Will,

Thanks!

I suppose that, in addition to the tenon, I could fit them into a 4x6x1 housing. Hopefully that will reduce the shear forces.

I'd sized the 4x9 joists to give me an l/d deflection of 1/523 (well within the 1/360 threshold). I guess I'd assumed that by limiting the deflection, I'd also be reducing the horizontal shear as well. Still trying to wrap my head around it. Thanks again for all your feedback.