Floor Joist Housings

[Rougespear]

In my plans I have floor joists spanning two tie beams.  One end of the joist is pocketed into the reference face of a tie beam, but the other end joins to a non-reference face of the other tie beam.  When cutting the joist pocket in a non-reference face, do I simply layout the housing from the reference face to account for any discrepancies on the non-reference side of the tie beam?  I'm assuming it would be silly to cut housings for all loft joists...

[Jim_Rogers]

The distance from one reference face on one tie beam to the reference face on another tie beam should be a constant dimension. So if your joist pockets are to be 2" deep into a 8x8 then you lay them out 6" off the reference face on the tie beam where they enter the non reference face. And on the tie beam where they are entering the reference face you lay that out two inches deep.  This should give you a constant dimension of your floor joist lengths.



 

Hope this helps.

Jim Rogers

[Rougespear]

Thanks for confirming that Jim.

On an 8" wide joint, is the common joist housing depth 2"?  Are there relational "rules of thumb" on sizing these pockets?

[Jim_Rogers]

It all depends on the load. One of my structural engineers likes 2" pockets but the load and joint should be looked at for sure.

When I look at it, I look at the total load on the joist. Then divide that by two as half goes to each end.
Next I look at the compression perpendicular to the grain. This value will determine how much surface area is needed to hold up the floor joist.

For example, in my above drawing the floor joist is a 5x8 and it's pocket is 2" wide. So the surface area of the side of the joist is 2" x 5" or 10 square inches.

If it was eastern white pine the compression perpendicular to the grain would be 350 lbs per square inch. So 10 square inches could hold up 3500 lbs.

In the above frame the tie beams are 12' from outside to center bent. So the floor joist unsupported span is 136 inches with the joist being 24" on center. The floor load is expected to be 60 lbs per sqft.
That puts a load on the joist of 1360 lbs and each end at 680. This tells me that two inches is plenty.

We could go with 1" but then you run the risk of the joist dropping out if shrinkage is a lot. It shouldn't be but I wouldn't want to run that risk. 1 1/2" would work, even 1 1/4" but two inch is nice.
Each floor joist should be secured with at least two timberlok screws with 2" of thread embedded into the tie.

Understanding the load on the tie beam is important and using the "net cross section" is important to make sure that what's left after the floor joist pockets are cut out is enough tie to hold up the loads.

At one of the Timber Framers Guild conferences the engineering council did some "joint busting". The joint they tested were two tie beams with a floor joist spanning between them. The pulled down on the floor joist to see what or where the joint would fail. They wanted to see if the wood below the floor joist pocket would "blow out" or what.
Almost all of the joints that I watched fail, failed in the exact same manner. The floor joist itself split and failed before the wood under the pocket blew out. It was a very interesting experiment.

The engineering council is still working up some guidelines for depths of joist pockets into tie beam, I believe. If they have published some info on this I may have missed that. I could check and see.

The rule of thumb that I was taught at an engineering course is shown in this picture:



 

This shows how deep the joist pocket should be based on the size of the tie beam and the joist itself.

Jim Rogers

[Heartwood]

The rules of thumb Jim mentions at the end of the last post may work for handling the shear in both members around the joint, but they ignore the bending stresses on the beam. As Jim says, the net cross section is greatly reduced with the pockets and the beam strength in bending must be considered. This calculation gets a bit complicated because a new centroid for the moment of inertia must be determined. I usually do 2" drop in pockets since 3" takes too much wood out, especially if there are joists on both sides. The building code requires 1-1/2" of minimum bearing for wooden joists, so I interpolate that to timbers and wouldn't do 1" even if shrinkage wasn't an issue, athough the engineers at the Guild's recent Schuylerville project OK'd 1" housings for joists. The safe way to do this without consulting an engineer or learning to do the calc is to make sure the beam section left between the back of the joist pockets is the size required for the load. Or use soffit/tusk tenons instead of drop-ins.

[Rougespear]

Thanks guys... very informative!  

On a related note (while mentioning timberlock screws) : my tie beams would span 18' carrying a loft above, and I was considering the need to have one of the central loft joists using a tying joint to keep the tiebeams from spreading apart from one another.  Now that I consider it further, if screws are used to secure every joist in its pocket, does this negate the need for a central tying joint?

[Heartwood]

I would say so. By the way, another good solution to avoid weakening the tie beam is to rest the joists on top of the beam rather than pocketing them in, or drop them only part way down. Increases height of frame, tho.

[Don P]

The safe way to do this without consulting an engineer or learning to do the calc is to make sure the beam section left between the back of the joist pockets is the size required for the load.

I remember from a presentation I think Dick Schmidt gave about 10 years ago, that was my takeaway rule of thumb for the beam.

[routestep]

I have to say that soffits are what I use for floor joists when I can. Love them.

[S.Hyland]

I saw another timber framer who used only a 1/2" housing for his joists. No tenon or joinery whatsoever, just a 1/2" housing. And this was in green hemlock. It scared the living heck out of me.