Monthly Archives: March 2019

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