Arching, or upturning the curve
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.
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.
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!