Octagonal structures are definitely possible. How to do it depends on your specific needs. What are the spans? What do you want it to look like? What size timbers are you going to use? Where are you building and do you need to worry about snow loads?
In the past we've done five sided post and half-lapped plates with an octagonal boss pin in compression at the peak, but there's more than one way to skin a cat. We've even done up to twenty sided polygonal structures. Just about anything is possible with a little geometry.
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