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Types of base isolation systems
There are two basic types of isolation systems; one that uses elastomeric bearings and the other uses sliding system.
The first type of the isolated structure is curved only in the isolation system and the structure above is rigid. The lofty form which develops deformation in the structure is rectangular to the first form and consequently to the ground motion. These higher forms do not share in the movement thus forbidding the high energy in the ground movement at these advanced frequencies not to be carried into the structure. This form of isolation system does not take in the earthquake energy, but rather resists it through the force of the system. A system which is un-damped and linear makes use of such a kind of isolation system.
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The 2nd kind of isolation system is the sliding system. This restrict the transfer of shear across the isolation boundary. A variety of sliding systems have been designed and some have been used already. In China at least 3 buildings use sliding systems whereas one made of lead-bronze plate sliding on stainless steel with an elastomeric bearing is used at a nuclear power plant in South Africa. The friction-pendulum scheme which applies a special kind of interface material sliding on stainless steel is utilised in a lot of constructions in the United States.
Base isolators are frequently proposed to absorb energy and attach damping to the system. This helps in reducing the seismic reaction of the building. In reality base isolation is not suitable for all buildings. Low to average rise buildings rested on hard soil below; high-rise buildings, buildings rested on soft soil are not suitable for base isolation.
Lead-rubber bearings are the normally used base isolation bearings. A sheet of rubber inserted together with levels of steel, is a lead rubber bearing. The middle part is of solid lead "plug". The top and bottom of the bearing are set with steel plates. These helps in binding the bearing to the building and foundation. The bearing is very inflexible and strong in the vertical direction, but flexible in the horizontal direction.