Designs of arches moved from one centre to 2 or more centre arches (ie the arcs that made up the arch originated from 2 or more centres). This article will explore 4 variations of arches based on 2 centres: The Equilateral Arch, The Obtuse Arch, The Acute Arch and the Segmental Arch.
Because of the aesthetic beauty of regular shapes, the first picture is that of an equilateral arch. The three points: two bases of arch and centre high point form the vertices of an equilateral triangle.
By moving the centres towards the inside of the arch, for the same width, the height of the arch can be reduced. The picture below shows an example of such an obtuse arch. Try experimenting by adjusting how far the centres can be moved towards each other. You can research also the limits placed on this by structural engineers. Perhaps someone can let me know if obtuse in this case means an angle of more than 60 degrees??? Unusual
For narrow passageways especially, the height of the centre needed to be relatively high compared to the width. By moving the centres outside of the base pillars, this could be achieved by what is called an acute arch. Acute Arches are also known as Lancet Arches.
The final example for this blog is a design based on lowering the centres of the circles below the top of the pillars. (See Arches 1 blog also) You can explore many variations but these were very commonly used when there was not a lot of space above the arch. For example bridges. These arches are called 2 centred segmental arches and this final picture shows an example. The less vertical the arch structure meets the pillars, the greater the need for buttresses or other means to support the lateral thrust of the structure.
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