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CATENARY – AN ARCH LIKE NO OTHER

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Versatility, affordability and ease of shaping bricks to the required size are its main features.

Every visitor to London knows of St. Paul’s Cathedral, but might not have thought how the dome is standing. Barcelona is synonymous with the Church of the Sagrada Familia by architect Antoni Gaudi but very few know what ensures it stands tall. They both use the principles of catenary arch, a rarely used but phenomenally unique concept in construction. Simply stated, if we were to hang a chain, it would hang loose in some curve to stay stable. If we draw up this curvature and reverse it to make an arch along the same profile, that arch will be stable with no other support. That’s precisely what masons of builder Arunkumar got done at Adivaram in Salem.

Once understood how to build, catenary arches are very simple, as has been proved at Auroville. Though the hanging chain has been a popular imagery, it is better to make a large scale template at site to get the arch profile as precisely as possible. Though mathematical explorations for this curve started from the 1600s, marking it physically at site is still the best with our local construction teams.

No single key stone appears at top, with the small curved profile there. With certain parts appearing as if straight, catenary does not have the same radius of curvature all along, but it varies. The first brick resting on ground is laid normal, slowly curving in later, so the load transfer finally happens at right angle to ground.

Contrast

Even for heavy loads, the top can be thin with thicker base, as such, it takes lesser materials to take the same load compared to segmental or semi-circular arch. To that end, catenary arch has its height or rise more than the span or the width. In contrast, most other arches have their span more than the rise. Centring support can come from a pile of dry masonry, wooden template or a moving framework.

Catenary can start right from the ground, making it easier for the mason to build it up, also enabling better head room heights especially in doorways. Of course, fixing door frames needs extra precaution. While it is very strong along the curve transferring the load, it may crack if any lateral load or side thrust is applied on it as it may happen while drilling for fixing and grouting door frame.

In principle, catenary arches become very handy and contextual where many other arches fail to fit. Being a rare sight, Europe often employed this typology as entrances for public buildings, making them appear grand.

While the visual reasons are still valid, what makes it more appropriate today are its versatility, affordability and ease of shaping the bricks to the required size. In an extended form, catenary vaults create rare interior spaces, while offering a real sight to the eyes. It’s time they start re-appearing in Indian buildings.

Arching, or upturning the curve

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The early arches were long poles bent with two ends fixed deep into the ground, series of which gave a perfect shelter.

The day early humans observed thin blades of grass curving down, may be with dew drops still wet and shining in the morning sun-rays, the discovery of arches must have begun. Cobwebs so commonly found then would have been hanging down in a curve, making people think of upturning the curve. There, the arch would have been discovered.

The early arches were long poles bent with two ends fixed deep into the ground, series of which gave a perfect shelter with what we call today as a vaulted roof. Incidentally, even now rural and poor people create make shift shelters in this manner! Arch starts right from the ground up.

Many kinds

However, majority of the later arches were erected upon certain height of the wall, called springing point which could be above the average human height, to avoid someone at the edge of arch from hitting the head into the arch. With such side supports and no more limits to height, there evolved semi-circular, elliptical, segment, pointed, multi foliated and even flat arch.

Among these, a special type is called centenary arch, which is among the very few arch types which start right from the floor like pointed arch, horse shoe arch and such others. With not much of side wall support or springing point needed, it can be erected in a smaller space, still enabling undisturbed movement of people.

European Gothic churches had evolved a system called pointed gothic arches, which is behaviorally different from the catenary curve. Incidentally, many churches of those days employed both the types, in the process popularizing the latter. The gently curving profile of catenary gets generated because of the very method of generating the curve.

When a chain is hung holding on to two ends, it hangs on a curved fashion to stay with equilibrium. Reversing the curve also maintains the equilibrium, giving us a profile that is stable and is called the catenary curve. The geometry of the curve is traced and repeated in building the arch.

Load transfer

Structurally, the load gets transferred in a catenary arch along the curvature and reaches the ground. No lateral buttress, side support and wall below is mandatory. As such, the upper parts can be thinner and lower parts thicker to take greater load or can be uniformly thin all along in case of nominal loads. However, if there is substantial weight to be supported, the wall parts edging the arch at bottom need to be wide enough to take the load.

Catenary arches tend to appear like an inverted English V with no sharp turn. Being a self- supporting profile, that too from the ground up, it creates a unique aesthetic statement. Considering their possible fit into narrow spans, they make excellent option for entrance doors without compromising on movement areas.

That’s precisely what made Sanjay choose this type of arch for his school building at Salem!

