2 8x8 beams join at 8x8 post

[rythma]

Hi guys,  Im new to the forum and relatively new at designing timber frame joinery. I am not the designer on this job, just the problem solver. I don't have the option of changing beam or post positions or sizes, I'm just trying to make a good joint with what they're giving me.

All the timbers are milled s4s to 7.5" square. Eastern white pine. This is a partial frame for the front and back porches on a new stick built house. Just a post and lintel frame with knee braces across the front and a beam on each side returning to the house. One side is a shed roof, the other is hip, both will be stick frame roofs on top of the timbers with a flat soffit ceiling directly on top of the beams. Im running the posts up past the beams to minimize any exposed end grain. This will be white pine exposed to the Vermont elements. The forces here are not huge. 8' and 12' spans across the front. 6' and 8' spans on the returns. I think the roof will be about 1/4 pitch.

Not sure how to get these pics to show in the post, but you can view them in my gallery.
All joints are 2" off outside face, with 1/2" shoulders. Tenons are 2" w x 6" h. 1" black locust pegs

This is the shouldered lap joint I'm thinking of for 2 beams in line meeting at a center post
https://forestryforum.com/gallery/displayimage.php?pid=212982#top_display_media

This is corner option #1. the tenons are mitered, so they have equal amount of relish. 3" length to heel, 5" length to tip
https://forestryforum.com/gallery/displayimage.php?pid=212983#top_display_media

This is corner option #2. The tenons are notched to lap past each other. This allows for one peg on each beam to have more relish behind it.
https://forestryforum.com/gallery/displayimage.php?pid=212984#top_display_media

any comments, suggestions or other ideas much appreciated.

-rye

[Ljohnsaw]

OK, Let me state I could be totally wrong. :-\

My understanding is the tennon's purpose is to keep the timbers aligned and for a place to peg to hold the joint together.  The shoulder of the joint is the weight bearing surface though a little is taken up buy the tennon, if properly fitted.  I would be inclined to do ¾ to 1" on the shoulder, but that's just me.  The mitered doesn't look like there is enough relish for the pegs to the end of the miter.  The half-lap version looks better and I would only expect 1 peg for each timber probably just before the half lap.  The short part wouldn't do much good with a peg in it, IMHO.

Jim?

[ChugiakTinkerer]

The residential code typically has a requirement for beam ends to have at least 1.5" of support.  If the tenon is load bearing that can make up for a shallower shoulder or housing.  For the first image I would design for a shoulder that is 1.5" deep on each side.  That leaves 4.5" of post for pegging tenons.  If that doesn't provide enough for tenons you could use a spline instead.  See here: http://timberframehq.com/spline-joinery/

My beginner's understanding of timber framing is that three-way junctions are challenging and complicate things significantly.  If given the luxury I would try to drop the tie beam below the top plate.  That way you could have full shoulders for each beam. 

[rythma]

All good points. I wish i could change the sizes, but i am not in control of this one. I just discovered the toolbox, what a great resource! You guys were right about the amount of bearing. I just ran the numbers on my longest span and it requires 4.75 sq" so if I increase the shoulder to at least 3/4" that brings me to over 5 sq" plus the tenons. I think I will go with the lapped tenon corners. I know it's not ideal, I'm working with a very conventional builder.