Shear Av,min in ACI 318-19 1

[DTS419]

In ACI 318-19 Table 22.5.5.1, when would case (c) ever apply? In other words, how could Av be less than Av,min without violating 9.6.3.1?

For cases where h < 10" such that Av,min is not required and no shear reinforcing is provided (Av = 0), does Vc/2 apply if equation (c) is used to calculate Vc?

 

[Celt83]

My understanding in reviewing the examples in MNL-17(21), Case(c) applies to any section that does not contain shear reinforcement - Walls, foundations, two-way slabs, one-ways slabs, Columns, etc.

 

[CDLD]

See summary by ACI:

https://www.concrete.org/tools/frequentlyaskedquestions.aspx?faqid=910

 

[DTS419]

That document from ACI is very helpful.

Now, what about 10.6.2 that gives the familiar Vc/2 limit for Vu without shear reinforcing. However, 9.6.3 gives phi*lambda*f’c^1/2*b*d. If you use equation (c) from 22.5.5.1 to determine Vc, then you have two different limits for Vu without shear reinforcing. Do we take the lower value, or does one prevail over the other regardless of the result?

My intuition is that 10.6.2 needs to be updated to reflect the new 9.6.3 but I’d like to know how others interpret this.

 

[Celt83]

Your best bet is to probably reach out to ACI for clarification but sure looks like they keep making reference to Vc as if it was the old Φ2λ√f'c b_wd throughout multiple sections before finally defining it as something else in 22.5.5, the way they reorganized 318 is like a really awful choose your own adventure book where you always die frozen in a cave. The commentary for 9.6.3.1 might provide some clarification.

 

[CDLD]

I'm not sure I understand your question fully.

For beams, if Vu > phi*lambda*f’c^1/2*b*d, you need to provide at least minimum shear reinforcing.
For beams that meet table 9.6.3.1 you don't need shear reinforcing provided that your concrete shear strength is sufficient for Vu.

For columns, if Vu > 0.5phiVc, you need minimum shear reinforcing.
I would use the Vc calculated by 22.5.5.1 (c) here, which is the same equation as 22.5.5.1 (b)

 

[CDLD]

The 0.5phiVc is roughly equivalent to phi*lambda*f’c^1/2*b*d for beams with minimum flexural reinforcing.

Since 8(0.0018^1/3) = 0.97