Experimenting with building design and construction precepts the people living in earthquake prone Himalayan region developed a construction algorithm with five distinct principles that ensured safety of structures during earthquake incidences.
Ongoing tectonic movements, ensuing subduction of the Indian Plate beneath the Eurasian Plate, and their consequent collision resulted in the evolution of the Himalayan mountain belt. The Indian Plate however continues to drift north-northeast, and the stresses accumulated in the process make the Hiamalayan region highly vulnerable to seismic tremors that are sometimes particularly devastating; Mw~7.5 September 1, 1803 Garhwal Earthquake, Mw~8.0 June 12, 1897 Shillong Earthquake, Mw~7.8 April 4, 1905 Kangara Earthquake, Mw~8.2 January 15, 1934 Bihar–Nepal Earthquake, Mw~8.6 August 15, 1950 Assam now Arunachal Earthquake, Mw~7.6 October 8, 2005 Kashmir Earthquake, Mw~7.8 April 25, 2015 Gorkha Earthquake, and Mw~7.3 May 12, 2015 Dolkha Earthquake.
The ground shaking during an earthquake induces instability in the structures, and these are often damaged and destroyed. The instability of the structures is a function of their height, as with increase in height Center of Gravity and Center of Mass of the structures move up, and away from the ground surface. Taller structures therefore have higher chance of getting razed to the ground.
In this background it is normal to expect low rise houses in seismically active regions, but then the Himalayan region despite being highly prone to earthquakes has a tradition of multistoried houses, and except for cowsheds (chani in local parlance) a single storied traditional house is often hard to come by. Moreover, four to five storied houses are commonplace in many areas. To add to it the local dialects of the region have distinct words to identify different floors of the house – ground floor, goth; first floor, chaak; second floor, paan; third floor, chaj in Kumaoni, and ground floor, kholi; first floor, manjua; second floor, baund; third floor, baraur in Garhwali. This stands testimony to common occurrence of multistoried houses as a language does not generally assimilate words unless these are required often.
These houses are at the same time quite old, and have withstood the ground shaking during previous major earthquakes. Radiocarbon (C14) dating of the wood utilised in the construction of two prototype multistoried houses at Koti and Guna villages of Uttarkashi district in Yamuna valley of Uttarakhand done at Birbal Sahani Institute of Palaeobotany, Lucknow suggests that these were built 880+90 and 728+60 years before present; i.e. 668–970 years before present, and since then these houses would have certainly witnessed many devastating earthquakes that include Mw~7.5 September 1, 1803 Garhwal Earthquake that caused damage as far as Delhi, Aligarh and Mathura. But for special measures taken for ensuring their safety, all these houses would not have survived.
Earthquake safe construction
The philosophy of earthquake safety revolves around the saying, “Earthquakes don’t kill, buildings do.” It is a reality that of the 25.8 lakh persons killed due to earthquakes around the globe in previous 120 years and many more altogether, not even one has been killed by earthquake induced ground shaking. All these persons have in fact been killed due to the collapse of structures as also other related incidences such as tsunami, landslide and the like. Constructing structures that can withstand ground shaking during an earthquake thus remains the key to ensuring seismic safety.
Based on experience, and accumulated knowledge of generations the people living in the earthquake prone Himalayan region understood this basic premise of earthquake safety, and developed a construction algorithm that ensured safety of structures from earthquakes. It is the merit of this algorithm that multistoried houses of the region could survive several high magnitude earthquakes.
It is important to note that the people of the region preferred higher elevations over in situ rocks for habitation, despite agriculture and water both being located in middle and lower slopes. Apart from landslides and floods, this provided safety from earthquakes as the impedance of the seismic waves over in situ hard rocks reduced ground shaking, and ensuing losses. This fact was revealed by Greek philosopher and polymath Aristotle (384-322 BC), and there is no reason for us to doubt if these people were aware of this fact.
Settling down over firm ground alone was however no guarantee of safety, particularly during intense ground shaking. So inspired by the zeal to protect the community these people experimented with locally available building material and evolved a unique architectural style that exhibits structural evolution trends whereby dry stone masonry, as also stone–lime/mud/clay mortar masonry was judiciously used with abundantly available wood to provide appropriate strength, and flexibility to the structures.
