ACI minimum area of steel 4

[nashe]

I have 3 feet deep pile cap. Due to lateral loads, I am placing rebars E.W. T&B. The Ast from analysis is less than minimum Ast = .0018 x b x h as Per ACI 318-05, Section 7.12, 10.5.4.
I am placing rebars each way both at top & bottom. Now I need this minimum Ast each way at top and bottom of pile cap.

 

[Lion06]

mitchelon-
You are right, but 10.5.3 clearly says that you can neglect these lower limits if you provide 4/3 req'd As.
That being said, as long as you are providing 4/3 req'd As at each face it is needed there is nothing wrong with combining the two faces to meet the minimum for T&S.

I agree exactly with 271828.

 

[271828]

LOL, StrlEIT, you were in the thread first, so I was agreeing with you!

 

[mitchelon]

StructuralEIT, I do not disagree. I was just explaining the reasoning behind the minimum requirement and how the equations were developed as it appears to be confusing some folks. 10.5.3 is very straightforward and I did not feel the need to explain it, but thanks for mentioning it as I can see some people thinking that the equations I described are the only ones you have to meet. I think we can close this case now.

 

[kslee1000]

All of above discussions are valid for regular beam action. However, for pile cap, I think it is more appropriate to consider deep beam action, since L/d is usually small, and reinforce accordingly.

To simplify this matter, we usually provide no less than 200/fy, or 4/3 Required As for the bottom, and T&S steel (0.0009bh, but no larger than the bottom steel) for the top and sides. The reason for wrap the cap with steel is that pile cap is one of the most critical component in the entire structural system. Once it starts to crack, the calculated capacity becomes unreliable. The problem is not easy to detect, let alone the repair.

Finally, the cost of adding a few rebars has very little impact on the over all budget. So, be conservative.

 

[miecz]

kslee1000-

Seems like your caps wouldn't meet 10.5.4 (7.12) if 4/3 Areq is less than .0009bh. Does that never happen?

 

[civilperson]

Never.

 

[Taro]

Wow. I always thought that the minimum flexural reinforcing provisions of ACI 318 were fairly straightforward. But from the discussion here, it seems that there is a great deal of confusion on the matter.

For those engineers who design slabs and footings with 0.0009bh tensile reinforcing, PLEASE run the calcs for the cracking strength of the plain concrete versus the ultimate strength of the reinforced section. I think it will be quite eye-opening. Regardless of the clarity of the code language, it is the duty of all licensed engineers to protect the safety and welfare of the public in their designs. It is well worth the few minutes it takes to run the calcs to clear up any confusion.

 

[miecz]

Taro-

Reviewing the above postings, none "design slabs and footings with 0.0009bh tensile reinforcing." All that have advocated this limit, myself included, also check 10.5.1/10.5.3 as well. Article 10.5.1 ensures that the strength of the reinforced section is greater than the cracking strength of the plain section.

 

[kslee1000]

Jmiec:

If my memory is correct, the .0018/.0009 bh is the minimum steel required for temp & shrinkage effect, which is more pronounced for structures subjected to temperature variations, and exposed to view, for which aesthetic is a primary concern. The placement of T&S bars thus is considered a practice, as opposed to the min steel required for flexural, 200/fy & 4/3 As required, which have more structural significance.

I concord, however, for pile caps and other mass concrete structures, it is quite often that T&S steel will exceed the minimum steel required for flexural, since "h" is large, and the calculated As can be quite small. However, for typical substructures with moderate thickness, either the 200/fy or 4/3 As required is capable of keeping the steel below yield, thus minimize the crack width, and prevent the crack from further propagation to cause large deformation, or losing entire section (I consider this is the main reason to provide reinforcing at all). For extreme thick/mass cast-in-place concrete placement, it is wise to check the stresses due to effect of heat, and find methods to limit the temperature differentials in between lifts and internal/external exposures, rather than just apply the min T&S steel, which does not work for mass concrete constructions. Please note that another set of rules will kick-in if corrosion & water-tight are in the concerns.

In our design, we never use T&S steel in place of min As for flexural for substructures, and no secondary reinforcing (so-called good practices)to be greater than the main reinforcement determined. However, I agree that the 4/3 rule should be applied prudently. It is useful in solving steel congestions while the concrete dimensions couln't be altered. And I wouldn't hesitate to apply it to the structures with low significance in protecting lifes and capital investments.

 

[Taro]

jmiec,

The very first response by StructuralEIT to the original post about the minimum 0.0018bh says simply "You need the minimum in each direction, but not at the top and bottom. You can combine your top and bottom (in each direction) to meet the minimum." Then there are other responses that say they agree.

I just want to make sure for the record that others who come across this web page in the future are aware that this is very, very wrong.

Also, article 10.5.1 does not apply to slabs and footings of uniform thickness, so most engineers are not checking that. If they did, it would always control over 10.5.4 and there would be no issue.

 

[Lion06]

Taro-
Let me make something clear. I never said I would use 0.0009bh as a minimum for tensile reinforcement. What I did say is that if you meet the requirements of 10.5.1 and 10.5.3 there is nothing wrong with combining your top and bottom reinforcement to meet T&S requirements.
I still stand by that statement. While I may never do it, and it may not be the best practice, ACI gives no lower bound on the application of 10.5.3 (only that provided As=4/3(req'd As).
Please tell me how this is very, very wrong.

 

[miecz]

Taro-

Ah, we get to the crux of the problem. It all has to do with article 10.5.1, and whether or not it applies to slabs and footings of uniform thickness.

