Spline Joint Drawing?

[jander3]

I am working up a plan where the tie beam for a bent will need to span 20 feet.   Since cutting > 20ft is too much for my mill, and I don't have trees size such that I can span 20 feet, I am thinking I will install a post at the centerline.   I want to intall both tie beams to the post with a spline type joint (i.e. tennons into post, with shoulder, and a spline board through the post and pegged into both beams).

Anyone have a drawing I can use for reference?  Or a better idea.   

Posts will be 6 x 8, tie beams will be 6 x 8.

[Jim_Rogers]

I have a gallery of pictures of splines in my general gallery.

Also, I have this drawing to use to figure out what size your splines should be and how deep they go into the tie beams:



Also, the last time I had an engineer review my spline drawing she told me to never peg the spline in the post only peg it in the tie beams.

Hope that helps.

Jim Rogers

[jander3]

Thanks Jim.  The beams you have pictured, do they have tennons?

[Jim_Rogers]

No.
But when I discussed "spline joints" with Jack Sobon he said he never uses splines. But what he does is an over and under tenon that is pegged to the post.

Like this:



And:



This type of tenon allows enough relish beyond the tenon peg hole to hopefully meet code.

Jim Rogers

[Brian_Weekley]

Here's a slight variation to Jim's where the tenons include half dovetails.  The mortise is cut higher to allow the two dovetails to mate in the post and then held tight with a wedge above.

[Jim_Rogers]

Nice...

[AMB30518]

Thanks for the spline specifics Jim and for Jander3 for asking. I thought about trying to put some in my cabin frame, when I get to that point.

I also like the half dovetail Brian.
Have you cut and used this joint?

I am using the standard over & under tenon in my carport and can use this as it looks to be a stronger lap with the dovetails against each other, rather than just the pegs and relish to hold it together.

A couple more open questions if you guys don't mind.
1) Would you draw bore to draw into the post and offset (up or down) to compress the dovetails together?
Draw the lower tenon in & down and the upper in & down for vertical shrinking?
2) What about a straight tight fitting hardwood block on top instead of a wedge?
Or would you want to be able to tighten tighten the wedge after some shrinkage?
3) If wedging what about double wedges, one from each side, and a straight through mortice instead of beveled?
4) Any advice on the amount of dovetail lap offset using 6x6s?
example: 2 1/2" at tenon base & 3" at end or 2" at tenon base & 3" at end.... etc

Jander3, I hope I am not intruding on your thread.
Thanks
Aaron

[Brian_Weekley]

Aaron,

I was planning to try this joint on a small shed I was planning to build where I needed two beams to come together at the same height.  However, I haven't started that particular project yet.  Originally, I was planning to use a spline as Jander asked about, but was persuaded against it because it wasn't a "traditional" joint.  There's a photo on the back cover of Jack Sobon's "Timber Frame Construction" book that shows an over/under dovetail.



I would think a wedge would be preferred over a uniform block so the joint could be tightened when the joint shrinks.  As far as the tapers used, I was planning to apply the ratios shown in a diagram in Sobon's book (1:5 for the dovetails and 1:10 for the wedge).  Therefore, in a 6" post where you might have 1/2" bearing surface for each beam, you're left with 5" of meat in the post.  To maintain the 5:1 ratio, I'd make one dovetail 1/2" above the centerline and the other 1/2" below the centerline for an overall 1" taper across the 5" length of the tenons.

Brian

[Satamax]

Or you can do a Jupiter line, which is the "traditional" joint for long tie beams.


And If you put a post in the middle of 20 feet, you won't have any space left!  I think I'd do this instead.






It's nowhere near as complicated as the joints showed either.

[grweldon]

I'm an engineer, but I would have never thought of doing a joint like this!  Amazing!  I love it!

[Thehardway]

Here is my splined interrupted tie-beam in a 24' span kingpost truss with 12/12 pitch.
The tie beams are 11'7" long and 7"X7"  the kingpost is 10"X12" the rafters are 7"X12"  All are white oak

The Spline is 6' long and 12" wide at the Kingpost and was made of quartersawn Black walnut. 

The ends of the tie beams are Half/dovetail and wedged into the rafters

Based on my personal experience, the biggest challenge you will face is the joints getting looser as they dry and shrink.  The wedges will need to need to be driven in regularly over the years and make sure they are deep enough that the wedges don't bottom out.  You can't remove the wedge to cut it off or upsize without bad results if the beam is in tension.

There are some problems with this but it is workable under the right circumstances.