The design and construction algorithm devised by these people for ensuring seismic safety of structures comprises of five principles.
Habitation at right place
Site selection is an important aspect of the structural safety of buildings, and other structures. Even today suitability of the site for siting of structures is assured through geo-technical investigations that include assessment of bearing capacity of the soil. The inhabitants of the region resorted to something similar wherein advice of Soil Examiners was sought before finalising the site.
Based upon experience, and accumulated knowledge of generations the Soil Examiners had mastered the art of assessing the suitability of a site for construction purposes for which they developed an objective algorithm wherein physical properties of the soil of the proposed site were correlated with the standard developed by these persons. Apart from intuition these persons based their assessment on soil texture, composition, odour, colour, density, porosity, moisture and presence of humus.
Elaborate rules were put in place for collecting soil from the proposed site, and advice of the Soil Examiners was religiously adhered to. The Soil Examiners at the same time commanded respect of the community, and their advice was also sought on various secular matters concerning the community.
This is still in practice, particularly in remote areas. With changed ground realities the common man is however not left with many options to choose from for constructing a house. Moreover, convenience, comfort, ease of access, aesthetics, and commercial opportunities are often given priority over safety which is often not accorded due attention. The traditional practice of site selection is therefore slowly losing ground.
Assessment of site suitability is however akin to pulse examination (Ayurveda Nadi-Pariksha) practice wherein pulse signals sensed at radial artery on the wrist below the thumb are utilised for diagnosing the ailment. This is widely practiced in India even today despite all sophisticated diagnostic tools. One therefore doesn’t have logic in outright rejecting this age old practice of site selection.
Research on this important aspect promises to provide a cost effective, convenient, and easy method of site selection, particularly for small structures. It is therefore recommended that finer details of this dying art of the region be documented in detail. Effort at the same time be made for scientific validation of these precepts.
Foundation bears entire load of the structure, and is another important component of the structural safety of the buildings and other structures. Particularly deep foundation was generally resorted to for providing stability to the structures. As a rule the foundation was dug until firm ground, in situ rocks or large exotic boulders (dal in local parlance) were encountered.
Unlike present times construction did not commence soon after digging the foundation. The foundation was traditionally left exposed for some rainy seasons which ensured ground settlement and kept the structures free of settlement cracks that are becoming increasingly common in present day constructions.
The layout and design of foundation becomes all the more important in tall or high rise buildings. It is a common knowledge that Centre of Gravity and Centre of Mass of an object moves upwards and away from the ground surface with increase in its height, and infuses instability in the object which tends to topple it down even with slightest application of force.
Structural engineers often utilise the design of foundation to overcome this problem in high rise buildings. Thick concrete slab resting on a large area of soil reinforced with steel, generally referred to as raft or mat foundation is therefore provided at the base of high rise structures. Besides redistributing the load of the building over a large area the weight of the raft at the base of the buildings keeps the Centre of Gravity, and Centre of Mass of the building close to the ground.
The people of the region understood the character of forces acting on particularly tall structures, and measures to enhance stability of these. Tall buildings were therefore invariably constructed over raised, and elaborate stone filled solid platforms that were the continuation of filled in foundation trench above the ground. In case of in situ rock being exposed the platform was raised directly over it. The height of such platforms constructed using dry stone masonry were observed to vary between 6 and 12 feet above the ground.
Massive solid platform at the base of the structure helped in keeping the Centre of Gravity, and Centre of Mass in close proximity, and close to the ground, which minimised overturning effect during seismic loading.
Simple, and symmetric buildings are often considered as being safe during an earthquake incidence. The traditional buildings of the region were observed to be constructed on a simple rectangular plan with the length and width varying between 4 and 8 meters, and the ratio of the two sides varying between 1.1 and 1.4. Height of the buildings above the platform was also observed to be traditionally restricted to double the length of the shorter side.
This is in keeping with the provisions of the building codes that prescribe buildings with a simple rectangular plan, and symmetrical both with respect to mass and rigidity, so as to minimise torsion and stress concentration.
