Something like this? R 200m
Build a one-degree arc segment module from a circle created with 360 sides, then cut half of it off, add another circle inside, then join one end segment across radially between the circles. Delete the rest of the circles.
Cut out some of the face to get the shape you want. Pushpull along Y/green direction to get depth. Add detail as required.
Rotate/copy 179x to get this. Needs a little tidying up at the end.
Arch model.skp (46.7 KB)
Add detail if you need it by making ‘beams’ of L or I shape cross section, and add diagonal bracing as required for stability and stiffness.
You can work on just editing one copy of the arc segment module and all of them will change to match.
PS. If you want to hide the join in the circumferential pieces, just hide the edges at the joining ends of the component.
Or add further joins, if you want to represent how the arch might actually be built and assembled ‘for real’.