If you had travelled in South India –especially in the provinces of Tamilnadu- Karnataka-Andhra, you would have come across several huge temples which date back to thousands of years- literally steeped in antiquity. A thousand years is a pretty long period by any reckoning. But this long age doesn’t tell on the structures- except some damages, that too primarily owing to lack of maintenance, these temples have withstood several earthquakes, floods, tempests and such natural perils, taking all these in their stride.
One of the main distinguishing features of the old temples, is their ability to withstand seismic events without any major visible signs of distress.
How has this been possible?
When we say temples, we mean the gopuram structures which are arranged in an increasingly descending height as we walk into the temple and move towards the garbagraha, the sanctum sanctorum.
For example, the official emblem of Government of Tamilnadu, is the gopuram of Srivilliputhur Andal temple; it towers upto a height of 193.5 ft; of course we have taller gopurams like Sri Ranganathaswamy Temple in Sirrangam at 239.5 ft and Murdeeshwara Temple at Karnataka which goes upto a height of 249 ft.
SRIVILLIPUTHUR ANDAL TEMPLE - 193.5 FT
Sri Ranganathaswamy Temple in Sirrangam - 239.5 ft
Murdeeshwara Temple at Karnataka - 249 ft
Of course many modern sky scrapers have achieved record breaking heights; for example, Burj Khalifa in Dubai, the tallest building in the globe today, is a whopping 2722 ft high. So we are not analyzing the old vs new equations here. The idea is to highlight the basic principles which have ensured the ancient structures survived the vicissitudes of time.
The general geometry of south Indian gopuram architecture is that it has a wide, broad and heavy base and it gradually tapers to progressively reducing sized tiers upwards.
An earthquake in simple terms is violent shaking of ground beneath a building; this exerts brief but powerful vibrations upon the structure trying to push it upwards and side wards on both perpendicular sides simultaneously. An analogy is the flour sieve which we use at homes. When we use it the normal way, you will see the flour and stones inside getting tossed around. Imagine a building being subject to such a force, but a million times more intense.
The extent of seismic force that acts upon a structure is proportional to its mass and how far that mass is away from the base; in other words how the mass is distributed over its height (analogy- bigger the person bigger his worries are and bigger will be the problems he would encounter).
As per this principle if the building has a heavy mass at higher levels, these will be subject to violent forces; so due to proportionally high level of vibrations, the top portions will tend to wobble heavily; this will create huge stresses which will create serious levels of inequilibrium that will either cause serious distress or might topple the entire structure.
Instead, if the mass is concentrated at low levels as in our temples, when the earth beneath shakes in an earthquake, given the regular geometry of the temple structure and heavy base, the base absorbs the maximum amount of shocks exerted as it has the inbuilt capacity and is closer to the earth and is substantially retained by earth all around. So it does not undergo much vibration sideways. When the base remains relatively stable and top portions are relatively lighter, the vibrations are well contained; the whole structure retains its equilibrium except perhaps some minor damages.
Another important aspect of temple construction is the symmetry of layout. If there are no sudden projections from the main frame which are not symmetrical on all sides, the vibrations in a building will be much less. Hence our temple builders made the structures quite symmetrical.
As an analogy, imagine you standing with both your feet pressed against each other and carrying a heavy bag on your head; if you are given a slight push near your shoulder, your chances of tumbling down or at-least swaying sideways is high; but if you keep your feet little apart and there is no weight over you, you would find you have gained better stability against lateral push.
Why is it so?
It is because of a factor called moment of inertia and the concentration of mass near the base. Moment of inertia goes up exponentially with the increase in dimensions of the base. Given the shape, that is reducing size of the tiers upwards, the mass gets concentrated in the lower levels. These two fundamental principles ensure the structure does not undergo unduly excessive vibrations in a seismic event.
So any earth quake can only create a whimper of stress in a gopuram type structure!!!
Now let’s see an interesting analogy!!
In human life, if your base – that is to say, knowledge in your chosen field, knowledge of history and society, cultural heft, familial relationships, friendships, faith in a superior force or let’s say a strong self-belief, good human values, generous and forgiving nature, philosophical attitude, other hobbies and interests and such positive things - is wide and broad, you stay well rooted to the ground. If any disturbance or sudden non-linearity in life (which is guaranteed for everyone), were to strike such a person, he would be in a much better position to take such shocks, as, such a wholesome personality can handle it without any serious long term damage and be resilient.
Coming back to the structures, in the present day residential and commercial buildings which are high-rises where we cannot have gopuram like structures routinely due to cost of space, there are precise modelling methods by which we estimate the forces likely from earthquakes of predictable intensity. The whole structure is well connected along the three axes and in some cases there are precisely made shock absorber kind of systems which ensure the structure remains safe. If such modern buildings collapse during earth quakes, it could be more due to failure to do proper design and construct to a proper plan or an earth quake of unexpectedly high intensity.
Dated 13th October 2016
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