Openings cause weakness in the walls, and additional reinforcement is therefore resorted to for overcoming the same. All the traditional houses were observed to have a single small entry, and relatively small openings. Strong wooden empanelment was at the same time provided around all the openings to compensate for the loss of strength.
Besides ensuring seismic safety, small openings also helped in energy conservation that was an added advantage in higher altitudes. The traditional houses of the region were thus solar passive and energy efficient. As communicated by the local people these houses remain cool during summers while conserving heat during the winters.
Joints between different building elements play an important role in ensuring safety of buildings during earthquake shaking. The people of the region utilised both nailed, and housed joints to put together wooden components while integrity of the walls was ensured by the use of specially carved out through-stones, and corner stones that ensured that the entire wall behaves as a unit and the adjacent walls do not fall apart during seismic shaking.
All windows, doorways, ventilators and floor-joists in the traditional houses were joined to the wooden logs incorporated in the walls, which provided additional safety to these structures.
Experimenting with the precepts of seismic safety the people of this region mastered the art of meticulously using wood and stone pieces of different shapes and sizes for the construction of the walls of the houses so as to improve their seismic performance.
Provision of wooden beams was generally provided in all the traditional structures but the art of raising walls of the multistoried buildings was particularly elaborate. These were raised by placing double wooden logs horizontally on the edge of the two parallel sides of the platform with thickness of the walls being determined by the width of the logs. The other two walls were raised with well-dressed flat stones to the surface level of the logs placed on the other two sides. The walls were further raised by placing heavy, flat, dressed stones upon the wooden logs on the two sides, and by placing another pair of wooden logs upon the stones on the other two opposite sides.
The four walls of the structure were thus raised using wooden logs, and dressed up flat stones alternately. The structure was further reinforced with the help of wooden beams fixed alternately and running from the middle of the walls of one side to the other, intersecting at the center. This arrangement divided the structure into four parts, and provided for joists supporting the floorboards in each floor of the building.
People of the region believe that the wooden frame of the traditional multistoried houses was finalised before filling up of the intervening voids with dressed stone pieces; similar to present day frame structures.
On the two sides of the multistoried structure wooden beams were often provided from outside. These beams inserted from above are identified as being shear keys that enhanced seismic performance of these structures.
The people of this region paid special attention to the construction of high rise buildings and salient features of the technology used was also used in other constructions.
The people of the region judiciously utilised the locally available building material. Being strong yet lightweight, wood offered distinct advantage as a structural material and enhanced seismic performance of the buildings as ground accelerations could not generate as much energy in wooden buildings as in other buildings. As an added advantage, wood-frame systems flexed more than other materials, thus absorbing and dissipating energy.
It is based on the strength of these earthquake safety features developed by the people of the region that multistoried houses of the region have survived many earthquakes that include the one of 1803 that caused damage as far as Delhi, Lucknow Agra and Mathura.
Transmission of the acquired knowledge
After having suffered losses during earthquake incidences the people would have observed differential impact on the buildings around them; some would have been razed to ground while others would have been spared or lightly damaged. This would have led people to investigate causes of this differential impact. In depth scrutiny and grouping the buildings according to the nature of damages people would have led people to conclude that differences in design and construction precepts result in different impact during the earthquake incidence.
This was an important conclusion as this would have led people to discard certain design and construction precepts while promoting and improving upon others. This process would have eventually helped people master the art of earthquake safe construction.
All this would have however not been that simple and straight forward, and before perfecting the design people would have experimented with different prototypes or improvised designs. But then, the efficacy of these prototypes could only be validated during an earthquake event and like present times earthquakes would not have been a frequently occuring incidence.
Long recurrence interval therefore warranted detailed documentation of both, the design utilised for construction of the building and performance of the building during subsequent earthquake. Without this such an experiment would not have been possible.
Due to transmission losses doing so through oral tradition alone was not possible. No written record related to earthquake related losses or design characteristics of the buildings has also not been discovered so far. It is therefore difficult to comment precisely on the methodology adopted by these people for evolving the design of earthquake safe structures. They would have certainly resorted to some form of documentation that has still not been unearthed.