How to Make Buildings Ductile for Good Seismic Performance?
Earthquake Tip 9
In India, most non-urban buildings are made in
masonry. In the plains, masonry is generally made of
burnt clay bricks and cement mortar. However, in hilly
areas, stone masonry with mud mortar is more
prevalent; but, in recent times, it is being replaced with
cement mortar. Masonry can carry loads that cause
compression (i.e., pressing together), but can hardly take
load that causes tension (i.e., pulling apart) (Figure 1).
Concrete is another material that has been
popularly used in building construction particularly
over the last four decades. Cement concrete is made of
crushed stone pieces (called aggregate), sand, cement
and water mixed in appropriate proportions. Concrete
is much stronger than masonry under compressive
loads, but again its behaviour in tension is poor. The
properties of concrete critically depend on the amount
of water used in making concrete; too much and too
little water, both can cause havoc. In general, both
masonry and concrete are brittle, and fail suddenly.
Steel is used in masonry and concrete buildings as
reinforcement bars of diameter ranging from 6mm to
40mm. Reinforcing steel can carry both tensile and
compressive loads. Moreover, steel is a ductile material.
This important property of ductility enables steel bars
to undergo large elongation before breaking.
Concrete is used in buildings along with steel
reinforcement bars. This composite material is called
reinforced cement concrete or simply reinforced concrete
(RC). The amount and location of steel in a member
should be such that the failure of the member is by
steel reaching its strength in tension before concrete
reaches its strength in compression. This type of
failure is ductile failure, and hence is preferred over a
failure where concrete fails first in compression.