pergola
Looking to build a pergola for my back patio. I’d love to cover the whole area (30’ wide x 24’ deep) without any posts in the center. I’ve attached a picture of the space, sorry it’s dark and a mess. Just bought the place and looking to lessen some of the sun without fully roofing it. I also attached a couple pictures of what I’m thinking. I’d love the arched roof, but I’m assuming that would be even tougher.
Well...I had to live and learn. I used 12 inch sonotubes with the same anchor and cheap cement (the fence post kind that was recommended to me) and 6 x6 posts, which turned out to be a bad idea. 2 of the 4 cement piers cracked part way down where the achor ties were. My solution was to build 21 inch squares around the base about 12 inches down and past the broken areas and leave the rest of the existing footing intact with the more expensive extra strong cement surrounding the existing pier that goes 3 ft down. Hopefully this holds up well..but I am a DIY person so there is trial and error on my end. Get quaility cement and do a wide base to limit lateral breakage IMO. Of course I dont think there would have been and issue if I had not used cheap post hole cement. I am just glad I discovered the issue before the structure became wobbly.
What you're trying to do appears possible, with a few extra parts. See here (forgive the crappy art, I made several edits):
The two red rectangles are the beam-post braces I would add (minimally) to handle the horizontal loads which can no longer be handled by your odd post.
The post I drew where there isn't one is just a short floating section used to fix the two beams together.
What you need to build so your post does not need to be buried in concrete is a cantilever. Extend the 45 degree horizontal beam some distance inside the pergola. The longer you make it, the less load you will have where you join it thanks to leverage. I would aim for as much inside as there is outside. Fix the end to another 45 degree beam (shown in red). This needs to be rather strong as it will be handling roughly the weight of that corner of the pergola. Except that end will be trying to move down, not up. Think of that joint as hanging a large weight on the red beam.
I've shown the red beam as resting on the 2x6 beam, but attached to the side of the 2x8 beam. It should be easy to find some kind of metal L you an screw to the 2x8 and rest the beam onto.
The connection between the two 45 degree beams could be a large bolt down the center, with appropriately large washer. They will be about 6 inches apart vertically as the red one is over the 2x6 and black one under, so that would likely be easiest. But it's also possible to insert a short section of wood and use steel plates to fix both ends to the beams properly.
I also suggest resting the 45 degree beam on top of the post, with some minimal fixing to keep it from moving around. I would drill a hole from the top and drop a rod which will not rust in it. Same deal where the 45 degree beam meets the tiny floating post. A lag bolt from below would do fine. Or any kind of screw you can get that's long enough, it won't be pulling anything but its own weight.
With that setup, your odd post does not even need a brace to the horizontal beam to hold everything up. But it will help overall stiffness to have one.
If you go the "10 foot post into concrete" way, I suspect you will find your post slowly tilting towards the pergola over the years. But that's just an educated guess.
Best Answer
Hire an engineer.
This is a non-trivial structure that could kill people if it collapsed. It may require trusses rather than simple beams to cover that span. Arched (bowstring) trusses are a possible "stock" solution depending what you want to do for an "arch", but in any case this is not something where off-the-cuff design seems at all advisable....