I would like to know how to construct a T joint with diagonal bracing on each side. The T itself will be a mortise and tenon joint. In my younger days I'm sure I saw such joints in post and frame barns, but can't recall if the diagonal braces were also mortised into the upright and crossbar. Seems like that would make much stronger bracing, but I can't visualize how the joint could be assembled with mortise and tenon joints everywhere. And sometime long ago I must have loaned out my book of woodworking joints.
In my younger days I'm sure I saw such joints in post and frame barns, but can't recall if the diagonal braces were also mortised into the upright and crossbar.
From watching the TV show Barnwood Builders, I seem to remember the diagonal braces were done with M&T.
Not knowing what you're working on, another approach could be to cut the diagonals to fit, screw them on, and cover the recessed heads with dowels to look like they're pegged.
I built a timber frame last fall, and yes the braces were all M&T. We used the 3-4-5 measures which simplifies and the proportions look right. That is, the brace bottom end was say 4’ down then the top was 3’ out, which would call for a 5’ brace length. The bottom of the post mortises were level , and into the beams were plumb. The strength is in the end grain to end grain compression, which is the highest of all the other structural properties.
As I recall from the wood Wright's shop (Roy Underhill) where they built the Anderson forge at Colonial Williamsburg, the braces had a combination of M&T and pegging.