India is one of the countries that suffers a great deal of natural disasters ranging from Earthquakes, Tsunamis, cyclones, floods, drought etc. These disasters have ravaged different parts of India at different times causing untold fatalities. Different preventive measures have been explored and adopted to minimize damage resulting from these disasters. In this article, we explore the built vernacular heritage of India especially in disaster prone areas and how these have helped minimize damage and withstand such disasters.
It is understood that 55% of India is prone to seismic shock (www.infinityfoundation.com/mandala/t_es/t_es_agraw_quake.htm). There is a saying that says, “Earthquakes do not kill, but buildings do”. As one of the ways of reducing the fatalities caused by these earthquakes, many regions of India have traditionally built houses largely built from wood, thatch, bamboos and clay. These kinds of buildings have the capacity to better withstand the earthquakes compared to other structures like those made of concrete.
For example, despite a history of earthquakes in the central Himalaya region of India, the Sumer and Chaukhat are some of the notable multistorey buildings in the Garhwal Himalayas-a highly seismic prone area in the Central Himalaya region. These structures are made of thick wooden logs, stones and clay and have withstood repeated occurrences of earthquakes including the famous ones of 1991 and 1999 that hit this area so hard
Despite being there for hundreds of years and standing up to about 17 meters high with 5 to 6 floors of 4 rooms each, the Sumers survive series of high magnitude earthquakes than many heavy masonry and concrete structures. A typical Sumer stands on a foundation trench of about 3 meters deep and 70cm wide. Its walls are basically erected using stones and wooden logs.
Likewise, according to http://nidm.gov.in/idmc2/pdf/abstracts/eq-a1.pdf , the North eastern parts of India are highly prone to earthquakes. According to seismic map of India (IS1893-2002), this region lies in the subduction zone and it has been hit by two great earthquakes namely; the Shillong (magnitude of 8.6) and the Assam (magnitude 8.7). Earthquakes keep hitting this region and people have adopted certain vernacular building styles that have gone a long way in minimizing the intensity of damage caused by these seismic activities.
The Assan type house and the thatch house are the housing typologies which are rife in North east India. They are basically built with light weight locally available materials like bamboos, wooden planks and thatch. These materials coupled with good building configurations i.e. its shape in plan and elevation, number of openings, location and connection details between individual elements have contributed a great deal to their resilience to earthquakes.
Since these vernacular structures like the Sumers, Assan type house and the thatch house are constructed with locally available resources (like wood, clay, bamboos, thatch and stones), these structures tend to be suitable for use in earthquake prone areas.
However, the question is, “What attributes of wood, thatch and bamboos make vernacular structures resist high magnitude seismic waves better than other structures?”
The following are some of the advantages of wood in vernacular heritage especially in earthquake prone areas of India and the world at large according to http://cenews.com/article/9036/wood-frame-construction-advantageous-in-areas-prone-to-seismic-activity
Firstly, wood is light. Most earthquake damage is caused by seismic waves that force the ground to move. When the ground motion is strong enough, it causes the building’s foundation to shake. Heavy steel and concrete structures experience greater forces than wood-frame which has higher strength to weight ratio. This makes vernacular heritage ideal in earthquake prone regions.
Secondly, wood is somehow ductile. Wood-frame structures have numerous nailed connections and joints. This provides inherent ductility compared to rigid masonry and concrete systems. Wood-frame can flex, thereby absorbing and dissipating energy when subjected to sudden earthquake forces.
Thirdly, the sheathing and finishes attached to wood joists and studs provide redundant load paths for earthquake forces. These numerous small connections and load paths dissipate seismic forces. Should some connections be laden or fail, neighboring connections will usually provide alternate load paths and help avoid ruin. This is usually absent in heavy masonry and concrete structure and they thus easily get ravaged.
Lastly but surely not least, there is usually high connectivity of individual parts in wood-frame structures. This connectivity enhances firmness as the structure stands as one solid unit. As a result, wood-frame structures tend to resist earthquake forces more effectively than other systems.
These characteristics of wood makes vernacular heritage suitable for use in disaster prone areas and that explains their prevalence. The fact that wood is relatively cheap and easily accessible than other materials adds to the advantages.