I have dug out some more pictures to describe the structure I think relates closely to Appolodorus' desgn.
Perronet designed this arch to carry the stones of an arch bridge until the ring was complete. The span is very similar to the Danube bridge. If it I were to try to make a picture of this in a small patch of stone, I think that the outcome would be much like the picture on the column.
These timbers are about 500mm square and 5m long. The ends are square cut and they meet in cut outs on the radial members which have wedges cut in the base. C is the top half of the radial at D.
The long notched timbers below link 6 of these frames together and there are diagonals to make a proper three dimensional structure.