Page 18 - EQTips_Eng

Learning

9 Earthquake Design
Earthquake Tip and

Construction

How to make Buildings ductile for Good Seismic Performance?

Construction Materials Concrete is used in buildings along with steel
In India, most non-urban buildings are made in reinforcement bars. This composite material is called
masonry. In the plains, masonry is generally made of reinforced cement concrete or simply reinforced concrete
burnt clay bricks and cement mortar. However, in hilly (RC). The amount and location of steel in a member
areas, stone masonry with mud mortar is more should be such that the failure of the member is by
prevalent; but, in recent times, it is being replaced with steel reaching its strength in tension before concrete
cement mortar. Masonry can carry loads that cause reaches its strength in compression. This type of
compression (i.e., pressing together), but can hardly take failure is ductile failure, and hence is preferred over a
load that causes tension (i.e., pulling apart) (Figure 1). failure where concrete fails first in compression.
Therefore, contrary to common thinking, providing
too much steel in RC buildings can be harmful even!!
Compression Tension Capacity Design Concept
Let us take two bars of same length and cross-
sectional area - one made of a ductile material and
another of a brittle material. Now, pull these two bars
Crack until they break!! You will notice that the ductile bar
elongates by a large amount before it breaks, while the
brittle bar breaks suddenly on reaching its maximum
strength at a relatively small elongation (Figure 2).
Amongst the materials used in building construction,
steel is ductile, while masonry and concrete are brittle.

Maximum
Force

F
Strong Weak Brittle Material

Bar Force F
Final Elongation is
small
Figure 1: Masonry is strong in compression but

weak in tension. 0
Elongation of Bar
Concrete is another material that has been
popularly used in building construction particularly F
over the last four decades. Cement concrete is made of
crushed stone pieces (called aggregate), sand, cement Maximum Force
and water mixed in appropriate proportions. Concrete
is much stronger than masonry under compressive
loads, but again its behaviour in tension is poor. The Ductile
properties of concrete critically depend on the amount Bar Force F Material Final
of water used in making concrete; too much and too Elongation
is large
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 0
reinforcement bars of diameter ranging from 6mm to Elongation of Bar
40mm. Reinforcing steel can carry both tensile and
compressive loads. Moreover, steel is a ductile material. Figure 2: Tension Test on Materials – ductile

This important property of ductility enables steel bars versus brittle materials.
to undergo large elongation before breaking.
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