today i spoke with the man who i apprentised with back in the 80's....
he suggested placing an additional timber across the corner of the beam connection(a relatively dry 6x6 should do[hardwood?])
this timber would dovetail to the top of the beams and receive the bottom of the hip...it would look a lot like a half-dovetailed collar tie
i like it a lot as a solution
for one thing it rounds the shape of the timberframe inside, essentially placing a second octagon shape within the main shape.
structurally the more weight, the tighter the connection -and the hip length is shortened somewhat.
At the post top/beam connection it allows very simple miter with a bit of indexing (t&g between the beams with an index of some kind on the post top should work)there is also the possibility of bringing a tenon from the post, between the beams and into the bottom of the hip(shrinkage in the beams and cross tie becomes a problem here - the peg could hang the hip as the timbers dry)

ok back to the drawing board!!!
-Mark.

Mark –

Your speaking of a dragon beam, an amazing and elegant system which both captures and reverses the thrust which would work to blow a corner joint apart and instead pushes it together and at the same time provides the space which would not otherwise be there, to land a hip on.

However, while a perfect solution for a rectilinear situation, the farther you splay from ninety degrees the increasingly less efficient this system becomes, by forty-five all advantage is lost. Neither still, do you need to create a place to land your hip.

Again, you need to capture your thrust up-roof, and if you have moved to a central post, you have, and the point is mute. Don’t get me wrong I’m not arguing for this as a solution. I think it both avoidable, and structurally and architecturally uninteresting.

Keep looking and keep the core open.

yes there is now officialy a central support
not a post but a masonry chimney
there will be flat steel rings around the circumference where the rafters(hips) bear to take the weight and steel pins will be welded or drilled into the ring for fasteners...just like earthwood.
it may be architecturally uninteresting but central masonry chimneys are very nice in a canadian winter...

as far as the dragon beams go, they do look good on paper, the designer likes them, would we be wasting wood and joinery time to use them? It's hard to see how the beams could escape once you place those ties down on them.

And, as far as daps go, are you thinking of a horizontal lapping of the beams on top of the post? i assume there would be a tenon of some kind rising from the post to index the joint?
the problem with this joinery is that one beam would be cut from the bottom up and this is bad for weight bearing timbers....do you have a way to half lap and get the bottom of both beams on the post top?
questions questions questions
i hope i'm not the only one benefiting from all these questions
a thousand thanks
-Mark.

A post would, yes be boring, your chimney, a far more pleasant story all together.

Hard fasten the hips, ( though it seems to me, this might need be massive or steel reinforced to deal with asymmetrical loads, be they wind or snow ) so they are now tensile members and you have much more freedom to do what you might desire below.

You could perhaps land your hips on dragons in those open areas, on ties in others.

The cant from which you saw the pentagonal posts ( like is often the case ) needs be large for joinery’s sake as much as for load bearing ability. Long story short, wider than the ties/dragon ties, the shoulders are proud of these, and provide full bearing no matter how you join the plates to them.

Seems like a great project you are pulling together…

Be your best and the luck will follow

ok
plan now shows a central masonry support
then rings of posts and beams(girts) at 8 and 16 feet of radius
the porch now has two rings of posts, one buried in the outside of the strawbale wall and a ring at the outside of the porch

The hips (principal rafters) are on top of the posts(through mortise and tenon)
The girts have moved down at least 6" from the hip/post connection (i'm considering timberlinx for the girt connections, they only need a shoulder connection plus a 1 1/8" hole and they can be tightened anytime-the trick will be assembly, the last timberlinx in a ring will have to have a very long hole to the outside face of the girt)
I see common roof purlins with hopefully a dovetailed ring at the 12 foot raduis or so...
I'm thinking 32 braces downstairs and 19 upstairs
The 5 northern posts at the 16 foot radius should be continuous to second floor

We would use dragon beams(or corner ties) on the inner ring if the client's budget allows....

Another word on the posts
i can see taking off the 22.5 degree cuts on the outside face of the posts(similar to the top cuts on a hip) but why remove more wood than that?
why not save the strength and cut sloped shoulders?
this may be something i see because of my layout technique, i use end level lines and quadrant snaplines between the timber ends, all layout as far as shoulders go is done with snaplines...so an angled shoulder is possible(although harder to cut)
i suppose the 11.25 degree post sides would look better?(except where the post meets the hip?)

ok
more later i'm sure
i'll probably exchange the design plans for the framing plans on the webpages when i'm done.
THANKS again to everyone who has helped me grapple with this, especially will truax and mark gillis
-M.

The octagon frame design is now on the web
There are two drawings, a cross-section elevation and a plan of the second floor deck.
The roof plan is quite similar to the second floor plan.
Again I appologise for poor quality on the images, I'm simply taking photos with our digital camera here(the scanner is coming).
Any comments or possible improvements would be welcome.
-M.
http://www.geocities.com/markola17/elevation.html?1082033225710
http://www.geocities.com/markola17/plan.html?1082033939740

hey gang, I wanted to dredge this thread up and post some photos of the octagon frame raising. The frame was in storage here over the fall and winter and last weekend we put it on it's foundation.....
here is a link to photos on a free web server called piczo...watch out for the ads!

http://www.piczo.com/wpltree?g=10263538

Mark

Just a note re: the chimney as masonary support.
You may want to run that by an engineer. It can of course be done but I think the work will most likely be done by a filled cell cinder block core over which you would have a stone facing. If you start with your flue liners (you show 2 flues) then the block and a natural stone facing, the chimney may dimention larger than the smallest dim of 3'
The good news is that the chimney could be engineered to resolve lateral force issues.
Looks like a fun project.

the project has run up a big engineering bill, to be sure.
the chimney is being built just as you suggest, John, block core with some rebar and stone facing(there will be a euro-style heater in the masonry work as well).
The chimney rings are 5' outside diameter for the second floor and 4' outside diameter for the roof ring. The chimney footing is over 8' diameter....
the lateral support is being provided by the strawbale wall system.... our timberframe includes radial bracing and the site-built porch frame has circumfral bracing, but both these bracing systems will be redundant once the strawbale walls are finished.