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IITK-BMTPC Earthquake Tip 8

What is the Seismic Design Philosophy for Buildings? page 2
damage is unavoidable. Different types of damage factors affecting the building performance. Thus,
(mainly visualized through cracks; especially so in earthquake-resistant design strives to predetermine
concrete and masonry buildings) occur in buildings the locations where damage takes place and then to
during earthquakes. Some of these cracks are provide good detailing at these locations to ensure
acceptable (in terms of both their size and location), ductile behaviour of the building.
while others are not. For instance, in a reinforced
concrete frame building with masonry filler walls Ductile
between columns, the cracks between vertical columns Performance
and masonry filler walls are acceptable, but diagonal
cracks running through the columns are not (Figure 2).
In general, qualified technical professionals are
knowledgeable of the causes and severity of damage Total Horizontal Earthquake Force on Building Brittle
in earthquake-resistant buildings. Collapse

Horizontal Movement of Roof of Building
relative to its base
(a) Building performances during earthquakes:
two extremes – the ductile and the brittle.

Figure 2: Diagonal cracks in columns jeopardize
vertical load carrying capacity of buildings -
unacceptable damage.
Earthquake Design Criteria, EERI, USA
Earthquake-resistant design is therefore concerned Photo from: Housner & Jennings,
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 (b) Brittle failure of a reinforced concrete
entire electrical wiring and appliances in the house, you column
sacrifice some small parts of the electrical circuit, called Figure 3: Ductile and brittle structures – seismic
fuses; these fuses are easily replaced after the electrical over- design attempts to avoid structures of the latter
kind.
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. Reading Material
Acceptable Damage: Ductility Naeim,F., Ed., (2001), The Seismic Design Handbook, Kluwer Academic
So, the task now is to identify acceptable forms of Publishers, USA
damage and desirable building behaviour during Ambrose,J., and Vergun,D., (1999), Design for Earthquakes, John Wiley
earthquakes. To do this, let us first understand how & Sons, Inc., USA

different materials behave. Consider white chalk used
to write on blackboards and steel pins with solid heads Authored by:
used to hold sheets of paper together. Yes… a chalk C.V.R.Murty
breaks easily!! On the contrary, a steel pin allows it to be Indian Institute of Technology Kanpur
bent back-and-forth. Engineers define the property that Kanpur, India
allows steel pins to bend back-and-forth by large Sponsored by:
amounts, as ductility; chalk is a brittle material. Building Materials and Technology Promotion
Earthquake-resistant buildings, particularly their Council, New Delhi, India
main elements, need to be built with ductility in them.
Such buildings have the ability to sway back-and-forth This release is a property of IIT Kanpur and BMTPC New
during an earthquake, and to withstand earthquake Delhi. It may be reproduced without changing its contents
effects with some damage, but without collapse and with due acknowledgement. Suggestions/comments
may be sent to: nicee@iitk.ac.in. Visit www.nicee.org or
(Figure 3). Ductility is one of the most important www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.

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