Hello experienced timber framers...

I am trying to figure out the geometry of doing an octagonal hammer beam with 8 rafters running into the top of it and 8 collar ties farther down. I know for the collar ties I can do a wedged half dovetail, but I'm struggling with the rafters. Everything I can find on hammer beams, king posts or about anything else says to use a mortise and tenon joint. However, on this particular piece there isn't a place to put pegs because every face has a member coming into it. Where do the pegs go? Are there pegs? Is there a way to design the interior of the post so that the tenons lock themselves? How does this work?

Check Timber Framing issue 32 for the article on the Sterne Park Gazebo, possibly the most complex small structure ever built in a Guild workshop. One of two sister buildings, designed to withstand Ontario snowloads and Gulf hurricanes utilizing bypassing freetenons through a big honking boss pin for tension connections on dual level ties and stainless steel knife plates connecting freetenons which tied the plates into a tension ring. Design and engineering credits to Joel McCarty, Ed Levin, Ben Brungraber annd Tim Chauvin.

Your building project sound very high risk and first needs high level engineering that can consider the structure as a functioning whole not the application of divers framer voodoo that might just work.

The project is a small roundhouse (10 foot diameter to the inside wall, 12 feet to the outside of the wall). We have looked at several ways to frame it and this is one that we didn't know enough about to discuss well.

To be clear, I have a masters in engineering and I am very familiar with structural analysis and the need for the same. I was trying to find out how the geometry works with the timbers and the joinery without metal. It sounds like the ends of the rafters become pegs for the other members which makes sense. Is there any way to see back issues of Timber Framing so that I can read up on the example you cited?

Thanks for the help!

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http://www.tfguild.org/publications/TF%201-102%20index-2.pdf

this is just a thought (i am no expert in timber-farming) think of an octagon(diameter 18"-24" now extend the sides on the clockwise side(or the counter clockwise side) out to whatever your radius of your structure minus the radius of the octagon, and wallah that is the plan view of your "Bents" put in a skylight in the center. this would resemble many classic Greek/roman dome structures. I would imagine joinery would be similar to a 'hip roof rafter" meeting a ridge beam. getting this all together would require many hands.