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#34266 - 09/26/17 04:03 PM Central Tenons (w/ Half Lap Sill Corners)
EPops Offline

Registered: 09/15/16
Posts: 5
Loc: Buffalo, NY
Hello all. I noticed the one topic on these forums ( Sill Plates ) about sill plates, specifically for the use of central tenons on half-lapped sill corners.

It was good info, but I would like to delve more deeply, and ask a few questions.

The frame I am considering:

First, I have drawn up a half-lapped sill corner (for both back corners) using a central tenon on the joining post. The sills and posts are all 6x6 pine. For this use only, there are no floor joists; rather, the floor will be separate from the frame.

As you notice, I am using square rule, so the laps are nominally 3" deep. The central tenon is square, 1-1/2" x 1-1/2". Since these are 6x6 timbers, I go by the thickness rule-of-thumb that Jim Rogers usually posts.

First question would be, can or should there be deviation to this rule-of-thumb for a central tenon, since it is not as strong as a regular tenon? More specifically, I am wondering how the thickness of the central tenon affects its capacity to resist any lateral or overturning forces that the post might see - basically, its shear and moment capacity. Along those same lines, how does the thickness of the mortises in the laps affect the bearing strength of the laps? As we increase thickness of tenon and mortise, the tenon becomes stronger but the bearing strength of the laps reduce. Yet in this case, since it is a sill (and not a post to top plate) connection, does the thickness of the central tenon really matter that much? Isn't it there just to prevent the sills moving apart, and won't really see much shear/lateral force?

My second question, which came up in the linked post above, is how long can or should the central tenon be? Right now, I have designed the bottom lap to have a mortise completely through it (for easier access of the mortiser and chisel, where I can come from the bottom face instead of the top face). But I have the post's central tenon only 4" long. Thus, the central tenon extends only 1" into the bottom lap.

The thing on my mind is, what is the "sweet spot" length for this central tenon, considering its thickness? I know that if the sills were to have joists and a floor attached to it, this would increase lateral forces. And having the central tenon only go 1" down into the bottom lap, will that be enough contact with the bottom lap?

The other thing connected to this: what if I made the central tenon 5" or 6" long, so that it made more contact with the bottom lap? Would that actually be worse for the tenon itself since there will be more of a moment arm on it? Again, I am curious both for my situation (where there is no floor on the sills) and if there were to be a floor.

Last point of data, in case it matters. This frame will be shed-style with single slope roof - as shown in the first picture. The slope of the roof is only about 2/10, and will have a metal roof, so maybe a slight snow load. The half-lap sill corners are for the back corners.


#34267 - 09/27/17 01:20 PM Re: Central Tenons (w/ Half Lap Sill Corners) [Re: EPops]
Jim Rogers Online   confused


Registered: 03/14/02
Posts: 1655
Loc: Georgetown, MA, USA
Are the posts going to be exposed to rain? if so then a through mortise would allow all rain water to drain.
Other wise at least 1 1/2" into bottom sill.

Jim Rogers
Whatever you do, have fun doing it!

#34268 - 09/27/17 06:39 PM Re: Central Tenons (w/ Half Lap Sill Corners) [Re: EPops]
EPops Offline

Registered: 09/15/16
Posts: 5
Loc: Buffalo, NY
Thanks Jim! No, not exposed, covered with shiplap siding. I wanted to keep this job a little quicker and easier, since the guy I am building it for needs it done soon. And I figured that this would be sufficient given that no floor is attached.

I do have one other question. As shown in my diagram, I have the back sill member having the top lap, while the shorter side sills will have the bottom laps. I did not know which configuration (as I have it, or the other way around) would be more stable.

It would seem that it depends on how the load is transferred, and any predicated failure modes. I figured in my design, the local failure in the mortise area might be worse than having that whole back "bent" overturn. I am not sure about that, since the whole back "bent" could rock the back sill. But I was thinking of using some strong tie plates between sills and posts as well.

Edited by EPops (09/27/17 06:50 PM)


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