I have a 12 x 14 pergola with 6x6 posts that are 4 ft into the ground with concrete underneath and surrounding each post. The frame is two 2x8s with 6x6 braces in between all around (so there are eight 6x6 braces). I recently built a twin bed size daybed that I want to hang from it on the one side that is possibly around 120-150 lbs. I want to know how much load this pergola can handle.
The situation as I understand it involves a 400-500 lb. dynamic (moving, jostling) load suspended from one or two 2x8 beams. (Unless you block the two beams together well they aren't a single unit.) Here are my concerns, in lieu of a yes or no answer. There are too many unknowns for that.
Your post backfill protocol is unclear. While 48" is a good depth, if you backfilled with poor quality soil or didn't compact it well it'll allow significant movement.
Your beams are carried on carriage bolts in shear. This is no longer allowed in deck situations, which your plan approximates, because they tend to sag and bend, especially as the wood begins to decay over time.
Your diagonals are not structurally attached to the posts or beams. I see what appears to be small screws holding them up, making them mostly cosmetic. You'd need to have them secured with bolts similar to what are in your beams to really make them do the job.
In short, I don't think anything will collapse under the proposed load, but I wouldn't be surprised if you get more sway than you expect, and you may see some displacement of the beam at the post connections.
