Since my early days in architecture school, I have been fascinated with masonry and stone arches. Arches are a wonder of both simplicity and complexity. How seemingly impossible is it that a group of stones, joined together oftentimes without any adhesive, span a distance and form a load-bearing vaulted structure, defying gravity and withstanding the elements for thousands of years?
When my son Asher was three I built him a wooden arch and hoped that he would enjoy assembling it and perhaps wonder how this magical structure, in particular the keystone at its head, manages to hold on. The arch that I came up with resembles an arched bridge where the weight of each of its elements is transferred to the heavy base stones on the right and the left sides.
As for the keystone, when it is inserted it pushes downward but is arrested by the flanking stones that push against it.
Building the arch was easy. All that I needed was a short segment of scrap from a 2×4 board, a compass, a sliding bevel, and a ruler.
The arch is made by drawing two arcs. A shorter arc (intrados) forms the arch’s inner opening, and above it, a broader arc, made by splaying the compasses legs to a longer radius and setting the new center to a much lower position than the center of the first arc (note that in the drawing the center of the first arc is about ⅜” below the baseline). This second arc (Extrados) forms the top…
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