Page 42 - EQTips_Eng

Learning

21 Earthquake Design
Earthquake Tip and

Construction

Why are Open-Ground Storey Buildings vulnerable in Earthquakes?

Basic Features Ahmedabad alone has about 25,000 five-storey
Reinforced concrete (RC) frame buildings are buildings and about 1,500 eleven-storey buildings;
becoming increasingly common in urban India. Many majority of them have open ground storeys. Further, a
such buildings constructed in recent times have a huge number of similarly designed and constructed
special feature – the ground storey is left open for the buildings exist in the various towns and cities situated
purpose of parking (Figure 1), i.e., columns in the in moderate to severe seismic zones (namely III, IV
ground storey do not have any partition walls (of and V) of the country. The collapse of more than a
either masonry or RC) between them. Such buildings hundred RC frame buildings with open ground
are often called open ground storey buildings or buildings storeys at Ahmedabad (~225km away from epicenter)
on stilts. during the 2001 Bhuj earthquake has emphasised that
such buildings are extremely vulnerable under
earthquake shaking.
The presence of walls in upper storeys makes
them much stiffer than the open ground storey. Thus,
the upper storeys move almost together as a single
block, and most of the horizontal displacement of the
building occurs in the soft ground storey itself. In
common language, this type of buildings can be
explained as a building on chopsticks. Thus, such
buildings swing back-and-forth like inverted pendulums
during earthquake shaking (Figure 2a), and the
columns in the open ground storey are severely
Figure 1: Ground storeys of reinforced concrete stressed (Figure 2b). If the columns are weak (do not
buildings are left open to facilitate parking –
this is common in urban areas in India. have the required strength to resist these high stresses)
or if they do not have adequate ductility (See IIT-
An open ground storey building, having only BMTPC Earthquake Tip 9), they may be severely
columns in the ground storey and both partition walls damaged (Figure 3a) which may even lead to collapse
and columns in the upper storeys, have two distinct of the building (Figure 3b).
characteristics, namely:
(a) It is relatively flexible in the ground storey, i.e., the
relative horizontal displacement it undergoes in the Earthquake
ground storey is much larger than what each of the oscillations
storeys above it does. This flexible ground storey is
also called soft storey.
(b) It is relatively weak in ground storey, i.e., the total
horizontal earthquake force it can carry in the
ground storey is significantly smaller than what
each of the storeys above it can carry. Thus, the
open ground storey may also be a weak storey.

Often, open ground storey buildings are called soft (a) Inverted
storey buildings, even though their ground storey may Pendulum
be soft and weak. Generally, the soft or weak storey
usually exists at the ground storey level, but it could
Stiff upper storeys:

be at any other storey level too. Small displacement between
Earthquake Behaviour adjacent floors
Open ground storey buildings have consistently Soft ground storey:

shown poor performance during past earthquakes Large displacement between
across the world (for example during 1999 Turkey, 1999 foundation and first floor

Taiwan and 2003 Algeria earthquakes); a significant (b) Ground storey columns severely stressed

number of them have collapsed. A large number of Figure 2: Upper storeys of open ground storey

buildings with open ground storey have been built in buildings move together as a single block –
India in recent years. For instance, the city of such buildings are like inverted pendulums.
41