CATENARY – AN ARCH LIKE NO OTHER
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.
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.