Shear Friction Development (Rehashed)

[DCBII]

Thoughts I've been pondering:

1. Shear-friction reinforcement must be developed for f_y on both sides of a shear plane.
2. Minimum bar cover in CMU is typically 2.25" (1.25" face shell + 1/2" grout space).
3. Minimum development lengths for Grade 60 bars in CMU are as follows (f'_m = 2000 psi assumed):

a. #4 bar: 0.13d_b²f_yγ/(K√f'_m)-13d_b = 12.9"​

b. #3 bar: 12" (code minimum l_d) - 13d_b = 6.0"​

4. Concrete diaphragms typically utilize shear friction to transfer loads to shear walls (see ACI 318-14 Fig. R12.5.3.7)
5. Anchor bolt connections to shear walls can provide only a fraction of even the thinnest concrete diaphragm's shear strength. If you doubt that run the numbers.
6. From point 3 above, the only bar that can be developed from a diaphragm into a 12" CMU shear wall is a #3.

Seems like concrete diaphragms and CMU shear walls don't mix. Open for discussion... I'll start:

Does wrapping the dowel hook around horizontal reinforcement count for anything? I know it's been discussed to death, but do we really need f_y? There's still tensile strain in the bar, and hence compression and corresponding friction across the shear plane if we're at say 75% of f_y.

 

[KootK]

Based on a strict reading of the standards, I agree with your assessment of things.

Does wrapping the dowel hook around horizontal reinforcement count for anything?

I would think that it does but the code doesn't give us an explicit way to account for it to my knowledge. By way of analogy, a small diameter beam stirrup may be considered to be developed at a transverse bar when it is hooked around such a bar.

I know it's been discussed to death, but do we really need fy?

Rationally, I would say that we probably do not need fy. But the code seems to make no allowance for exceptions on that and specifically directs us to develop for fy. The only mechanical justification that I can think of for requiring fy is that, owing to the irregularity of the shear transfer surface, perhaps bars in different places attract tension at different rates and, therefore, some must be able to yield while others are still ramping up. This is just pure conjecture on my part though. Who knows, maybe testing has only been done for fy developed bars at that's the motivation for the limitation.