The mechanics of historic (South Indian) temple architecture: Stone masonry walls
In one of our previous blogs we had said that ‘historic buildings are constructed of natural materials and hence they behave differently’. In this blog we discuss ‘what is different’ through one example: stone masonry in construction of historic structures, particularly South Indian temples.
Typically in South Indian temple architecture stone masonry is constructed either as a single veneer or as a three veneer composite masonry. By single veneer I mean masonry with single layer of stone blocks in regular bonding. Composite masonry has inner and outer veneers in stone masonry and a core filled with rubble and mud or in few cases brick masonry in lime mortar.
In both types the important point to note is that the veneers are of dry stone masonry – meaning there is no mortar used in construction of the stone walls. Then how are the stones joined? How do they stay in place? Would the blocks not collapse with a strong push to the wall? What happens in case of earthquake? Well, these historic constructions have been standing there for centuries together this way. Then, what is the knowledge that enables the structure to stand? To answer this first one needs to understand few things; 1) construction system of the wall, and 2) physical characteristics of stone as a material.
Typically in South Indian temple architecture stone masonry is constructed either as a single veneer or as a three veneer composite masonry. By single veneer I mean masonry with single layer of stone blocks in regular bonding. Composite masonry has inner and outer veneers in stone masonry and a core filled with rubble and mud or in few cases brick masonry in lime mortar.
In both types the important point to note is that the veneers are of dry stone masonry – meaning there is no mortar used in construction of the stone walls. Then how are the stones joined? How do they stay in place? Would the blocks not collapse with a strong push to the wall? What happens in case of earthquake? Well, these historic constructions have been standing there for centuries together this way. Then, what is the knowledge that enables the structure to stand? To answer this first one needs to understand few things; 1) construction system of the wall, and 2) physical characteristics of stone as a material.
Dry masonry, box effect construction system
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The above might sound a bit technical but let us try and understand it together. One important factor that plays a role in stability of the masonry is the shape and size of the stone blocks. If most of the wall has large-sized stones the masonry wall performs well. In addition a large size stone enables a better interlocking between the orthogonal (right-angle) joints of the wall [1]. These orthogonal joints create a box-effect such that the structure can withstand horizontal movements (e.g. earth quake). When this box-effect is created the structure moves as a single mass. Requirement of mortar comes into picture only when stone blocks are smaller, where the interlocking behavior or a box-effect cannot be achieved for creating a monolithic structure which can sway as a single body of freedom. This was one of the driving factors for the use of lime mortar in medieval period buildings of the Deccan Sultanate or the ones in British administration when smaller size stone blocks were used.
Physical characteristics
Stone, more specifically granitic gneiss and schist (typically used in South Indian temples as structural masonry) were naturally formed billions of years ago under high pressure and temperature due to volcanic activity. Such stones naturally have a very high compressive strength (comparable to that of the more modern and contemporary period high strength concrete); meaning the strength required for the stone to fail or crush is much greater.
Coming back to temple walls and why they don’t fall when pushed (even when there is no mortar) when we try to crush a stone by applying load from one direction, there would be restraining force acting in the opposite direction. Meaning the stone uses some energy to resist the load so that it doesn’t collapse under the load – this resistance is high for stone. Lab experiments have been conducted to understand the failure of stone masonry (with mortar joints) under compression by studying its failure pattern[2]. The results showed that the stone blocks of the masonry wall remained intact while the wall failed at the bonding between stone blocks. This demonstrates that the typical purpose of mortars to bond stones into a solid mass is not essential and doesn’t play much of a role in historic stone masonry built with large stone blocks.
This also explains why when patch repairs of cracks along the mortar joints of such masonry walls (we often see such patch repairs on temple walls) are undertaken with mortar, either lime or cement, the problem is not solved and the cracks tend to re-appear. In fact, cement mortar repair worsens the problem because the stone wall is no longer able to relieve pressure buildup at the original crack location, it leads to fresh cracks elsewhere (for unlike lime cement forms a hard, brittle mortar–see our blog).
The other physical characteristic of stone as a natural material is its an-isotropic property, i.e. it has different properties in different directions, which plays an important role in its prolonged use or its failure. As the stone is naturally formed in layers or bands, it has different properties along its parallel and perpendicular bands. Generally stone has higher compressive strength when it is parallel bedded rather than when it is bedded perpendicularly. Building craftspeople knew this empirically and very rarely do we find a historic wall with perpendicularly bedded stone masonry. In fact such blocks are more likely to be later day insertions.
Coming back to temple walls and why they don’t fall when pushed (even when there is no mortar) when we try to crush a stone by applying load from one direction, there would be restraining force acting in the opposite direction. Meaning the stone uses some energy to resist the load so that it doesn’t collapse under the load – this resistance is high for stone. Lab experiments have been conducted to understand the failure of stone masonry (with mortar joints) under compression by studying its failure pattern[2]. The results showed that the stone blocks of the masonry wall remained intact while the wall failed at the bonding between stone blocks. This demonstrates that the typical purpose of mortars to bond stones into a solid mass is not essential and doesn’t play much of a role in historic stone masonry built with large stone blocks.
This also explains why when patch repairs of cracks along the mortar joints of such masonry walls (we often see such patch repairs on temple walls) are undertaken with mortar, either lime or cement, the problem is not solved and the cracks tend to re-appear. In fact, cement mortar repair worsens the problem because the stone wall is no longer able to relieve pressure buildup at the original crack location, it leads to fresh cracks elsewhere (for unlike lime cement forms a hard, brittle mortar–see our blog).
The other physical characteristic of stone as a natural material is its an-isotropic property, i.e. it has different properties in different directions, which plays an important role in its prolonged use or its failure. As the stone is naturally formed in layers or bands, it has different properties along its parallel and perpendicular bands. Generally stone has higher compressive strength when it is parallel bedded rather than when it is bedded perpendicularly. Building craftspeople knew this empirically and very rarely do we find a historic wall with perpendicularly bedded stone masonry. In fact such blocks are more likely to be later day insertions.
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| Temple at Pattadakal (observe the horizontal bands) |
A lab experiment demonstrated this traditional knowledge: With regards to the granitic gneiss found in Bangalore region it was found that when load is applied to a stone block in its parallel and perpendicular bands the results were 104.9MPa and 86.1MPa[3] meaning the compressive strength of the material was higher when the block was loaded parallel.
Knowledge of physical characteristics is extremely important when attempting restoration particularly in the case of sedimentary stones such as sandstone - the bedding direction in which the blocks are laid plays a vital role[4] due to the deteriorating nature of sandstone, which is enanced by prolonged exposure to atmosphere and loading. Though there is heavy deterioration in most stone blocks of the historic temples in Aihole and Pattadakal it has withstood the onslaught of time as the blocks were loaded in parallel bands (see image).
References
[1] Dipasquale, Letizia & Rovero, Luisa & Fratini, Fabio. (2016). Ancient stone masonry constructions. 10.1016/B978-0-08-100038-0.00011-1.
[2] K. Venu Madhava Rao, B. V. Venkatarama Reddy and K.S.Jagadish, Strength characteristics of stone masonry, Materials and Structures, (1997) 30 pp.233-237.
[3] K. Venu Madhava Rao, B. V. Venkatarama Reddy and K.S.Jagadish
[4] http://suvratk.blogspot.com/2020/01/sedimentary-structures-building-stones.html?m=1
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