Pile Cap Design - One way shear using ACI 318-19: More specifically corner pile one-way shear check

[GA_Engineer]

I have seen a few threads on here regarding the new one-way shear (reduced) capacity for slabs and pile caps in ACI 318-19. This change is seemingly only now coming to the forefront because more municipalities are adopting the 2021 IBC. I have not seen a thread yet regarding one-way shear check a pile cap and more specifically the corner pile. Most design programs use the CRSI equations for one-way shear of pile caps and have now input the new one-way shear equation into the old CRSI equation. This is seemingly making pile caps slightly thicker, which isn't a huge deal. However, when checking the corner pile one-way shear check, can you use the modified CRSI equation since your d>w or since you really aren't engaging any other piles you can't use the CRSI equations and need to just use the new ACI 318-19 one-way shear equation? If the latter is the case, most pile caps that have ever been designed using old code and are designed such that you are close to maxing out your pile capacity now suddenly fail. With this new size effect factor and the fact pile caps have substantial "d" values (as compared to slabs), the capacity of this corner pile is greatly reduced.

Personally, when I design pile caps, I just determine the design for a maximum PC-2, PC-3, PC-4, etc (i.e. maximum load in piles)... and then just make sure my gravity column loads are less than the maximum allowable load for each of my PC2, PC3, PC4, etc... design (I will input a load factor for the strength design). I mean this was the process that CRSI uses. In looking at a program like TEDDS, when I input the load to max out my pile loads using ACI 318-14 I get a reasonable sized PC-4 based on CRSI manuals and just practical experience:

I change the code to ACI 318-19 and nothing works, especially the corner pile one-way shear. I can live with making the pile cap slightly deeper due to reduced shear in the old CRSI equation, but if I want my piles to have maximum load, there is no possible way to get one-way corner shear to work if I just use the new ACI 318-19 equation. My capacity has been reduced by over 50% just by changing code. So does this mean a whole lot of pile caps out in the field right now, across the country don't work (rhetorical question)?

 

[GA_Engineer]

As a follow up, could you interpret the code that a pile cap is a two-way isolated footing or a mat foundation (if it is large pile cap for a shear wall for instance

 

[GA_Engineer]

Bumping this thread to see if anyone will respond. I got back into this project and am essentially getting that one-way corner pile shear will never work using ACI 318-19 with anything reasonable depth.

 

[bones206]

I change the code to ACI 318-19 and nothing works, especially the corner pile one-way shear. I can live with making the pile cap slightly deeper due to reduced shear in the old CRSI equation, but if I want my piles to have maximum load, there is no possible way to get one-way corner shear to work if I just use the new ACI 318-19 equation.

So are you saying corner shear is now governing the thickness for the same pile cap layouts where it did not govern previously?

 

[Celt83]

The new shear provisions are really not good, I’m surprised more folks haven’t looked at this stuff yet and inundated the 318 committee with backlash.

The size effect factor applies to pile caps because they are not continuously supported elements.

Spread foundations will need an increase in F’c and even then will still need to be 3-10” thicker than foundations designed using 318-14.

Basement walls balloon in thickness

Retaining walls balloon in thickness with some larger infrastructure walls being unattainable.

Pretty much any element that doesn’t have stirrups in it takes a massive hit to the tune of 60-70% reduction in capacity between 318-14 and 318-19.

Many design firms will see a drop in design efficiency as decades of in service design tables are rendered useless by the new provisions.

 

[GA_Engineer]

@bones206 (sorry I don't know how to quote your post)yes in looking at calculations it seems that way that one-way corner pile shear is controlling in some cases. Based on the good old CRSI Pile Cap Design equations you can get "deep beam" shear action for the one-way shear at the face of the column. But for the corner shear you would have to use the ACI one-way beam shear equations (at least that is how most programs check it and how I've always done it) and with the new equations utilizing the size effect factor, there is a great reduction in this capacity, so much so that it can control. I never had issues before with the corner pile one-way shear check because the good old 2*sqrt(f'c)*b*d was always good enough for just the capacity of a single pile.

 

[GA_Engineer]

@celt83 I reached out to ACI to ask if the size effect factor can be neglected in pile caps and their response was no it can not but in simple one-way and two-spread footings that bear on soil you can indeed neglect the factor so spread footings shouldn't be affected too much but the new equations for shear without shear reinforcement still decrease the capacity. ACI just published the following excerpt in the October 2023 edition of Concrete International. I was told by ACI that this question (and questions about one way shear in ACI 318-19) are on the rise.

 

[Celt83]

From the quick numbers I’ve run on things the size effect factor isn’t causing most of the reduction instead I’m seeing the rho^1/3 term providing the most significant reduction so spread foundations experience about a 40-60% reduction in shear capacity even though the size effect factor is 1 for these elements. This was for slabs, walls, foundations so elements that wouldn’t normally be exceptionally thick.

With a rho = 0.002, that essentially results in a 1/8 reduction meaning, for elements where it applies, before the size effect you are starting at 50% less capacity than 318-14 for elements where 0.002 would provide As,min.

 

[GA_Engineer]

@Celt83, You are indeed correct after I looked into it a bit more. While the size effect factor does reduce capacity, the new rho term has a much larger effect. In looking at the numbers from the situation in my first post if I were to see the size effect factor to 1, I still have roughly a 40% decrease in capacity just from the new equation utilizing rho^1/3.

 

[bones206]

If you search around, there have been a handful of similar posts regarding the new shear provisions. There was one where somebody mentioned having lunch with SK Gosh, who apparently mentioned they were trying to do something to address the shear provisions for the next ACI code cycle.

I did a semi-deep dive on this last night, and read this paper that describes the whole effort that went into the new provisions:

https://www.concrete.org/publications/internationalconcreteabstractsportal.aspx?m=details&i=51716739

I can see from the paper what they were trying to do with the research, trying to increase the reliability of the one-way shear design equations based on a large database of test results. However, I think the focus was on beams and they perhaps failed to appreciate the impact to other structural member types. Maybe just a little myopic in their rush to get this work incorporated into the 2019 code.

Here is another link for some of the testing that went into the new provisions:

https://www.nrc.gov/docs/ML1926/ML19262F141.pdf