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In architecture one of the most pleasing support structures is the Arch. Simple elegant and strong they are the blend of refinement and purpose.
I use three kinds of construction methods for my arches. The simplest one and the only true structural arch I make is cut from PT pine into strips approx. 3/8 thick by 1 1/2 out of a clear 2x8 or 2x10 material you can get at any lumber yard. Using a form for the 48" arches I use each 10' strip laid in place to make up enough material for perhaps 3 inches of material stacked, clamped and then secured together from top and bottom.
Once everything is nicely secured the holes are drilled for the arch to post bolts and it is put aside. Some sanding is done if I had difficulty lining up the pieces so everything looks well.
Another arch is made in quadrants with a device I secure to my router and plunge cut each 3" wide piece out of the same typ of material, pressure treated pine. When these are done and I have three to make an arch they are glued end to end and a 3/8 strip is secured over the top of the arch and under the bottom for binding strength.
The third method is used on my bridges where we take a ful 2x12 or 2x10 and using the router jig, cut a long lazy arch cutting about 1/3 of the board away and use the left over piece to make the upper section when added to the same board. Careful figuring is used so as not to destroy the boards strength beyond its abilities. The end result is one part of the board has the bottom arch cut out of it and the other part is the top of the arch. When they are mated together they make a bridge arched side rail.