Advantage Arches

The indoor spaces and outdoor styles created by arches are unparalleled both in history and contemporary designs.

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The most fascinating chapter in the history of architecture could be to find how humans managed to keep the roof up there. The idea of the shelter must have started then.

It is believed that the earliest method of forming space could be by watching two branches kept inclined to each other supporting each other. So multiple such tree branches inclined together could form a secured space between them, creating a cone-shaped hut. Next four branches kept vertical with horizontal members on top could have created a flat-roofed structure, but this would still be a small space considering the kind of branches one could get – not always straight, often bending in the centre, roof collapsing from the edge and such other mishaps happening.

All this would have changed with brick making, among the earliest technologies discovered by humans, shaping it with sun drying. Soon wall making complete with varied openings like doors, windows and perforations would have followed. One of the challenges was to support the wall part above the openings. That’s when, possibly, arches were discovered.

Arches transfer the wall load by compression, i.e. loads move from one member to another by pushing them vertically or diagonally, and not by tension where they move horizontally too like in a beam. How humans learnt about the strength of curves could be an exciting research topic, for there were very few naturally curved objects to learn from. It’s possible, a playful curving of a twig could have led to trying out a wall opening with a curved profile, creating the first arches in history.

In comparison to horizontal opening topped with flat lintel, the arched top offers a multitude of benefits. Its profile changes with every variation in width and height, with a new look every time. There can be dozens of arch types, while flat lintel is flat forever. Aesthetic theories associated with arches far outweigh those with horizontal ones. The indoor spaces and outdoor styles created by arches are unparalleled both in history and contemporary designs. Of course, along with advantages, also come challenges. If built upon a support system, its strength is unknown until the supports are removed. To construct the arch without such support takes some skill. A basic knowledge of geometry applied to construction is necessary to plan them out, depending upon the span, rise, springing point and keystone on top. The joint between the arch edge and wall needs to be well thought out. Based on the width of opening and load from above, arches may be in one or multiple courses. They also pose problems in fixing frames and shutters, including for windows or doors.

Despite challenges faced, arches continued to rule the world of architecture. When masonry construction declined, being replaced by frame systems, arches were in reduced demand. Now we are realising that framed buildings, especially those in RCC, have higher embodied energy, hence less sustainable. It’s time to revisit arches

Arch as a foundation

With earth below, the arch would be even more stable and hence can take greater load, supporting the building walls. 

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It is interesting to note how we tend to forget what we learn. Just leaf through an old book of medicine, construction, cooking or stitching, documenting certain ideas. Only after proving itself, that idea would have been codified, written about and became part of the book. As such we may assume the written idea is a proven idea.

Let us check how many of the ideas explained in the texts are followed now. Maybe a handful or little more, not because the rest of the ideas have failed, they have been either replaced by newer ideas or have simply been forgotten.

The art of constructing a building foundation using arches is one among such ideas – simply forgotten. We continue making arches above ground but why not below ground? With earth below, the arch would be even more stable hence can take the greater load, supporting the building walls.

There are two methods to do an arch foundation – if the full foundation would be underground, the foundation trench is dug directly with arch profiles with adequate spacing between them to build foundation piers to spring the arches. This space where arches start would need a normal footing with enough width to take on two arches. The unexcavated earth stays underneath the arch as if that’s the centring support.

If the site is lower than the road, we dig only to erect the foundation piers, taking them deep enough to reach hard soil. From the top of piers which is at the site level, we can start the arches, which should be within 5 to 8 feet span for general safety. In case of uneven sites, the rise of arches can also be varied to suit the context. Arch centring is generated by filling in mud in the required profile, where using an M.S. template ensures the curved profile is perfectly suited to take the arch action.

The curved mud profile is topped with water mud mix and manually consolidated. If the top mud layer appears very brittle, a thin cement or cement-stabilised mud mortar can be applied, ensure there is a level base layer below the arch.

Now, stone or brick arches are built just like we do in any wall and cured well. Plinth beam is placed above the arches and backfilling the earth up to plinth level completes the foundation work of the building.

Architect Rajesh Jain from R-LEEF has been reviving this forgotten technology for many years now, improving it from its textbook days. Of course, it needs an engineer’s supervision, masons with arch construction skills and contractors with inclinations to explore. If the team is not interested in exploring, the idea may fail due to a badly done job.

