of Vasuki - How do we cope?
article was first published in the Development Alternatives Newsletter
of January 1996.
It is being reprinted with permission from the Eco-Building Advisory
Unit, Development Alternatives.
are sudden geologic movements - tremors that create shaking at
the surface. Earthquakes have been objects of great superstitions
and awe throughout recorded history. The severe quakes wreak catastrophic
havoc in the human community because of destruction of structures
- houses and buildings, bridges, roads, railways and uprooting
of transmission towers. Death comes in a violent form - at times
to hundreds and thousands.
Earthquakes occur in all continents - in Mediterranean Europe,
North Africa, Central Asia, Indian sub-continent, East Asia, the
Americas and even part of Australia. Everything happens suddenly-without
warning. The first distressing factor is collapse of dwelling
units. Although measuring instruments at the Seismological Laboratories
are able to measure the geologic disturbances, nothing has been
invented that can forecast an "earthquake" as we understand it.
Human knowledge has yet to cross this frontier. But death and
destruction can be prevented or vastly minimised if the houses
are structurally sound. In a poor country such as India, which
is struck by severe earthquakes every now and then - the last
two major destructive ones bring in Uttarkashi region of Uttar
Pradesh and Latur district of Maharashtra - the
problem of appropriate 'safe' housing must receive adequate attention
from architects, engineer, builders, and owners of property.
India is tentatively divided into six seismic zones. Each place
finds itself in one of these six zones, which represent increasing
probabilities of earthquakes of hazardous magnitudes. This division,
however, does not rule out possibilities of major earthquakes
in "safe zones" also - for example Latur was in 'low risk' region
when it hit the headlines. In such cases only the map is modified,
thus bringing Latur from Zone 2 to Zone 5. Earthquakes being so
unpredictable and so dangerous necessitate precautions even in
so called "safe" zones.
are no earthquake-proof houses
misconception commonly shared is that some houses could be made
earthquake proof. The reality however is that by taking precautions,
the earthquake resistance of the house is increased finitely,
to make them resist quakes of specific magnitudes. These houses,
too, may fail once they face quakes having more intensity than
their design took care of. A recent earthquake in Japan could
cause so much destruction in an otherwise 'prepared' Japan because
the quake had a vast component of unprecedented magnitude thus
making most structures vulnerable.
The movement within
the earth's crust manifests itself in the form of waves that reach
earth's surface and cause vibrations in structures. The structures
fail and collapse under the action of these vibrations. These
vibrations may be in horizontal direction, in vertical direction
or combination of both, which generally is the case. The vertical
component of seismic force creates repeated changes in the weight
of structures while the horizontal component induces. These forces
get commanded in each cycle.
Houses collapse as a result of these inertial forces. Under the
action of earthquake the lower portions of the structures, which
are in direct contact with the ground, tend to vibrate more; whereas
due to inertia, the upper portions of the structures tend to remain
static. The resultant stresses build up fast with the increased
frequency of vibrations leading to failure of the structures.
Latur disaster came about when huge stone masonry walls gave way
under the earthquake, bringing down with them the roofs that were
overlaid with thick clay. The walls and roofs were heavy; the
masonry was badly made with round stones. The houses were not
'engineered' units, but more an assemblage of materials. The mud
mortar used was weak and could not provide enough cohesion to
sustain the walls on that fateful night. The huge mass of construction
material led to a massive disaster.
The magnitude of the inertial forces increase as directly as the
weight of the houses. The heavier the house, heavier is the destructive
force. One important consideration in making a house more earthquake
resistant is to reduce its mass and making it as light as possible.
poor man's houses in the tropics have mostly wattle and daub walls
with thatched roofs - these houses consume little material, and
are light by weight. The inherent limitations of the houses do
not permit higher storeys. Additional advantage these houses afford
is that due to their lightness, the houses, even if they collapse,
may not kill people.
way of improving earthquake resistance is by way of making the
houses rigid, viz, making sure that the houses vibrate together
as one unit. This prevents unnecessary absorption of energy by
the structural members and improves the quake resistance of buildings.
Schematically, in such houses the top portions of the houses are
so joined to the bottom that all movements are transferred immediately
from lower levels to the entire building and the entire house
vibrates as one rigid body. Consequently no disharmonious stresses
are set up and the house remains safe.
bamboo construction in North-East India follows this principle
of Rigid house. Bamboo being
light gives added natural advantage. In these houses rigidity
is achieved by means of cross bracing and triangulation. All joints
are strengthened by means of cross members that can transmit earthquake
forces directly to the remaining portions of the house.
concrete 'frames' are rigid by design and their rigidity can be
improved further by small increases in steel used. The 'framework'
of such beams and columns can be made to resist earthquake-induced
vibrations of considerable magnitudes. Almost all the multi-storeyed
buildings in towns and cities are framed, thus
the structure finds great support. It must be noted here that
though the RCC frames take care of major portions of earthquake
induced forces, the forces generated in the non-load bearing walls
could still lead to damages byway of wall collapses. In places
like rural India where concrete technology has little reach, masonry
in brick and / or in stone takes major share in the form of load
strength of masonry depends to a great extent on the strength
of the mortar joining them to distribute the earthquake-induced
forces equally throughout. One modern method 'Foamed Concrete'
construction combines principles of structural masonry together
with the advantages of light-weight foam concrete.
Foam concrete is prepared by aerating the concrete profusely as
it is mixed to create a highly fluffy, lightweight substance.
Building blocks and units made out of this material lead however
to centralisation and are subject presently to patent laws.
when properly constructed, possesses good resistance to seismic
forces. Bad construction, however, means disastrous consequences.
Improving the rigidity of load bearing masonry by way of three
RCC bands one each at plinth, lintel and roof levels, is
the most common technique employed in a country's earthquake resistant
housing programmes today. These houses, when constructed under
supervision can resist earthquakes of moderate intensities quite
better resistance to earthquake today involves access to steel
and cement concrete if not to ultra modern aerated concrete blocks.
There have been several attempts at local levels to make use of
bamboo and mud instead. These attempts
have not all stood the test of time; most of them have not got
the chance to face further quakes of high magnitudes - but these
attempts have always met with criticism by the scientific fraternity.
In our rural housing, local materials have to play an important
role. Even poor man's materials can be used to provide extra strength
to a dwelling unit with incorporation of a few simple engineering
principles. Total safety can not be assured even by use of high-technology
- but all dwelling houses, big and small, can be made safer.