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.

 

[miecz]

UcfSE-

I don't get your point. Could you elaborate?

 

[Lion06]

Taro-
No offense taken, but it is simply my interpretation of the code. Based on everything I and several others have written in previous posts, you haven't changed my mind yet. I don't disagree that it probably isn't good practice, but I still don't see the code disallowing it. 271828's post lays it out pretty well.

 

[271828]

So as jmiec pointed about about 15 posts ago, it seems that the issue is whether 200*bw*d/fy applies to footings and slabs.

If not, then 0.0018*b*h becomes the only lower bound so needs to exist at any tension face like taro argued.

I think I could go for taro's argument and just provide 0.0018 at tension faces and forget 200*bw*d/fy if not for its inclusion in my textbook's footing example and the wall examples others typed about. Perhaps these guys are just wrong--examples writers mess up frequently.

For now, I think I'd put 200*bw*d/fy on tension faces and be done with it (0.0018 automatically satisfied if 200/fy is). I haven't designed a footing in a couple of years, but I always used that as a limit and the steel didn't get out of control.

Anyway, unless somebody has a definitive source that states that 200/fy doesn't apply to footings, that's what I'm gonna assume is the best thing to do.

 

[UcfSE]

If you derive the equation for minimum rho, you get less than the code minimum for a rectangular beam by quite a bit. This is a very long thread about something that really shouldn't be so long, IMHO. I've always thought the code was pretty straight-forward about minimum steel.

After reading through it I thought I would mention what's in my earlier post. Code should not be taken as gospel though we probably all know some engineers who do that, but should be understood to really get the intent of the what and the when, when there is some confusion as to what to do and whether more stringent values should be applied just to be safe. I'd go with 271828 and Taro on this.

 

[swivel63]

same here...

ACI and CRSI are pretty much in line with each other.

 

[271828]

Sorry to regurgitate the thread, but I talked to a concrete design professor about this subject today. She's been around a while and is active in ACI, so I think she'd know.

She told me that the 200/fy stuff does not apply to slabs and footings, period. 0.0018 is it.

Due to redistribution, 2-way action, etc., ensuring that the cracking moment doesn't greatly exceed the reinforced moment just isn't important for slabs and footings. This aspect as historically been neglected and horrible things haven't happened.

So if I ever design concrete again (doubtful, LOL), that's what I'm gonna do. Oh the steel I've wasted by also using 200/fy.

 

[haynewp]

That is right. And the 0.0018bh can be placed all in the middle of the slab because it is temperature-shrinkage and not minimum flexural.

But for a lateral resisting pile cap, I would not go with the 0.0018 and I would have reinforcing at both the top and bottom each way.

 

[Taro]

Once again (and hopefully for the last time)...

The 0.0018 IS minimum flexural reinforcing for slabs. It cannot be placed in the middle of the slab. It cannot be placed half on top and half on bottom. All of it MUST be placed near the tension face.

Only for one-way slabs in the direction perpendicular to the span can you place some or all of the 0.0018 outside of the tension zone. It is unfortunate that ACI uses the same value of 0.0018 for both T&S and minimum flexural reinforcing because a lot of people are getting confused here.

 

[Lion06]

When I was in school, my professors always made us ensure that phiMn>Mcr. This made sense to me and still does for the reasons stated by others earlier in this post.
When I started at my first job, I asked a few P.E.'s in the office where this provision was in teh code. No one knew and hadn't really heard of it. One of these guys has a master's from MIT, mind you. I did a little code searching myself and couldn't find anything that required phiMn>Mcr.
Is this in the code anywhere or is it just good practice? Also, if the 200/fy min is meant to ensure this very thing, why are you allowed to be less than that if you use 4/3 Req'd As? It is certainly within the realm of possibilities that you could have phiMn<Mcr if you use this provision. Once you are into this area, why wouldn't the code point you to the chapter on plain concrete?
Is it possible that in reality this only occurs (outside of very rare instances) in footings because the thickness is based on shears rather than what is needed for flexural capcaities?

 

[swivel63]

this is verbatim CRSI chapter 13

"the requirement for minimum areas of flexural reinforcement (ACI 10.5 and 7.12) have been satisfied by the following conservative interpretation, where As is the calculated area for flexure;

if As > 200/fy, use As
if As < 200/fy < 4/3As, use 200/fy
if .0018bh < 4/3As < 200/fy, use 4/3As
if 4/3As < .0018bh < 200/fy use .0018bh"

 

[haynewp]

Per ACI 13.3, "Area of reinforcement in each direction for 2 way slabs systems shall be determined from moments at critical sections, but shall not ne less than required by 7.12"

And 7.12 is all about temperature and shrinkage reinf. which lists 0.0018bh.


Where does it say that the 0.0018bh reinforcement is actually a flexural minimum and it has to go near the tennsion face and cannot be placed at the center of the slab (As long as it still meets the moment capacity and deflection requirements when placed at the slab center.)

 

[271828]

Taro, so my very experienced, and active in ACI, concrete professor at a very reputable engineering school is confused?

 

[miecz]

271828-

I don't see exactly how your prof and Taro disagree.

swivel63-

I don't think a "conservative interpretation" is gonna carry much weight here...

haynewp-

10.5.4.

 

[haynewp]

jmiec,

I saw 10.5.4 also, but I thought that was referring to one way slabs (under the "flexural members" heading)?

I was under the impression you are given more engineering leeway with 2 way slabs (as an example, provision 13.5).

 

[271828]

We only disagree at this point on the need to have a reinforced phiMn exceeding the unreinforced phiMn. My understanding is that we simply should not give a rip about this for slabs and footings.

It's temperature & shrinkage only. It does not all have to go on the tension face.

 

[271828]

jmiec typed: "swivel63-

I don't think a "conservative interpretation" is gonna carry much weight here..."

LOLOL! That's the truth.

 

[swivel63]

hey, don't shoot the messenger

LOL

like i said, that's verbatim CRSI

i hope you guys realize that one or two more bars is really gonna cost about.......1 buck.

 

[swivel63]

ok, so how does this apply to pile caps....particularly those that have both downward and upward forces on them? the tension face would be at both the top and bottom for varying load cases. do we apply .0018bh to both? effectivly doubling the T & S?

 

[Taro]

271828, I seem to be saying the exact same thing your professor is saying. 0.0018bh is the minimum tensile reinforcing for slabs and footings of uniform thickness. This amount of reinforcing produces a strength that is approximately the same as the cracking strength. Because there is usually more redundancy in a slab than a beam, this is a sufficient amount of reinforcing and the higher reinforcing ratios (200/fy, etc.) are not required. It is NOT just temperature and shrinkage steel. It DOES all have to go on the tension face.

swivel63, yes. If there is tension at top and bottom, you need to provide 0.0018bh at top and 0.0018bh at bottom.

My head hurts. Anyone have an aspirin?

 

[haynewp]

I checked my concrete text and it calls the 0.0018 "flexural reinf. for 2 way slabs" so I think Taro is right from the beginning. ACI calls it temperature- shrinkage and then references it again in the flexural provisions. Ibuprofen is better than aspirin.