Imagine a small plot with a dozen RCC columns and next to it a dozen arches – both take the load. To live a green future, we need to revive older ideas

Practice makes perfect

It is time arches get revived, before the skills vanish completely.

archIt goes unnoticed that a large number of people actually love arches and may like to see them in their homes and offices, but end up without one. The blame may lie upon the architects and engineers who hesitate to build them for fear of defects and mistakes. The mistakes could be blamed upon the masons, who had no chance to learn about arches with no proper instruction, ending up spoiling it all. While the trend of senior masons teaching the juniors has diluted, there are not many centres like Auroville, where one gets exposed to constructing arches.

However by studied observation, basic knowledge of arch action and sincere willingness, anyone can learn to build a simple arch, which can then be perfected across time. The critical point about an arch is not in the final product, but in the design process and in the making of the arch. Firstly, one needs to finalise the shape, be it semi-circular or segmental, depending upon the clear width, the rise and the strength of the side supports possible. This shape should fit well within the springing point at the bottom and key stone at the top.

The arch profile should be marked on the ground on actual scale using a thread as a compass, from the centre point of the arch, and double checked for accuracy. The central line should be drawn and the key stone marked, such that rest of blocks can be fitted equally on the either sides of the key stone. With the same thread, each stone or brick should also be marked, keeping the size of the said masonry block in mind. Now, each individual block should come in a tapering shape, all blocks should be in full size and the first block near the springing point clearly sized.

Dummy

Now a dummy arch with open joints can be placed on the ground for final verification. This done, each block can be lifted piece by piece, building the arch on the wall, upon the temporary centering support provided. Unskilled masons also use a template of the curve, made from plywood, holding it across the arch under construction to ensure the profile is maintained. Over the time, skilled masons may do arches without such elaborate preparatory procedure.

Though all arches are curved, the specific type of arch may vary – with one or more layers, bricks projecting out, different placement patterns, mix of materials, pointed arches, twisted profiles, arches overlapping on each other and such others.

Providing just one arch in a building may appear out of place, hence using arches as a repetitive design element may be more pleasing, but it is important that the owners should appreciate arch as an idea, before trying them out. A flat RCC lintel comes with no choice, but arches come with dozens of variations, often at lower prices and different aesthetics. It is time arches get revived, before the skill levels vanish completely.

Building an arch

The earliest brick arches were built possibly around the Mesopotamian civilisation, more than 3,000 years ago. Both abroad and in India, we still see arches standing tall, often bereft of the roofs, open to rain and sun, yet lasting for centuries.

19HAB-greensens_19_1851562gIt is strange to see how the horizontal and straight line has come to dominate building construction, especially in lintels, beams and roofs. Most of us may not know that early humans did not create shelters only with the horizontal, instead built sloping roofs, conical forms, domed profiles, arched windows, corbelled projections, vaulted shelters and a range of such forms where there were ideas beyond the horizontal and this continued until a few hundred years ago. Of course there are region-specific exceptions where long timber members or stone lengths were available; however the idea of horizontal lintel and beam has been gaining universal application only for a few centuries.

The simple example to discuss the effectiveness of a curved lintel is to hold a book horizontal and place a weight in the centre – of course it will bend down in the centre, or deflect, as a technical person may explain. If it is held in a curved profile, the same book will take the weight.

Depending upon the type of curve, masonry material, construction skills, strengths of the side walls, columns holding the arch and such others, the clear width of the arch gets decided by expert masons and engineers. The load gets transferred to the sides, enabling a support-free space under the arch.

Free standing

The earliest brick arches were built possibly around the Mesopotamian civilization, more than 3,000 years ago. Both abroad and in India, we still see arches standing tall, often bereft of the roofs, open to rain and sun, yet lasting for centuries. Arch is among the few forms around us that can be free standing unlike a vertical post and horizontal beam which may fall sideways. Hence many gates, bridges and openings in forts were arch shaped.

Unbelievable but true – many hundreds of years ago, many masonry bridges in stone and bricks were built with many hundreds of feet clear width!

Arch performs under a concept termed as load transfer by compressive method, where the weight of the building comes on the arch, trying to push it down, but ends up moving along the curved profile to the edges, creating a side thrust there. Thicker the arch and stronger the side wall, the wider can be the arch with greater load. Accordingly, people in the past used very thick walls; however, with the advent of modern technologies today an arch can be built with steel and cement spanning across thousands of feet.

Most Indian buildings predominantly use masonry materials even today, though concrete and steel buildings are making inroads mainly for larger complexes. With the load bearing wall anyway in place, it makes tremendous sense to support the roof over an arch. Even in a house with RCC framed system, arches can be judiciously introduced at appropriate places to reduce the cost. We only need to re-consider it as a viable option.