Timber Framers Guild Forums General General Forum Questions Which side of the beam to mortise?

Is it better to cut a 1" mortise into the (a) inside of center cut beam or (b) the outside. In the case of a boxed heart beam, they are both
the "outside" (c & d)

The beam is an 8x12 (7.5" x 11.5") and the mortise is for 6x8's (5.5" x 7.5"). The wood is Coastal Dougfir, #1 and structural select.

Thanks in advance,

-srj

Your drawing suggests that these mortises are joist pockets for drop in joists. Is this true that they are joist pockets?
Is the 8x12 a tie beam or wall girt? And the 6x8 a floor joist?

Jim Rogers

I would respectfully suggest that 1" is not deep enough for these connections. At least 2" would make me more comfortable.

Yes, these are joist pockets for drop-in joists (6x8s). The 8x12 would be considered wall girts.
I probably could've done a better job of illustrating that. :rolleyes:

And yes, the 1" deep pocket would seem shallow if it weren't for the straps of metal on top(Simpson Strong Ties) keeping the joists from sliding/shrinking/dropping out of the pocket. Oh, I can just hear the purists now...

The main jist of my question is about the wood directly under the pocket where there is more direct force.

If the grain of the wood goes (more or less) vertically, like this:

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Is the strength characteristic different than when the grain runs horizontally?
Like this:

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to some degree, I think you may be splitting hairs on your grain orientation...
I'd still like to see more than 1" bearing, even with steel connectors. If your building should shift for some reason(usually caused by foundation change), the steel could tear out and the joist could pop.

First of all a joist pocket should follow some standard rules of thumb.
One is that the depth of the joist pocket shouldn't be greater than 5/8's the height of the girt. So, 5/8 x 11.5" = 7 3/16". Cutting a full 7.5" deep joist pocket exceeds this rule. I'd cut it 7" deep to leave 4.5" under the pocket for support. And modify the end of the joist to match by cutting away a curved slope from the 7.5" surface down to the 7" tenon.
As to how deep into the girt to cut the pocket, you should look at the load being carried by the joist and whether or not the 1" x 5.5" surface can support that load. 2" x 5.5" would be better and would most likely be ok, maybe not. Tough to call without knowing all the facts.
Granted that the joists aren't going to shrink any in length but the 8.5" girt is going to get narrower by at least 3/32" to 1/8" which will leave even less to hold up the load of the floor above, if there is a floor above.
And if you do use the FOHC beams it could distort more and move enough for a joist to drop out of a 1" floor joist pocket.
If it was me, I'd cut the joist as shown in the drawing on side B for the free of heart center beams.
But when inspecting girts for use you should look at the crown and make sure that it is up and then you can look at grain patterns for which face. If the girt is an outside wall girt then it will be carrying less load as it will only have joists on one side and then the joist pocket can be cut deeper into it.
If the girt has joists from both sides you'll need to review your design and check your load before you decide how deep into the girt your joist pockets should be cut.

Good luck with your project.

Jim Rogers

Thanks Jim and Mark for your advice and points to consider.

Initially, I was also very skeptical and I remember having this specific conversation with my structural engineer (about the 1" pocket) so I know the load calculations have been done.

This was especially what I was looking for:

Thanks again for the help.

-Steve

nvh-srjones,

Good question. I believe you are referring to the orientation of the annual rings, not the orientation of the grain.

The grain of wood is the direction the wood fibers are running. This direction is more or less lengthwise up the tree or down the timber. Slope of grain is a way to measure the deviation of the grain from the timber or board's long axis.

Grain is often confused with figure which is the pattern that the growth rings make on the face of a timber or board as they surface and re-submerge.

Now to the heart of your question. Does the orientation of the growth rings relative to some load affect the strength properties of the timber?

In a species that is very prone to shake (the seperation of the wood fibers along an annual ring), I think that this might be important to consider, as a bad case of the shakes might pop a lot of joists.

I don't know about Doug fir off the top of my head as we don't use it. Maybe someone familiar with the characteristics of Doug fir could give their 2 cents.

Gabel

For what it is worth, the bearing of a Large span header (6 ft or more) needs a double cripple (jack stud I believe the Yanks call it)under each end to give a three inch bearing, so for a floor joist, more bearing surface would be better as well...

Thanks Gabel and everyone.

Yes, you're absolutely correct, I'm confusing grain with the annual growth ring orientation.