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]

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.

 

[nashe]

Can you please refer where in code or commentary it is clearly specified

 

[Lion06]

See 7.12.1. If you used As,min for top and bottom you would get twice the required area for T & S.
Just as a side note. I would be sure that you are increasing your actual required As (from analysis) by 1/3 before using using the minimum for T & S. If your required As is less than 200/fy, you can get out of the 200/fy min by increasing the required by 1/3. If that is still less than T&S steel, you can use the T&S steel, but check the 1/3 increase first. See 10.5.3 in ACI for the 1/3 increase provision. All my references are to ACI 318-05.

 

[Taro]

nashe, you DO need the minimum 0.0018bh both top and bottom. Note that 10.5.4 requires the minimum area of TENSILE reinforcement to be the same as that required by 7.12. So if you split the 0.0018bh between the top and bottom, half of it will be in compression and will not satisfy the TENSILE reinforcement requirement. This is the interpretation that every publication I have seen (PCA notes, CRSI etc.) uses.

 

[swivel63]

it can be combined. but don't do nothing crazy like space bars at like 18" c.c., lol.

 

[miecz]

nashe

I agree with you, ACI is not clear at all on this point. I believe Structueral EIT is correct, based on the following;

ACI 350 for Environmental Structures has a similar provision, i.e., 10.5.4 refers you to Article 7.12. Article 7.12 has minimums for T/S like ACI318, but based on length between wall joints. PCA publishes a design guide based on ACI 350. Example 1 for a Single Tank bases the minimum steel on half the wall thickness.

I think the confusion arises because most design examples for pile caps do not have two laysers of reinforcement, so all the T/S reinforcement must go to the single layer.

 

[swivel63]

StructuralEIT is correct.....and i think the actual guidlines are also specified in the CRSI manual as well.

 

[Taro]

If you use half of 0.0018bh in the tension zone for slabs and footings, you may up with an UNSAFE design!

Run the numbers to prove it to yourself. 0.0018bh in one layer will provide a strength that is fairly close to the cracking strength of the plain concrete section. But using half of this amount of reinforcing will provide a strength that is much less than the plain concrete. So after the concrete cracks, it can collapse suddenly.

 

[rapt]

Taro is correct on this.

There is a minimum reionforcement requirement for strength and this is required at the tension face. It is based on the requirement that the ultimate strength of the member be greater than the cracking moment. For a rectangular section with a concrete strength of about 4500psi, this comes out to about .0018bh. The aim is to ensure that the reinfrocement does not have a sudden increase in strain above the failure strain on the onset of cracking which would result in failure of the reinforcement when the tension fore is transferred from the concrete to the reinforcement. If both faces can be in tension under different load conditions then the minimum is required at each face.

There is also a requirement for minimum shrinkage and temperature reinforcement. This can be split between the faces.

 

[swivel63]

i thought that tension face minimum requirement was the 200/fy*b*d minimum for flexure. if memory serves me correctly, .0018*b*h is something completely different (temp and shrinkage). i'll check tommorow.

 

[271828]

I vote for StrlEIT's argument and second the last post by swivel63!

The total needs to satisfy 0.0018, but tension steel needs to satisfy 200/fy*b*d (or 4/3 Asreq).

 

[271828]

Taro Typed: "...So after the concrete cracks, it can collapse suddenly."

Doesn't the 200bd/fy business take care of this?

ACI 318-02 (sorry--don't have the 05) Commentary to 10.5 seems to indicate that the 200bd/fy stuff is designed to prevent failures similar to what you're typing about.

FWIW, Nilson, Darwin, & Dolan's textbook shows the 200bd/fy check for footings.

 

[miecz]

The way 10.5.1 is written ("except as provided"), it seems as though 10.5.4 is to be used instead of 10.5.1 (and 10.5.3) for structural slabs and footings. I don't think that was the intent of the code. Both need to be checked.

 

[UcfSE]

The 200/fy check is a legacy of old codes to provide a minimum value not necessarily corresponding to providing strength greater than cracking moment.

 

[swivel63]

well, 200/fy is the minimum....but the actual minimum based on concrete strength is 3*f^.5/fy*b*d or something to that effect i believe.

 

[Dinosaur]

If it is in flexure or tension, I place at least 200/fy in each tension face. If I am just placing Temperature and Shrinkage steel I place the required amount on any face I wish to protect against T&S. Reinforcing steel is not so expensive as a crumbling piece of concrete that needs rehab. Of course, if my numbers indicate, I place more steel to accommodate the loads.

 

[mitchelon]

Ok. 10.5.4 applies to slabs and footings, and in this case pile caps as well. The minimum reinforcement of flexural members requirement is for every tension face and shall not be distributed on the cross section. Hypothetically, if you have a member with 10 sides, and these sides can all be in tension under different loading conditions, you have to apply this minimum requirement to all sides. As rapt said, “It is based on the requirement that the ultimate strength of the member be greater than the cracking moment.” Here is how it was developed:

Mcr = S*fr = (bh^2/6)*approx 600psi for fr
My = As*fy*jd
= rho*bd*fy*approx 7/8d for jd
My > Mcr
7/8*rho*bd^2*fy > (100/0.81)*bd^2
rho > 140 / fy

rounded up to 200/fy

The 3*fc^0.5/fy is similar, but instead of assuming 600 psi for fr it uses 12*fc^0.5.

 

[271828]

So after all this, what's the answer?

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

AND (whichever controls)

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

That gets my vote, LOL.

 

[271828]

Actually, maybe I should've typed "OR" instead of "AND," but I think it's clear enough.