Page 16 - EQTips_Eng

Learning

8 Earthquake Design
Earthquake Tip and

Construction

What is the Seismic Design Philosophy for Buildings?

The Earthquake Problem may sustain severe (even irreparable) damage, but
Severity of ground shaking at a given location the building should not collapse.
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 Minor Shaking
design and construct a building to resist that rare
earthquake shaking that may come only once in 500
years or even once in 2000 years at the chosen project Moderate Shaking
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 Strong Shaking
earthquake effects? Or should we design the buildings to be
“earthquake proof” wherein there is no damage during the Figure 1: Performance objectives under different
strong but rare earthquake shaking? Clearly, the former intensities of earthquake shaking – seeking
approach can lead to a major disaster, and the second low repairable damage under minor shaking and
approach is too expensive. Hence, the design collapse-prevention under strong shaking.
philosophy should lie somewhere in between these
two extremes. Thus, after minor shaking, the building will be
Earthquake-Resistant Buildings fully operational within a short time and the repair
The engineers do not attempt to make earthquake- costs will be small. And, after moderate shaking, the
proof buildings that will not get damaged even during building will be operational once the repair and
the rare but strong earthquake; such buildings will be strengthening of the damaged main members is
too robust and also too expensive. Instead, the completed. But, after a strong earthquake, the building
engineering intention is to make buildings earthquake- may become dysfunctional for further use, but will
resistant; such buildings resist the effects of ground stand so that people can be evacuated and property
shaking, although they may get damaged severely but recovered.
would not collapse during the strong earthquake. The consequences of damage have to be kept in
Thus, safety of people and contents is assured in view in the design philosophy. For example, important
earthquake-resistant buildings, and thereby a disaster buildings, like hospitals and fire stations, play a critical
is avoided. This is a major objective of seismic design role in post-earthquake activities and must remain
codes throughout the world. functional immediately after the earthquake. These
Earthquake Design Philosophy structures must sustain very little damage and should
be designed for a higher level of earthquake
The earthquake design philosophy may be protection. Collapse of dams during earthquakes can
summarized as follows (Figure 1): cause flooding in the downstream reaches, which itself
(a) Under minor but frequent shaking, the main can be a secondary disaster. Therefore, dams (and
members of the building that carry vertical and similarly, nuclear power plants) should be designed
horizontal forces should not be damaged; however for still higher level of earthquake motion.
building parts that do not carry load may sustain
repairable damage. Damage in Buildings: Unavoidable
(b) Under moderate but occasional shaking, the main Design of buildings to resist earthquakes involves
members may sustain repairable damage, while the controlling the damage to acceptable levels at a reasonable
other parts of the building may be damaged such cost. Contrary to the common thinking that any crack
that they may even have to be replaced after the in the building after an earthquake means the building
earthquake; and is unsafe for habitation, engineers designing
(c) Under strong but rare shaking, the main members earthquake-resistant buildings recognize that some