i've been presented with plans that call for eight posts in a ring carrying 12 foot beams.
so two beams meet on each post and the connections are at an angle.
could anyone provide some joinery possibilities here?
thanks
-Mark in Ontario.

A very simple technique that works for us is to rip a post lengthwise, in this case with a saw bevel of 22.5 degrees. Then flip over one of the halfs end to end and glue the two ripped edges together (the glue makes it slippery and the "halves" will start to warp immediately so plan on using fastners to hold them together while the glue sets up. This allows the surface of the post presented to the beams to be a simple, traditional plumb, square surface for simple joinery. We use this in conjunction with our splines and it has saved untold hours in detailing, cutting, and head scratching.

Alternatively, if the posts are big enough then the beams could be housed into the sides of the posts, the 22.5 degrees would be cut off the end of the beams and the mortise would slice thru the post at the same angle. Remember, tenon grain should be straight, a smaller extension of the beam so that the wood fibers remain continuous thru the tenon and into the main timber.

clever trick to split and glue like that...
but yes these posts and beams will be at least 8x8 and possibly as big as 10x10
are you suggesting an open mortise and tenon at the top of the post?
also would it not be better to angle the post shoulder rather than the tenon shoulder?

as far as assembly goes something that drops in from above will work best

are there fastener alternatives here?

i know there are places like this being built as the plans are modelled after earthwood.....

THANKS!!!!!
-Mark.

by the way Paul, how big are the posts you split, flip and glue?
-Mark.

Mark—

Polygons are big fun, and I know from experience, jumping into one blind is the kind of thing that can and should make the hair on your arms stand up, so…

No hard advice here, but some simple points to help point you towards the right path

Your five sided posts should be milled at half your angle of interface, in your case 11 ¼,
Less acute angles make for joinery which is simpler to cut, with both less short grain and better edge distances.

The key to simply and traditionally joining a polygon (which oddly requires ignoring there is little such tradition) is to resolve thrust completely before it reaches your plates, this allows you to join them, be it to each other, a tie or the posts themselves as simply as possible, by eliminating tension from the quotient. First ask how to achieve this, then decide how to join it.

In my opinion, the only way to layout anything compound, and most especially anything polygonal is to scribe it. I know if you are not familiar with scribe technique this in no way helps prepare you for your jump…

All the same, I hope this helps

will... could you expand a bit on the five sided post? we have a sawmill and i have some experience in taper sawing and sawing clapboard

as far as thrust goes i'm a novice on circle engineering for sure
i know that this building will have a living roof and it is designed with no roof slope in the timbers, just the overlay materials. there are three rings of posts carrying beams in polygon shapes and the roof members sit on radial lines on top of the beams....the center meeting place of the roof members will be mitered around a continuous roof member spanning the diameter (does this resolve thrust?)
however, i don't feel right about a flat interior ceiling surface
my instinct tells me there should be some kind of rise toward the center
i'm thinking of sawing tapered roof members and keeping the tops flat, let the bottom rise toward the center of the frame.

speaking of layout i'll be starting at least with a snapline and square technique
i will try to lay out the angles on the ends of the posts and work from lines snapped between them
can you point to some reading material on scribing?

do you use slanted bearing shoulders?
i'm thinking of a wider shoulder on the inside of the circle than the outside
my main concern here is the joint at the top of the posts... a 3 way joint between 2 beams and the top the post
are open mortise and tenon joints strong enough here?
is there a way to use a continuous spline through the post here?
how about a bent steel plate?
could this be done as a mitered connection with lag bolts drilled and set from the top of the beams?
the wall system is strawbale and plaster so there will be shear strenghth provided by that system
where do we need braces, if any?
i've already asked the engineer about this but if you could comment?

thanks and more thanks
this forum and the writers here is are a big support!
-M.

Mark we rip 6x10 posts to make our resulting six sided posts, allowing all our plates to connect to the posts in an orthogonal plane. Will's approach makes the most sense if you want to keep your posts as one piece, and are relying on traditional joinery. Also, I doubt there is a better way to fit a joint tightly and cleanly than scribing if your customer is willing to pay for it.

It sounds like you need to employ a designer and engineer for this job, with no central post your rafters will distribute significant horizontal thrust to the outside walls and you will need steel to hold everything together. I have done this with wood in a gazebo since the spans were short enough to overcome thrust, but a building of any significance will exceed the capacity of a wood joint in tension.

There are a number of knowledgeable engineers and designers in the guild resource guide all capapble of advising you in this venture. Will is absolutely correct, be afraid, be very afraid. Paranoia is an engineer's best friend!

as far as fear goes
i've put in a lot of time milking holstien cows and lost most of my fear
those cows can get pretty rowdy, especially the first time you milk them.
the design is being engineered and i have sent several questions through the designer to the said engineers(timber sizing/rafter placement/placement of bracing)

perhaps a bit more info on the design would help
the first ring is a 6 foot radius
contains 8 posts and 8 beams

second ring is at the 16 foot raduis
again 8 posts and 8 beams

the third ring, (which falls outside the wall system and forms the outside wall of a wrap-around porch)is at the 23 foot radius
this third ring contains 16 posts and 16 beams

by the way i saw an interesting solution to the joinery last night.....someone did it with 4 "normal" bents and spaced the bents with 45 degree pieces

-Mark.

I recently worked on a similar building but with an exterior ring and a center post. I was told the living roof support structure needed a minimum slope of 1 on 12 even with the roof membrane. This project was not a residence but rather a classroom so it was required to resist seismic forces, hence knee braces were used. This one used cordwood instead of straw for the infill. There is no code accepted shear values for either straw or cordwood. (The third pig had no problem) Be careful, the living roof system is very heavy.

christopher-
this design does have an exterior ring
at 23 feet radius/46 feet diameter
we are trying to avoid a center post as it would be in the middle of the kitchen

-M.