I have shown this bridge before, but am doing so again because several people have asked how the arch was made. The pictures below show several stages during construction; a close-up of an end tower which shows one critical location -- the establishment of the initial angle; and my inspiration -- an old anonymous photograph from the 1920s
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This view shows the supporting structure. Note that there are gaps which allowed me to slide the supports towards the center before removing them. None of the supporting structure was touching the arch when I started to remove the support stones. First slide the stones inward to cause the compression forces to support the arch. Then remove the stones. Start at the top and work down.
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The first crossing layer. Five stones. These stones must be placed very carefully, starting with the center stone and working outwards, placing the stones on each side at the same time. Weight added in a non-symmetrical will cause the arch to fall.
A close-up of one of the end towers. Enough weight to prevent sliding sidewise is critical. Thus the towers are not hollow, but solid stone. The correct angle at the intersection of the stone tower and the arch is not easy to create. One does not have an unlimited selection of angles in the Anchor stone selection. If you look closely you can see the stone against the lower circular stone, which is needed to prevent the arch from spreading sidewise and coming apart.
I took my inspiration, and the basic arch design from this picture. You can see the sidewise movement prevention stones in this picture also. I did not realize how important they were, at first, but I quickly found out.
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