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Wrath of Vasuki - How do we cope?

Author: Narayan Sarma

This 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.

Earthquakes 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.

Presently 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.

There are no earthquake-proof houses
Another 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.

The 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.

Traditional 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.

Another 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.

The 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.

Reinforced 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 bearing walls.

The 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.

Masonry 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 efficiently.

Creating 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.

See also:
The art of building a "bhonga"
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