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What is the Seismic Design Philosophy for Buildings?
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The Earthquake Problem |
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Severity of ground shaking at
a given location during an earthquake can be minor,
moderate and strong. Relatively speaking,
minor shaking occurs frequently, moderate shaking
occasionally and strong shaking rarely. For instance, on
average annually about 800 earthquakes of magnitude 5.0-5.9
occur in the world while the number is only about 18 for
magnitude range 7.0-7.9 (see Table 1 of IITK-BMTPC
Earthquake Tip 03 at www.nicee.org). So, should we
design and construct a building to resist that rare
earthquake shaking that may come only once in 500years or
even once in 2000 years at the chosen project site, even
though the life of the building itself may be only 50 or 100
years? Since it costs money to provide additional earthquake
safety in buildings, a conflict arises: Should we do away
with the design of buildings for earthquake effects? Or
should we design the buildings to be “earthquake proof”
wherein there is no damage during the strong but rare
earthquake shaking? Clearly, the former approach can
lead to a major disaster, and the second approach is too
expensive. Hence, the design philosophy should lie somewhere
in between these two extremes.
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Earthquake-Resistant Buildings |
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The
engineers do not attempt to make earthquake proof
buildings that will not get damaged even during
the rare but strong earthquake; such buildings will be too
robust and also too expensive. Instead, the engineering
intention is to make buildings earthquake resistant;
such buildings resist the effects of ground shaking,
although they may get damaged severely but would not
collapse during the strong earthquake. Thus, safety of
people and contents is assured in earthquake-resistant
buildings, and thereby a disaster is avoided. This is a
major objective of seismic design codes throughout the
world.
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Earthquake Design Philosophy |
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(a) Under minor but frequent
shaking, the main members of the building that carry
vertical and horizontal forces should not be damaged;
however building parts that do not carry load may
sustain repairable damage..
(b) Under moderate but
occasional shaking, the main members may sustain
repairable damage, while the other parts of the building
may be damaged such that they may even have to be
replaced after the earthquake; and
(c) Under strong but rare shaking, the main members may
sustain severe (even) irreparable) damage, but the
building should not collapse.
Thus, after minor
shaking, the building will be fully operational within a
short time and the repair costs will be small. And,
after moderate shaking, the building will be operational
once the repair and strengthening of the damaged main
members is completed. But, after a strong earthquake,
the building may become dysfunctional for further use,
but will stand so that people can be evacuated and
property recovered. |
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The
consequences of damage have to be kept in view in the
design philosophy. For example, important buildings,
like hospitals and fire stations, play a critical role
in post-earthquake activities and must remain functional
immediately after the earthquake. These structures must
sustain very little damage and should be designed for a
higher level of earthquake protection. Collapse of dams
during earthquakes can cause flooding in the downstream
reaches, which itself can be a secondary disaster.
Therefore, dams (and similarly, nuclear power plants)
should be designed for still higher level of earthquake
motion. |
Figure 2: Performance objectives under different
intensities of earthquake
shaking
–
seeking low repairable
damage under minor shaking and collapse-prevention under
strong shaking.
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Damage in Buildings: Unavoidable |
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Design of buildings to resist
earthquakes involves controlling the damage to acceptable
levels at a reasonable cost. Contrary to the common
thinking that any crack in the building after an earthquake
means the building is unsafe for habitation, engineers
designing earthquake-resistant buildings recognize that some
damage is unavoidable. Different types of damage (mainly
visualized though cracks; especially so in concrete and
masonry buildings) occur in buildings during earthquakes.
Some of these cracks are acceptable (in terms of both
their size and location), while others are
not. For instance, in a reinforced concrete frame
building with masonry filler walls between columns, the
cracks between vertical columns and masonry filler walls are
acceptable, but diagonal cracks running through the columns
are not (Figure 3).In general, qualified technical
professionals are knowledgeable of the causes and severity
of damage in earthquake-resistant buildings.Earthquake-resistant
design is therefore concerned about ensuring that the
damages in buildings during earthquakes are of the
acceptable variety, and also that they occur at the
right places and in right amounts. This approach of
earthquake-resistant design is much like the use of
electrical fuses in houses: to protect the entire
electrical wiring and appliances in the house, you sacrifice
some small parts of the electrical circuit, called fuses;
these fuses are easily replaced after the electrical
over-current. Likewise, to save the building from
collapsing, you need to allow some pre-determined parts to
undergo the acceptable type and level of damage. |
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Acceptable Damage: Ductility |
Figure3:Diagonalcracks in columns jeopardize vertical load
capacity of building -
unacceptable
damage |
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So, the task now is to identify acceptable
forms of damage and desirable building behaviour during
earthquakes. To do this, let us first understand how
different materials behave. Consider white chalk
used to write on blackboards and steel pins with
solid heads used to hold sheets of paper together. Yes…
a chalk breaks easily!! On the contrary, a steel
pin allows it to be bent back-and-forth.
Engineers define the property that allows steel pins to
bend back-and-forth by large amounts, as ductility;
chalk is a brittle
material.
Earthquake-resistant buildings, particularly their main
elements, need to be built with ductility in them. Such
buildings have the ability to sway back-and-forth during
an earthquake, and to withstand |
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earthquake effects with some damage, but
without collapse (Figure 4).Ductility is one of the most
important factors affecting the building performance. |
4(b)
Brittle failure of a reinforced column. |
4(a)
building performances during earthquakes: Two extremes-the ductile
and the brittle |
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Figure 4:
Ductile and Brittle structures seismic design attempts
to
to
avoid structures of latter kind. |
Thus, earthquake-resistant design
strives to predetermine the locations where damage takes
place and then to provide good detailing at these locations
to ensure ductile behaviour of the building.
Resource Material
Naeim,F., Ed., (2001), The Seismic Design Handbook, Kluwer
academic publishers, Boston, USA.
Ambrose,J., and Vergun,D., (1999), Design for Earthquake,
John Wiley & sons, Inc., New York.
Authorized by: C.V.R. Murthy
Indian Institute of Techonology kanpur, Kanpur, India
Sponsored by: Building Materials and Technology Promotion
Council, New Delhi.
Suggestion/comments may
sent to
eqtips@iitk.ac.in
To see previous IITK-BMTPC earthquake Tips, visit
www.nicee.org |
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