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IITK-BMTPC Earthquake Tip 10

How flexibility of Buildings affects their Earthquake Response? page 2
Importance of Flexibility
The ground shaking during an earthquake
contains a mixture of many sinusoidal waves of
different frequencies, ranging from short to long
periods (Figure 3). The time taken by the wave to
complete one cycle of motion is called period of the
earthquake wave . In general, earthquake shaking of the
ground has waves whose periods vary in the range Earthquake Shaking

0.03-33sec. Even within this range, some earthquake (a) Buildings in a city lie on different soils
waves are stronger than the others. Intensity of
earthquake waves at a particular building location
depends on a number of factors, including the 50
magnitude of the earthquake, the epicentral distance, and
the type of ground that the earthquake waves travelled 40 10-14 Storey Motion and Soil Liquefaction During Earthquakes,
Buildings
through before reaching the location of interest. 30 3-5 Storey Adapted from: Seed and Idriss, (1982), Ground
Structural Damage Intensity (%) Buildings
Short 20
Period 0 Time EERI, USA.

Wave T short 10

0 0 50 100 150 200 250 300
0 Time Depth of Soil (m)

(b) Intensity of damage depends on thickness of
underlying soil layer: 1967 Caracas Earthquake

Amplitude Figure 4: Different Buildings Respond Differently
Long to Same Ground Vibration.
Period 0 Time
Wave T long

Flexible buildings undergo larger relative
Figure 3: Strong Earthquake Ground Motion is horizontal displacements, which may result in damage

transmitted by waves of different periods. to various nonstructural building components and the
contents. For example, some items in buildings, like
In a typical city, there are buildings of many glass windows, cannot take large lateral movements,
different sizes and shapes. One way of categorizing and are therefore damaged severely or crushed.
them is by their fundamental natural period T. The Unsecured shelves might topple, especially at upper
ground motion under these buildings varies across the stories of multi-storey buildings. These damages may
city (Figure 4a). If the ground is shaken back-and-forth not affect safety of buildings, but may cause economic
by earthquake waves that have short periods, then losses, injuries and panic among its residents.
short period buildings will have large response.
Similarly, if the earthquake ground motion has long Related Tip
period waves, then long period buildings will have IITK-BMTPC Earthquake Tip 2: How the Ground Shakes?
IITK-BMTPC Earthquake Tip 5: What are the Seismic Effects on
larger response. Thus, depending on the value of T of Structures?
the buildings and on the characteristics of earthquake
ground motion (i.e., the periods and amplitude of the Reading Material
earthquake waves), some buildings will be shaken Wiegel,R., (1970), Earthquake Engineering, Prentice Hall Inc., USA
more than the others. Chopra,A.K., (1980), Dynamics of Structures – A Primer, Earthquake
uring the 1967 Caracas earthquake in South Engineering Research Institute, USA
America, the response of buildings was found to
depend on the thickness of soil under the buildings. Authored by:
Figure 4b shows that for buildings 3-5 storeys tall, the C.V.R.Murty
damage intensity was higher in areas with underlying Indian Institute of Technology Kanpur
soil cover of around 40-60m thick, but was minimal in Kanpur, India
areas with larger thickness of soil cover. On the other Sponsored by:
hand, the damage intensity was just the reverse in the Building Materials and Technology Promotion
case of 10-14 storey buildings; the damage intensity Council, New Delhi, India
was more when the soil cover was in the range 150-
300m, and small for lower thickness of soil cover. This release is a property of IIT Kanpur and BMTPC New
Here, the soil layer under the building plays the role of Delhi. It may be reproduced without changing its contents
a filter, allowing some ground waves to pass through and with due acknowledgement. Suggestions/comments
may be sent to: nicee@iitk.ac.in. Visit www.nicee.org or
and filtering the rest. www.bmtpc.org, to see previous IITK-BMTPC Earthquake Tips.
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