Thanks again for all the comments. I do have a couple more questions and wanted to add a few details on the project.
I am using pressure treated lumber for the post, I let them dry out over the Summer, they are 4" X 4" oversize so they are 4-1/8" X 4-1/8" actual. They are already set in concrete about 28" deep and measured square corner to corner, 28" is way below my local frost line. Inside width about 44", outside adds 8-1/4" so about 52 1/8" outside. Overall depth about 34"
I have picked up a couple WRC 1 X 6 X 8 boards to go on top of the post, my plan was to use both boards (so 1.5" thickness total) on top of the post and then attach the actual arch to the top boards.
Here are my questions;
You mentioned using 4/4 vs. 6/4 for the top, since I am targeting a 3" thick on each arch piece (double 2 x 6 thick) that is extra cost extra lumber, will you please elaborate on the advantages of why you suggest this approach.
Slot screwing the connecting pieces on the arch is a great idea, thank you.
I can see how weak a butt joint would be, I can see the advantage of using splines. I am debating the Freud Slot Cutter vs. just using my table saw with a jig, I am undecided but here is my next question.
My plan was to off set the 2 x 6's on top of each other as in the attached picture, this should add a lot of strength, would you still recommend the splines. I am guessing yes and if so I will add splines.
OK, here is where I am lost, you recommend a birdsmouth joint. I looked it up and understand the joint and its use. I assume you are recommending this for attaching the arch bottom to the frame, if other than that, I need you to provide me some more details.
Thank you once again.