T&S Steel in Plain Concrete?

[ToadJones]

If I am designing a footing and the it works according to plain concrete design in ACI 22.7, is T&S Steel still required?

My Concern is that the footing/block will crack, but T&S Steel @ 0.0018 still seems heavy.

The design is basically a ballast block sized for overturning.
4ft x 10ft in plan and around 20" thick.

 

[dik]

If a strip footing, the plan dimension is too short to worry about T&S rfg...

Dik

 

[marinaman]

I don't know if its required (provisions for plain concrete v/s r/f concrete portions of the code) but for me personally, I always provide reinforcing. Concrete is going to crack.....that's a given. When it cracks, you will only be engaging the amount of uncracked concrete that's still attached to your structure. I'm sure some folks have done it (meeting the provisions of chapter 22), but I never use unreinforced concrete.

I also think about the provisions of ACI that have been added regarding cracked concrete and the use of anchors. Not that this applies here....but that the whole basis for that addition, the way I understand it, is that concrete is definately going to crack, and as a result, this should be taken into consideration when designing anchors to concrete. From a big picture standpoint, doesn't that imply we ought to reinforce all concrete?

I guess what I'm getting around to is that its always in good practice to provide reinforcing....in my opinion. I am uncomfortable that there's even a plain concrete section in the code.

Also, the way I read ACI 318, Chapter 22, is that really, plain concrete isn't counted upon as doing much of anything structurally. In other words, the plain concrete must either 1. Be supported itself by soil or structure that will provide adequate support for the plain concrete itself. 2. Must be in compression under all load circumstances or 3. Wall or pedestals (which have additional restrictions in their own portions of chapter 22). Its my opinion, from a big picture review of chapter 22, that, we need to be reinforcing concrete. I can't think of a reason why I'd want to not reinforce something.....as the cost is small to do so.

 

[ToadJones]

I was really just trying to find a way to reasonably reinforce a section that is simply sized as a dead weight ballast very little stress.

 

[spats]

"plain concrete isn't counted upon as doing much of anything structurally"... I guess that's why the provisions are included in a document specifically "for Structural Concrete"! This sounds like a deadman, which are used all the time as temporary anchors, and normally demolished and carted off site when they are no longer required. If so, the contractor will cuss you out if you put steel in it. If it's more of a permanent application, I would have no problem designing it as plain concrete, and just throw in some minimual reinforcing to hold it together... no reason for 0.0018, in my opinion. A few bars at mid-depth in the long dimension would be appropriate. No reason for bars in the 4' direction at all. I always design my permanent strip footings as plain concrete in the short direction if they're less than about 5' wide. It takes 22" minimum just to develop a #4 bar in 3000 psi concrete.

 

[TXStructural]

ACI classifies unreinforced concrete members and members with less than minimum reinforcement as structural plain concrete. Members must comply with ACI 318 chapter 22. (as you noted)
T&S reinforcement is only required in the perpendicular direction to flexural reinforcement in reinforced concrete. In the flexural direction, other minimums apply. Neither of these would apply to under-reinforced or unreinforced.

Plain concrete should generally not be reinforced less that the ACI minimums, since doing so will likely lead to poor performance and under-reinforced concrete may behave unpredictably. The minimum ratios are selected to provide restraint crack width, but this will not be provided by very low ratios.

While the industry would love to sell you a little rebar or wire, there is no guideline which suggests this is good practice. When I was designing, I always designed with minimum reinforcement where the stresses did not demand more.
--
Maybe I should suggest that you go with the minimum for architectural concrete columns...1%. (each way, of course) <wink>

 

[hokie66]

If it is plain concrete, it has no reinforcement. But in my mind, a 4' x 10' x 20" thick footing should be reinforced.

 

[spats]

hokie66... if you go along with the principle of plain concrete, then what would the footing size have to be in your estimation to qualify to be unreinforced? ToadJones says there's "very little stress".

 

[TXStructural]

Wait, that's what I get for reading the OP on a small screen (iPad). If you are resisting overturning, do you not foresee tension where uplift could occur? How will you attach the footing to the structure above, and maintain integrity of your "ballast block" without reinforcement? If it is a steel structure with a baseplate or embed, you will probably want to provide supplemental, confining reinforcement for the anchors. In this case, even if it is not designed for flexural moments, I would recommend you use at least ACI minimums to hold cracks tight and assure adequate aggregate interlock during uplift events.

 

[hokie66]

spats,
I would not use plain concrete in a footing where the width is more than twice the thickness. No reference, that is just me.

 

[PMR06]

0.0018 each way on that footing works out to about 100# of steel. Assuming $2/lb placed, that's about $200.

Now assume each of the responses above amounted to 10 minutes of time: reading other posts, typing a response, talking about it in the office, etc. That's about 100 minutes. At a billable rate of $125/hr that's about $208.

Macro-economically speaking, it would have been cheaper to put the bars in by this point... then again I'm no economist or contractor, just a sarcastic engineer

 

[ToadJones]

PMR06-
I very much appreciate the sarcasm. I honestly do.
But imagine designing and specifying a few thousand of these blocks every year, many of which are for the same customer(s).
Now all of the sudden you've spent a million dollars.
I was trying to optimize the design.
If this was one building or foundation, I wouldn't bother to post the question.

 

[frv]

A few thousand of these gives you a few thousand opportunities for the foundations to crack. Are you willing to assume the liability of a failed foundation for an additional $200 to the owner?

 

[RFreund]

I think it is a good question regardless if it is one or a thousand. Why does it matter if it cracks, as long as it all stays together, right? Not that I'm saying I would do it, but I'd atleast like to know what is 'realistic' and what is not.


EIT

http://www.HowToEngineer.com

 

[ToadJones]

frv-
Again, trying to optimize...not just leave the steel out.

 

[PMR06]

ToadJones, I completely understand. I have encountered similar situations. In the end it's all about perception to the owner/client... the perception of savings or overdesign, regardless of the actual costs or calculation issues. I would still try my best to require the bars though. Good luck!

 

[spats]

"Why does it matter if it cracks, as long as it all stays together"... T&S steel does not assure the section will stay together if it cracks. I did a quick calc figuring cracking at 7.5 x sq.root(3000 psi) = 410 psi, and reinforcing at mid depth needs to be 2.6 sq. in., or 6#6. This is definitely more than T&S steel = 0.0018 x 48" x 20" = 1.73 in. sq. Reinforcing top & bottom actually requires more total steel (approx. 3 in. sq.). Seems to me if you want to assure "it stays together", you would make it stronger cracked than uncracked.

 

[TXStructural]

Taking a second look at ACI 318-11 22.2 "Limitation", structural plain concrete is limited to members continuously supported on soil (or similarly supported), members under compression under all loading conditions, and wall and pedestals.

Does this application meet these limitations?

 

[ToadJones]

nope....there is an OT moment and uplift.

 

[spats]

I don't necessarily think that means your situation doesn't qualify for Chapter 22. 22.7.2 talks about a plain concrete footing transfering moment to the soil, and as long as your footing doesn't lose contact, you ARE continuously supported by soil.