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From my earliest days in architecture school, I have been fascinated by masonry and stone arches. Arches are a marvel of both simplicity and complexity. How seemingly impossible is it for a group of stones, often joined together without any adhesive, to span a distance and form a load-bearing vaulted structure, defying gravity and resisting the elements for thousands of years?
When my son Asher was three I built a wooden arch for him and hoped he would have fun mounting it and maybe I wonder how this magical structure, especially the keystone at his head, can hold up. The arch I have devised resembles an arch bridge in which the weight of each of its elements is transferred to the heavy base stones on the right and left sides.
As for the keystone, when it is inserted it pushes down but is stopped by the side stones that push it against.
Building the arch was easy. All I needed was a short scrap segment from a 2 × 4 board, compass, sliding bevel, and ruler.
The arch is made by drawing two arches. A shorter arch (soffit) forms the interior opening of the arch, and above it, a wider arch, made by spreading the compass legs to a longer radius and setting the new center at a much lower position than the center of the first arc (note that in the drawing the center of the first arc is approximately ⅜ ”below the baseline). This second arch (Extrados) forms the upper circumference of the arch. Once the rays have been drawn, it’s time to create the stones. Place a ruler or sliding bevel in the center of the short arc and section the arc a few times to create an odd number of sectors that fan out. Now cut the design on the bandsaw, shape and smooth it, and also apply a finish on it.
Assembly of the self-supporting arch.
After attempting to set the bow, you will immediately realize that this process requires a minimum of three hands, so it fits perfectly as a fun and educational activity for a parent (or grandparent) and child. You will also notice that trying to erect the arch on a smooth surface is challenging as the weight of the stones will push the stones of the base of the arch sideways and cause it to collapse. However, if you build it on carpet or any other material that provides enough traction to cancel out lateral forces, the arch will prevail. The latter feature can generate a conversation about the forces of nature such as gravity, friction and vectors.
Good work and construction.
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