The form ( often called the 'centering') used in supporting the stones for a dry stone bridge (or an arch) while it is being built, often ends up looking a lot like a large lobster trap.
The one we are building for our latest dry stone foot bridge project will be for an eight foot span.
One sheet of plywood will make two of the four equally spaced curved ribs needed to make an eight foot wide bridge form.
The arc that the voussoirs will travel is in the shape of a segmented arch, 24 inches in height. To get the proper curve, a point 60 inches is determined at a right angle from a line running longways through the middle of the plywood and the curve is drawn using that as the radius.
Don't forget to draw radiating lines at this stage while the plywood is flat, so you have guidelines for determining the angle that each of the voissoirs should be oriented when the form is in place. It is better to draw the lines now than try to guess the angles later. Trying to re-establish the the centre point of the arc (usually somewhere in the centre of the stream) later is pretty difficult.
A jig saw does the trick for cutting each of the four curves. Go slow and keep the line smooth. Pretend you are in a truck painting the white lines along the side of a curve in a road.
Notches are cut into the two inner ribs for seven 93 inch 2x4 spacers to fit in and then the out side ribs are screwed to the ends.
You can cut the notches of both sheets of plywood at the same time.