ACI 319-19 One way shear provisions for walls and slabs 1

[manstrom]

There have been a few posts on this, but I don't think this has been properly addressed by the ACI.

In the ACI 318-19, there is a new table for calculating one way shear. Table 22.5.5.1 shows three formulas.

Formula (a) and (b) simplify to the familiar equation phi*2*sqrt(f'c)*b*d times a new shape factor of lambda. This equation applies where Av min is met.

Formula (c) is the newer formula which applies where Av min is not provided. This utilizes rho. Assuming 0.0018 for rho, this simplifies to phi*1*sqrt(f'c)*b*d. Essentially half.

In previous versions of the code, there were exceptions to Av min for slabs, walls, wide beams etc. The intention is that Av min applies to beams, but walls and slabs are wider and can redistribute shear. My concern is that if I use formula (c) for one way shear (since I dont have stirrups in my walls), this is greatly impacting the design of isolated footings, laterally loaded basement walls, heavily loaded podium slabs.

In code revision summaries, they address the shape factor as a revision, but don't say "by the way we are cutting one way shear in half unless you put stirrups in your footings and walls"

So it Av min actually required in walls and footings? Table 9.6.3.1 is "Cases where Av Min is not required" which includes beams integral with a slab (but not a slab), shallow beams (but not walls) and wide beams (but not footings). The slab, wall and footing chapters refer directly to Chapter 22 and do not state a requirement for Av min for their respective elements.

Even so, if Table 9.6.3.1 says Av min is not required, my design is penalized in Table 22.5.5.1 for having met Av min. The following Technical question seems to address this.

An ACI Technical question doesn't address walls and footings (I think they misunderstood the question)

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

A previous thread for reference.

https://www.eng-tips.com/viewthread.cfm?qid=499654

Long winded. My question is - how are you interpreting the ACI 318-19 one way shear provisions for footings, walls, and heavily loaded slabs?

 

[Celt83]

see this thread: Link

Case(c) applies to any section that does not contain shear reinforcement - Walls, foundations, two-way slabs, one-ways slabs, Columns, etc.

 

[manstrom]

Thanks

I found that thread previously. It addressed beams, but I'm more concerned about slabs, walls, footings.

I also appreciate your quote there:

the way they reorganized 318 is like a really awful choose your own adventure book where you always die frozen in a cave

 

[Celt83]

My understanding is all of the testing was done on beam sections and the provisions extrapolated to slabs, walls, footings, etc., since those elements aren't typically found with shear reinforcement they default to the case (c) formula.

 

[ClearCalcs]

We've heard from quite a few of our users about this. We also base our calcs on the understanding that Case (c) is the one that applies for walls and footings. You do get the benefit of ignoring the size factor (lambda) for footings at least (Clause 13.2.6.2).

The typical solution we've seen has been to increase thickness, even up to doubling it. In some cases bumping up the concrete strength is good enough, but has diminishing returns since the formula is based on the square root of f'c. But still, if you always assume f'c = 2500psi but bump it up to 4500psi, it's a 50% increase in shear strength, so you can save on a fair bit of concrete.

-Laurent

http://www.ClearCalcs.com

 

[bookowski]

We can finally stop worrying about those rampant one way failures in basement walls and slabs that we're always seeing.

 

[manstrom]

I received the following from ACI

Yes, the values can be significantly different for non-beam sections that don’t provide Avmin in ACI 318-19. ACI 318 commentary provides a brief overview of the reasons for change along with several references if more in-depth information is desired. ... An On-Demand Course: Changes in One- and Two-Way Shear (ACI CODE-318-19) is also available that discusses the changes. ACI 318 is considering some changes to the shear design provisions during the current code cycle that will be forthcoming in ACI 318-25.

 

[JedClampett]

In our business (water and wastewater treatment), we design wall thickness based on shear, then reinforce based on bending. So every design we've done in the last 90 years (the time we've been in business) is now inadequate. Never mind the fact that we've never had a shear failure and our designs are working swimmingly (pun intended).

Hypothetical Case (that happens all the time): We have a basin for some kind of treatment. Client is expanding so they want an identical basin. But now, an 18-inch wall needs to be 30 inches thick. How the bleep do you explain that to an owner? Well, the code changed, so your existing wall doesn't work anymore. So our existing walls that have been up for 30 years are not adequate? Well, no....

So, what I've been doing and encouraging my cohorts to do is to put in the minimum shear reinforcing. It's very hard to calculate and I'm sure, harder to install. But a few #3 or #4 hairpins puts you exactly back where the code used to be.

I know, I could defer back to ACI 350, which hasn't caught up the ACI 318, but when it does, they're not taking that section out.

 

[JAE]

Jed,
With your 18" wall all you have to do is use #8 @ 3.6" o.c. to get your rho up to a point where Vc is 2 x sqrt(f'c). (I jest).
With ACI 350 requiring more rebar than "typical" walls the Vc value is higher than 1 x sqrt(f'c) but I don't think you use #8 @ 3.6" right?

 

[JedClampett]

With ACI 350 requiring more rebar than "typical" walls the Vc value is higher than 1 x sqrt(f'c) but I don't think you use #8 @ 3.6" right?

No, we never exceed 6 inch spacing, so it would probably be #9's at 6" If there's a case where I need to thicken the wall, I just add the shear reinforcing. For whatever reason, adding reinforcing is not noticed, but thickening a wall is a capital offense.

 

[manstrom]

I'm not sure this issue has sunk in yet at the real world level. I'm just getting into projects using IBC 21 / ACI 318-19. I believe (and hope) the intention was to have an exception for walls, footings, one way slabs since they can't really accommodate stirrups. As Jed said, how do I explain to a client that walls now need to be 50%-100% thicker than before the code changed.

What funny is that the Av min requirement is surprisingly low. For a 12" wall, Av min is 0.01 si. If you provide that, then you double your shear value. If enforced, I'm thinking about adding wire mesh as "stirrups" @ 12" o.c. in walls 6x6 W1.4 mesh running perpendicular to the plane of the wall would tick the box and be relatively inexpensive. Still, I don't think this is intended to apply.