Tennon surface area

[danreed76]

In reviewing our plans with the engineer, the only change I had to make was to increase the surface area on some of the tennons.  On a few of them, there just isn't enough room to get a traditional tennon that goes the full depth of the beam.  Attached is the best option I could come up with to get the required surface area.  Per the inspector, this meets their expectations (sufficient surface area per the calcs), but I wanted to weigh in with some of the voices of wisdom here.  The posts and beams are SYP, and the pegs will likely be red or white oak (those and maple are my available hardwoods).

I don't have the bandwidth to upload the whole frame drawing right now, but hope to post it soon.

-Dan

[Rooster]

Have thought about a through spline?

Or even side by side through tenons, each only 1.5 in. thick?

Rooster

[Jim_Rogers]

Dan:
Here is a picture of your tenons, with x-ray view turned on.



 

The first thing that stands out to me is that the tenons aren't long enough. You need to have so many peg diameter's between the end of the tenon and the peg hole in order to have enough relish to prevent the end of the tenon from "blowing out".

When I drew a sample tenon for a joint like this, I made the mortise a through mortise with an over and under tenon.



 

Above is my example.

Here you can see that the tenons go all the way through the post and end at the housing on the other side. For example if this was an 8x8 post with a 1/2" under rule the width of the post at the housings would be 7". So each tenon would be 7" long. And with the housing area, (1/2" x 8") you may have enough "surface area" to support you load.
The surface area that I'm talking about it the area shown in "blue" in the drawing below:



 

When an engineer figures the load that needs to be supported he would/should look at this "blue area" of my drawing to see the entire surface area that will support the load.

Sometimes you have to increase the size of the timber in order to make it big enough to have enough wood there for all the joinery.

Does that make sense to you?

Jim Rogers

[danreed76]

Jim:

Thanks for posting the pictures; I had exported some last night, but due to my amazingly slow connection right now, I couldn't get them to load to my gallery, so attaching the sketchup model was about my only option.  I had figured the tenons to have 2x the peg diameter for relish; I can surely make the tenons longer no problem (a through mortise seems as though it would be easier to cut anyhow).  I mainly wanted to make sure that the over and under technique was considered acceptable and that I wasn't creating some additional weak point by having a "step" between the tenon and the housing.

Rooster:

I'll give the side by side tennon idea a shot this evening when I have some time to play with it.

I hadn't really thought about putting a spline in there, but that may be my next best option.  Can I assume that I would need equal surface area for the mortise in each adjoining timber as is required in the post connection to use a spline?  Should the spline be the same species as the timbers, or would a hardwood be preferred?

The beams are 6x8 joining into an 8x10 post. 


Thanks, as always for the feedback, gents!

Dan

[Jay C. White Cloud]

Just my two cents, but I would really use a spline in this situation.  It is by a great margin the strongest and very traditional, (just not so much in European framing, though they do occur.

Regards,

jay

[Ljohnsaw]

Jim,

DISCLAIMER: I've never built a timber frame, so I may be too green for this: :

In your drawing of the under/over showing the blue area for support, I think that is true for the bottom tenon.  However, I would question that for the upper tenon.  As both of the tenons shrink, I would expect there to be  gap between them, supported only by the shoulder/housing of the beam/post and the peg.

I would vote for a side-by-side or a spline.

In calculating my support area, I was shooting for the shoulder/housing to be the supporting element, neglecting the area of the tenon, and I sized mine to 1" where I see 1/2" often used.  Plus, it scares the heck out of me thinking all this load is resting on 1/2"!!!    In my mind's eye, I see the post shrinking and the intersection of the housing and beam getting smaller and smaller until the structure is relying on the tenon for support.  Maybe I'm just paranoid! :D

[Jim_Rogers]

It is possible for a post to shrink enough that it maybe a problem.
You engineer will specify how much "shoulder" you'll need to support the load.

I feel you're kind of spinning your wheels until he tells you the amount you'll need.

You can do a trough spline. But that will have to be designed correctly to support the tie beams and hold them together to the post. Never any pegs into the spline in the post, according to one of my engineers.

Side by side tenons may take out too much "post" to be a solution, depending on the details. Again your engineer should specify what he wants to see there.

And as I said you may have to increase the size of the post to compensate for joinery.

Jim Rogers

[Jay C. White Cloud]

As Jim said, wait for PE feed back.  Make sure you have a PE that knows this craft.  Also, I still stress the spline as being your best choice, otherwise, I would do it pretty much they way Jim has outlined it (and have,) if you want to use tenons.  We use splines almost exclusively for things, except pass through and wedged tenons.  The PE love them and they aren't that much harder to do.  Housing are again up for PE specification.  We tend you use wider ones, but on vintage frames, they are often 1/2" to 3/4" in size.

Regards,

jay