In this blog, there are two variations of drawing arches (arcs of circles) from 1 centre.
The first and simplest arch was the design used commonly by the Romans....The semi circular arch- so named for obvious reasons
The stone at the top of the arch is called the key stone. The other stones that create the arch are called the voussoirs. By ensuring these are cut so their edges are 90 degrees to arcs of the semi circle, maximum strength can be obtained. The weight above the voussoirs force them together and so create the structural strength of the arch from the pressure or compression exerted on these stones. Where the arch meets the columns on which they sit, buttresses and other techniques were used to maintain or balance the forces within the arch.
Clearly on a semi circular design, the rise of the arch is required to be half the distance between the supporting pillars. In order to be able to construct arches with a lower rise, by dropping the centre of the circle below the level of the top of the supporting pillars, a segmental arch was invented. The diagram below shows a segmental arch
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