Taro:
The confusion is that I am getting apples and oranges. To be specific, let us use the following numbers as an example: (again, using the 3rd edition of LRFD)
bolt diam d = 0.625 nominal area Ab = 0.307 phi = 0.75
Fnt = 90 Fnv = 48 (from Table J3.2)
(As an aside, I don't quite understand why the full shaft area, Ab, is use to compute shear stress instead of the root area if the threads are in the shear plane. But, that's what the equations call for)
NOTE: instead of following these numbers here, you can also download the attached MathCad sheet and see the work.
Factored tension load Tu = 7k, shear Vu = 6k
Tensile stress = ft1 = Tu/Ab = 22.82 ksi
Shear stress = fv = Vu/Ab = 19.56 ksi
From the interaction equation:
ft = phi*sqrt[Fnt^2-(Fnt/phi*Fnv)^2*fv^2] (Fig C-J3.3 from Page 16.1-244)
The result of this is ft = 56.67 ksi
Now, going to Table J5.3, Page 16.1-65:
Ft = 117-2.5*fv <90 (with the threads in the shear plane)
Ft = 68.1 ksi, and phi*Ft = 51.1 ksi
If this difference is the result of the Ft equation being a 3-line approximation of the elliptical equation, then that would explain it. That being the case, then using this lower Ft value would be a bit more conservative.
A Unity check of this would be (ft1/phi*Ft)+(fv/phi*Fnv)<1
which resolves to 0.990
Now, let us fast forward to the specifications for A325 bolts, Section 5.2 on Page 16.4-32 where we have a combined shear and tension unity expression for ulitmate limit state (Equation 5.2).
Here, Tu & Vu are the applied loads (same as above) and Rn is based on the same relative Fnt and Fnv values.
phi*Rnt = phi*76.56*Ab = 17.63k phi*Rnv = phi*48*Ab = 11.05
The ratios become:
Tu/phi*Rnt = 0.403 Vu/phi*Rnv = 0.543
Now if these are added linearly, we have 0.946, which would be just slightly less conservative than the 0.99 figure as above.
HOWEVER, Equation 5.2 says that each of these quantities is to be SQUARED, then added. That produces a sum of 0.457, about half of the other sum.
So, to conclude this rather long story, my confusion is the inconsistency between two interaction equations that should yield the same result. Unless it is related to the fact that A325's are supposed to be pretensioned, it would almost lead one to believe that Eq 5.2 is a misprint.