ACI 318-19, Table 22.5.5.1, Reduced Shear Capacity 1

[McCalf]

Local jurisdictions in my area recently started to adopt IBC 2021, so we are implementing ACI 318-19 on our projects for the first time. I have concerns about the new shear provisions. Updated equations are included in ACI 318-19, Table 22.5.5.1. At first glance it looks like the familiar shear equation (Vc = 2*sqrt(f'c)*b*d) is still effective for structural elements that do not require shear reinforcement, but the following ACI link clarifies that unless shear reinforcement is provided, the designer must default to equation (c):

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

One-way slabs, basement walls, footings, etc. are not traditionally detailed with any shear reinforcement, so it is my understanding that they must now be designed using equation (c). Is anyone else finding that the use of equation (c) yields a significantly reduced shear capacity? In the conditions I have checked, I am finding that Vc is reduced to roughly 1.0*sqrt(f'c)*b*d, or nearly half! It gets even worse for deep sections due to the size effect factor (22.5.5.1.3). Suddenly our footings, walls, and slabs will need to be nearly twice the thickness to satisfy these provisions!!

Please tell me I am interpreting this incorrectly and ACI did not just reduce the capacity of our concrete by this shocking amount! I am going to have a very difficult time explaining these increases to our clients.

 

[ClearCalcs]

It won't help for walls and slabs, but there is an exception for the size factor used in footings - see Clause 13.2.6.2 that applies for "one-way shallow foundations, two-way isolated footings, or two-way combined footings and mat foundations."

http://www.ClearCalcs.com

 

[BulbTheBuilder]

The shear capacities in ACI 318-19 (from what I have seen so far) is less than 318-14 thus requiring deeper or bigger member. I don't think you are interpreting it correctly wrongly.

 

[BulbTheBuilder]

Checkout this document

http://www.concreteros.org/uploads/2/4/0/2/24020264/aci_318-19_presentation.pdf

 

[McCalf]

It won't help for walls and slabs, but there is an exception for the size factor used in footings - see Clause 13.2.6.2 that applies for "one-way shallow foundations, two-way isolated footings, or two-way combined footings and mat foundations."

Good to know, but even after excluding the size effect factor, footing shear capacity is dramatically reduced. Consider a 12" thick footing (f'c=3000 psi) reinforced with #5 at 12"o.c. with d = 8".

ACI 318-14
V[sub]c[/sub] = 2*√(f'[sub]c[/sub])*b*d = 10516 lbs per ft

ACI 318-19
V[sub]c[/sub] = 8*(λ[sub]s[/sub])*(ρ[sub]w[/sub])[sup](1/3)[/sup]*√(f'[sub]c[/sub])*b*d
V[sub]c[/sub] = 8*1.0*(0.00323)[sup](1/3)[/sup]*√(3000)*12*8
V[sub]c[/sub] =1.18*√(3000)*12*8
V[sub]c[/sub] = 6204 lbs = 40% reduction in capacity!

λ[sub]s[/sub] = 1.0
ρ[sub]w[/sub] = A[sub]s[/sub]/bd = 0.31 in[sup]2[/sup]/(12"x8") = 0.00323

 

[McCalf]

Checkout this document

http://www.concreteros.org/uploads/2/4/0/2/2402026...

This document is very helpful. Thanks.
The example on page 309 is similar to the example I posted above. The example confirms that the one-way shear calculated using ACI 318-19 is indeed significantly reduced. I was certainly hoping there was something in the code that would exempt footings and walls from equation (c), but no such luck.

I am surprised that there are not more engineers commenting on the dramatic concrete increases that will be required to satisfy these new provisions.

 

[BulbTheBuilder]

Glad it helped. When I am designing foundations in the initial phase for "heavy" loads I tend to use ACI 318-19 just in case we'd have to go back and make revisions. If the size is way higher than I anticipated, then I revert to ACI 318-14. For foundations like crane and other massive loads I stick with ACI 318-19 regardless. There's a paper that talks about the reduction of the shears and reasons behind it....I'd have to search that online.

 

[cooperDBM]

If you're running into a problem with a low rebar ratio then you have the option of adding more reinforcing instead of concrete, which may also reduce the size effect.

 

[RWW0002]

I am surprised that there are not more engineers commenting on the dramatic concrete increases that will be required to satisfy these new provisions.

I'm betting many engineers are practicing in areas where IBC 21 has not been adopted yet.. Give it time and the complaints will come in. I think this is one issue with lagging code adoption is that by the time engineers digest the significant code changes, the code and adopted references are one or two iterations down the road, and the "ship has sailed" for feedback on changed provisions.

We should all probably be more involved in the revision process, but who has time for that??

 

[Settingsun]

Shear reinforcement is something that seems to be tightening up generally. The Australian code removed a relaxation on maximum spacing a couple of versions ago, tweaked some parameters, then switched to the modified compression theory. I hear that Eurocode always requires shear reo in beams, even when the calculated shear force is small.

There was a recent discussion here (which I didn't reply to) about whether to provide shear reinforcement when just under the limit where minimum reinforcement is required (ie half of unreinforced capacity). I would provide it in that case, otherwise any renovation/change-of-use down the track will likely be starting from a point of non-compliance with the prevailing future (more stringent) code even under the original use/loading.

 

[DaveAtkins]

This is the first time I have heard of this. Very disturbing. I played with the equation a bit, and found rho must be almost equal to rho max to get the shear capacity back up to 2*sqrt(f'c).

This will affect a lot of designs. Shear DOES control quite often in slabs and walls.

DaveAtkins

 

[McCalf]

Yes, this change is going to facilitate some interesting designs.

For example, I decided to add single-leg shear stirrups in the lower portion of my basement wall rather than double the thickness of the wall or add absurd amounts of vertical reinforcement. The wall thickness is sized for V[sub]c[/sub] shear capacity at the top. I am sure that I will get odd looks and complaints from the contractors when they review the details, but the alternative options are just as abnormal. Doubling the thickness of the wall will be impossible to explain to the architect, especially when there is currently a huge push to reduce the concrete carbon footprint.

 

[slickdeals]

Had lunch with SK Ghosh a few weeks back and he mentioned that there is a lot of activity inside the ACI 318 subcommittees to address this in ACI 318-25, but he said they may not have enough time to go through the balloting process. Let's see.