Hi all,

I'm laying out tenons on floor joists (drop-in's where the 8x8 sill is fully supported and tusk tenon's where not supported) and have a question about calculating sheer stress at the joint.

The joists are 4 x 9's and I'm considering 4"x4" tenons for the drop-in's and 4x2" tenons below a 2" upper shoulder for the tusk version (with the underside tapered on all).

Looking at Ed Levin's calculations on page 72 of the TFG Timber Framing Fundamentals book, it appears that a simple 4 x 4 joist would have less shear stress on it (at the joint) than a 4 x 9 joist reduced to 4 x 4 at the tenon.

My question is this: Where is that additional force coming from when the ratio of joist depth to tenon depth increases?

Thanks in advance!

(For those of you who also frequent the Forestry Forum, you'll note that I've asked the same question there. Just trying to reach all with the question.)