After much reading, I have to admit I still don't understand the discrepancy. Perhaps we're referring to different shear values, or applying them to different (and perhaps inappropriate) situations?

From what I can tell, "Fv" = the NDS "Design Value for Shear Parallel to Grain"; Fv seems to be used to determine whether the "actual" (parallel, horizontal) shear stress within a horizontal beam is allowable, or not. You would think that Fv should equal the "Shear Parallel to Grain", yet Fv is about 10- or 20-fold lower, on the order of 100 psi.

By contrast, the tabulated values for "Shear Parallel to Grain", for Black Locust, are 1,760 psi [green] and 2,480 psi [12% moisture content]. These values do not vary more than about 2- or 3-fold, with locust being one of the highest, and some cedars & firs being the lowest, ~600 psi.

The *actual* beam shear stress, "fv", is calculated according to a standard formula. In a rectangular beam, for instance, fv is based on the (vertical) shear force P (e.g., half the load if the beam is supported only at the ends) and the beam breadth & height, b & h: fv = 3P/2bh. fv less than Fv is OK; fv greater than > Fv is bad, signalling possible failure.

I have not been able to find exactly how the low NDS Design Fv values are determined or estimated. One document suggested that Fv = 40 + 266*G, where G is the wood specific gravity. Another, from the USDA Forest Service, says:
(My locust has essentially no checking; it's solid like a rock.)

Does anyone know how the Design Fv is really determined, or why it is so much lower than the measured (literally, by shearing wood blocks) Shear Parallel to Grain value?

Or are Roger and I just mixing apples & oranges, and the shear I originally was worried about (e.g., a mortise giving way) is not the same shear as what the NDS is talking about (because it's assuming worst-case-scenario beams)?

Thanks,
David