The concept
of base isolation is explained through an example building resting on
frictionless rollers. When the ground shakes, the rollers freely roll, but the
building above does not move. Thus, no force is transferred to the building due
to shaking of the ground; simply, the building does not experience the
earthquake. Now, if the same building is rested on flexible pads that offer
resistance against lateral movements, then some effect of the ground shaking
will be transferred to the building above. If the flexible pads are properly
chosen, the forces induced by ground shaking can be a few times smaller than
that experienced by the building built directly on ground, namely a fixed base
building.
The flexible
pads are called base-isolators, whereas the structures protected by means of this
device are called base isolated buildings. This main feature of the base
isolation technology is that it introduces flexibility in the structure. As a
result, a robust medium-rise masonry or reinforced concrete building becomes
extremely flexible. The isolators are often designed to absorb energy and thus
add damping to the system. This helps in further reducing the seismic response
of the building. Several commercial brands of base isolators are available in
the market, and many of them look like large rubber pads, although there are
other types that are based on sliding of one part of the building relative to
the other. A careful study is required to identify the most suitable type of
device for a particular building. Also, base isolation is not suitable for all
buildings. Most suitable candidates for base-isolation are low to medium-rise
buildings rested on hard soil underneath, high-rise buildings or buildings rested
on soft soil are not suitable for base isolation
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