ABOUT 60 per cent of India's land area is under
the threat of moderate to severe seismic hazard. Nine earthquakes
in the Indian tectonic region over the past 17 years have provided
constant reminders of the vulnerability of Indian construction.
It was the Bhuj earthquake of 2001 that stirred-up maximum concern
on the vulnerability of Indian constructions, particularly the
modern reinforced cement concrete (RCC) constructions.
The Indian public is under-informed and under-educated
on earthquake resistant design and construction. This paper highlights
the seismic vulnerability of the large number of a class of reinforced
concrete multi-storey buildings being constructed in the urban
areas. . These buildings have open ground storey to accommodate
parking and 230 mm size columns in the ground storey. The typical
RCC buildings in India are made of frames consisting of vertical
and horizontal members. The former are called columns while the
latter members consist of (a) slabs, that is, the flat, plate-like
parts of the building on which people stand, walk and conduct
themselves in the building and (b) beams, that is, the stiff,
rib-like parts of the building running under the slab but between
columns which support the slabs.
Stilt structures
The typical buildings being built in India accommodate
parking in the ground storey. Most buildings having parking in
the ground storey have no masonry walls built between any columns.
While having parking in the ground storey is not the concern,
having all masonry walls removed and leaving the columns bare
is the crux of the matter.
Many unprofessional players in the real estate
development industry are doling out substandard civil engineering
products. and exploiting the ignorance of the common man on many
technical aspects of civil engineering. For example, most RCC
frame buildings in the present context use the same 230mm(9-inch)
wide columns in them irrespective of the number of storeys they
support above them. This article focuses only on one aspect of
the oft-constructed RCC buildings, namely the use of 230 mm wide
columns. .
A stark feature of these columns and also the most
unsafe aspect of the buildings they hold is that their size is
fixed- 230 mm. Irrespective of the overall height of the building
(1, 3, 5, 7, 11, 14 or even 21 storeys), the location of the building
(in severe seismic zone or in low seismic zone), the type of soil
strata underneath the building (soft soil or hard rock) and the
distance between adjacent columns (2m, 3m, 4m or even 5m), the
column size is unbelievably the same- 230 mm. The person who built
your apartment may not necessarily be qualified to make earthquake
resistant buildings. Open ground storey buildings have consistently
shown poor performance during past earthquakes across the world;
a large number of them have collapsed.
The presence of walls in upper storeys makes them
many times 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. There are several features associated with the
open ground storey buildings with 230 mm wide columns that make
them vulnerable during earthquakes. The vertical members in buildings
support the horizontal members. So vertical members must be stronger
than the horizontal ones. But open ground storey buildings on
230 mm wide columns do not have the feature of strong column-weak
beam. Usually, beams are deeper than the columns, have the same
width as the columns but have more steel than the beams. This
makes them stronger than the columns.
After the Bhuj earthquake, the Indian Seismic Code
IS: 1893 (Part 1) - 2002 has included special design provisions.
First, it specifies when a building becomes soft
or weak. Second, it specifies higher design forces for the soft
storey. For all new RCC frame buildings, the best option is not
to have such sudden and large decrease in the stiffness and/or
strength in any storey of a building; it would be ideal to have
walls (either masonry or RC walls) in the ground storey also.
(THE AUTHOR IS PROFESSOR, DEPARTMENT OF CIVIL ENGINEERING,
INDIAN INSTITUTE OF TECHNOLOGY
