE and a stainless steel plate finished with smooth surface as a mirror in the Photograph. It works as bearing to sustain the weight of the building and is able to move the building laterally on the surface of the plate with a certain amount of friction.
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3. Rotating Ball Bearing:

	It consists of ball bearings with retainers and rails or plates as shown in the Photograph. It works as a bearing to sustain the weight of the building and is able to move the building laterally without friction.
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4. Damper:
Usually for a damper is used to suppress vibration, damper is used usually.
Mainly three types of dampers are used as follows;

a. Steel damper:

		It utilizes the elasto-plasticity property of steel. The hysteretic loop contributes to energy dissipation. There are two types of steel dampers, one is square in section and other is round in section of the rods as shown in the Photograph.
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b. Lead damper:

	It utilizes the plasticity property of lead. The hysteretic loop contributes to energy dissipation. There are two types of lead dampers, one has a round section and a curved shape as shown in the Photograph and other has a cylindrical plug in core of the elastomeric isolator.
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c. Oil damper:

	It utilizes the viscous property of fluid. Oil or silicon are used as the fluid. Viscous resistance contributes to energy dissipation. Oil damper is a cylindrical shape as shown in the Photograph.
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