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Learning

24 Earthquake Design
Earthquake Tip and

Construction

How to reduce Earthquake Effects on Buildings?

Why Earthquake Effects are to be Reduced them look like large rubber pads, although there are
Conventional seismic design attempts to make other types that are based on sliding of one part of the
buildings that do not collapse under strong earthquake building relative to the other. A careful study is
shaking, but may sustain damage to non-structural required to identify the most suitable type of device
elements (like glass facades) and to some structural for a particular building. Also, base isolation is not
members in the building. This may render the building suitable for all buildings. Most suitable candidates for
non-functional after the earthquake, which may be base-isolation are low to medium-rise buildings rested
problematic in some structures, like hospitals, which on hard soil underneath; high-rise buildings or
need to remain functional in the aftermath of the buildings rested on soft soil are not suitable for base
earthquake. Special techniques are required to design isolation.

buildings such that they remain practically If the gap between the

undamaged even in a severe earthquake. Buildings building and vertical wall of

with such improved seismic performance usually cost the foundation pit is small,

more than normal buildings do. However, this cost is the vertical wall of the pit Rollers
may hit the building, when

justified through improved earthquake performance. the ground moves under

Two basic technologies are used to protect the building.
buildings from damaging earthquake effects. These are (a) Hypothetical
Base Isolation Devices and Seismic Dampers. The idea Building
behind base isolation is to detach (isolate) the building
from the ground in such a way that earthquake Building on rollers without any friction
– building will not move with ground
motions are not transmitted up through the building,
or at least greatly reduced. Seismic dampers are special Forces induced can be up to Small
devices introduced in the building to absorb the 5-6 times smaller than those movement
of building
energy provided by the ground motion to the building in a regular building resting
directly on ground.
(much like the way shock absorbers in motor vehicles Large
movement
absorb the impacts due to undulations of the road). Flexible pads in isolators
Base Isolation
The concept of base isolation is explained through
an example building resting on frictionless rollers Original

(Figure 1a). When the ground shakes, the rollers freely Lead plug Isolator
roll, but the building above does not move. Thus, no Flexible
force is transferred to the building due to shaking of Material
the ground; simply, the building does not experience the
earthquake. Now, if the same building is rested on Stainless
flexible pads that offer resistance against lateral steel plates
movements (Figure 1b), then some effect of the ground (b) Base Isolated
shaking will be transferred to the building above. If Building Isolator during Earthquake
the flexible pads are properly chosen, the forces Building on flexible pads connected to building
induced by ground shaking can be a few times smaller and foundation – building will shake less
than that experienced by the building built directly on
ground, namely a fixed base building (Figure 1c).

The flexible pads are called base-isolators, whereas Forces induced are large. Large
the structures protected by means of these devices are movement
called base-isolated buildings. The main feature of the of building
base isolation technology is that it introduces
flexibility in the structure. As a result, a robust (c) Fixed-Base
medium-rise masonry or reinforced concrete building Building
becomes extremely flexible. The isolators are often Building resting directly on ground
designed to absorb energy and thus add damping to – building will shake violently
the system. This helps in further reducing the seismic

response of the building. Several commercial brands of Figure 1: Building on flexible supports shakes
base isolators are available in the market, and many of lesser – this technique is called Base Isolation.
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