I see that PCA Notes on 318 Example 22 does not check 10.5.1, and does check 10.5.4, as you say. Also, I have to agree that the code reads that 10.5.4 overrides 10.5.1 for slabs of uniform thickness.

However, I have a PCA publication that does check 10.5.1 for a slab of uniform thickness. This is Example 1 published in the PCA Publication "Rectangular Concrete Tanks."

Example 1 in the Design Guide for a Single Tank calculates the required reinforcing in a tank wall for strength, and checks the minimum according to Articles 10.5.1 and 10.5.3. Example 1 then checks the minimum steel according to 10.5.4 based on half the wall thickness.

Also, my old college textbook (Wang and Salmon) checks 10.5.1 for a cantilever retaining wall.

So, I'm admittedly confused. Do you have a reference that definitively states that Article 10.5.1 does not apply to slabs of uniform thickness?

 

[271828]

"Also, my old college textbook (Wang and Salmon) checks 10.5.1 for a cantilever retaining wall."

So does mine.

I think this is really pretty easy, so I'll re-post what I typed before. Somebody tell me what's wrong with this.

Put 200*bw*d/fy or 3*bw*d*Sqrt(f'c)/fy or 4/3Asmin on tension faces

AND SATISFY

0.0018*b*h in the cross-section, some top and some bottom if desired???

 

[Lion06]

That is EXACTLY what 271828 typed before and I still agree with it.

 

[271828]

"...If they did, it would always control over 10.5.4 and there would be no issue."

That is an interesting point. For 60 ksi steel and d approximately equal to h,

200*bw*d/fy=0.00333*bw*d which approximately equals 0.00333*b*h. This is a lot more than 0.0018*b*h.

Commentary to 10.5.1 (ACI 318-02) seems to indicate that 200*bw*d/fy is meant to prevent the brittle failure of concern here. It even states that 200/fy isn't enough for some concrete strengths, so 0.0018 is even worse.

So if one satisfies 0.0018 (forget 0.0009), which is << 0.00333, then isn't that allowing this brittle failure?

Perhaps ACI isn't too concerned about this mode for footings and slabs, with all the 2-way action, redistribution, etc., hence only making us satisfy T&S steel.

AND, no I don't have time at the moment (LOL) to go run numbers on it. Perhaps the next time I design some concrete...which will probably never happen.

 

[mitchelon]

271828,

That is correct. I think it is important to reiterate that these are two completely different requirements. 0.0018*b*h is a T&S requirement for stresses perpendicular to the main flexural reinforcement. For example, this applies to one-way slabs in the long direction. The main flexural reinforcement is governed by 200*bw*d/fy or 3*bw*d*Sqrt(f'c)/fy or 4/3Asmin. Assuming fy=60ksi and f’c=3ksi, the first equation will result in 0.0033*b*h an the second in 0.0027*b*h, which proves that their values will always be greater than 0.0018*b*h and also that the requirements are completely unrelated and are to be used for different purposes. Now, if the 4/3Asmin controls the minimum flexural reinforcement requirement, I would still verify that the minimum T&S requirement is met.

 

[Lion06]

Something I also find interesting is the 4/3 req'd As in 10.5.3 now that we have opened this discussion up a little. If 10.501 is meant to prevent brittle failure (because with rho less and 200/fy or 3F'c^0.5/fy this will be likely), what good really does 10.5.3 (if 4/3 req'd As is still less than 10.5.1) do if the max moment that can be resisted by the cracked section is still less than that resisted uncracked plain concrete section?
Presumably, once the section cracks failure will occur (if rho is less than 10.5.1. That being the case, why is 10.5.3 allowed whether it is 0.5req'd As, or req'd As, or 4req'd As?? Either way will cause the same failure mode at the same strength level.
Just a question.

 

[Taro]

StructuralEIT, I don't mean to offend you but your first response could easily lead someone to believe that 0.0009bh is an appropriate minimum reinforcing for slabs and footings. It may clear in your mind, but it's not at all clear in what you wrote.

271828, ACI 318 used to have a sentence in the commentary that explained the beam vs. slab issue quite well. It said, "The minimum reinforcement required for slabs is somewhat less than that required for beams, since an overload would be distributed laterally and a sudden failure would be less likely". In other words, a higher minimum is warranted for beams because there is no redundancy or alternate load path. They took this sentence out of the commentary in the 1995 code cycle and it appears to be causing quite a bit of confusion for younger engineers.

mitchelon, 0.0018bh is not just a T&S requirement perpendicular to the span. It also happens to be the minimum flexural tension reinforcement ratio in the direction of the span. The other minimums (200/fy, etc.) are not applicable to slabs or footings.

 

[UcfSE]

Keep in mind also that the 3 i the 3[&radic;](f'[sub]c[/sub])/f[sub]y[/sub] is very conservative for beams, more than 50% over what si required for the cracked section to be stronger than uncracked.

 

[miecz]

Taro-

ACI is causing some confusion with some of us older engineers as well. I remember when everyone, myself included, used to design slabs to 10.5.4.

I remember the commentary that you refer to. I can see how it would apply to a two way slab with a point load. But it never made sense to apply the redundancy line of reasoning to a cantilever retaining wall, a tank wall, or a footing on piles. In these cases, the bending moment is uniform across the entire section, and there is no alternate load path.

Besides that, codes change. Perhaps the commentary verbiage was purposely changed. I know this much. I have two current PCA publications that disagree on this point.