Min Area of Steel for Spread Footing 4

[ToadJones]

For minimum steel in spread footings I refer to section 10.5.4 of ACI.
That section refers to 7.12.2.1 which states that for Grade 60 bars rho = 0.0018.

Is this what others are using for footing design minimums?

I am checking a reinforcing design for a footing that is mostly sized for overturning/stability and the design moment for flxure is very small. So, I am left checking for minimum steel.

"d" for the footing is 33".
I get a min area of steel = 0.0018 x 33" x 12" = 0.71 in^2/ ft of width.
This would require a #8 bar 12" o.c.
the footing has #5 bars 12" o.c.

Is there a lesser AS min I can use for this case since flexure really is of no concern?

 

[msquared48]

33" seem awful deep. Are you sure some extra concrete was not added as dead load to resist overturning?

Oh, and .0018 is used for #5 and smaller, .002 for #6 and larger.

Mike McCann
MMC Engineering

 

[ToadJones]

Mike-
As I stated, The footings is basically designed purely to resist OT.

"Oh, and .0018 is used for #5 and smaller, .002 for #6 and larger. "

Where is this stated?

 

[delagina]

"Generally, pump foundations do not need reinforcing for stress requirements. Placing a nominal reinforcing mat of straight bars such as #5 at 12 inches C/C each way is
sufficient to prevent cracking of concrete surface at the top."

I got this from my guideline for pump foundation design. Weird the example is also 33" thk footing with #5 @ 12" rebar at the top.

 

[msquared48]

I am looking for the ;002 - I think it is in one of the older codes too.

But take a look at section 10.5.3 where if 1/3 more steel is provided than needed for flexure, the As minimum provisions do not apply. That is your answer.

Mike McCann
MMC Engineering

 

[ToadJones]

thanks mike!
For got about that one!

(I hate ACI Code!)

 

[msquared48]

As for the #5 and #6 comment, ignore that as I was mixing slab, wall and fy requirements. Brainfreeze I guess.

Mike McCann
MMC Engineering

 

[ishvaaag]

Just as a comment, Spain's EHE 2008 establishes the minimum ratio of reinforcement for

-foundation mats
-reinforced footings

in both X and then Y direction of a bottom mesh reinforcement as 1 or 0.9 per thousandt of the total section of concrete. See note 1 in jpg attachment. Other notes are not reproduced.

 

[ishvaaag]

Again the attachment doesn't upload, I don't know what is the matter.

 

[TXStructural]

If it is NOT a flexural member where reinforcement is required, then none is required. But it sounds like you need some flexural reinforcement, so the 0.0018 minimum must be met.

As a quick exercise, estimated cost for reinforcing in a 10'-6"x10'-6" (3" cover) footing with bars at 12" each way (at $1/lb). For #5@12" the bars would cost about $200, for #8@12" bars would be about $500.

This is really not a significant cost increase in the scheme of an entire building, even for 20 similar footings.

 

[spats]

Design as structural plain concrete using ACI 318 Section 22.7. Then throw in some reinforcing if you want to.

 

[steellion]

Msquared, if you provide 4/3 the required As for flexure, then you do not need to meet 10.5.1, but you still need to provide 0.0018bH for reinforcement for a flexural member for shrinkage and temperature reinforcement. If it is not a flexural member, then 0.0018bH does not apply, but I would always provide this minimum steel for a spread footing.

 

[ToadJones]

If designed to 22.7 do I still need T&S steel?>
I have never designed a plain concrete footing.

For T&S on a 36" deep footing (gross depth) the Min Area of Steel is still 0.0018 x 36" x 12" = 0.78 in^2/Ft which would still be #8 both ways ....but
(dumb question here)....this footing has #5 bars 12" o.c. Top and Bottom, can I count both the top and bottom mats in calculating T&S steel?

 

[steellion]

Yes, for S+T reinforcement it is total reinforcement, so you can consider top and bottom at the same time.

If it truly is a deadman-type foundation that you're using for weight, then reinforcement isn't required. I use "plain concrete" provisions for small wall footings to get around the .0018bH in the short direction.

 

[JAE]

No - the min. reinforcement required for footings (based on the chapter 7, 0.0018Ag) is for the tension side of the footing only. You can't split this into top and bottom steel.

 

[TXStructural]

R7.12.2 specifically notes that reinforcement could be placed in top and/or bottom of slabs. BUT, R10.5 discusses that for flexural members this is a minimum for tensile reinforcement.

JAE is correct that the requirement is for tension reinforcement, in the direction of the span, as stated in 10.5.4. Further, the nomenclature "As" used in 10.5 refers specifically to longitudinal tension reinforcement. If the footing is under two-way flexure, then 0.0018 would apply both ways at the bottom for gravity. The minimum would apply on the top if there is moment or uplift (not just gravity) applied to the footing.

10.5.4 does not permit the 1/3 over requirement in footings (10.5.3 applies ONLY to 10.5.1 and 10.5.2)

--
As an aside, ACI is contemplating removing the commentary from all codes, with the idea that if the code is complete and understandable, nothing more is needed. I personally dislike that idea, since there still much to learn from the commentary.

 

[ToadJones]

ACI 318 should consider a new committee; one capable of writing a decipherable code and commentary.

I checked the footing as plain concrete and it is well below moments/stress permitted by 22.7.
I guess my min steel problems go away with that, eh?

 

[bmagdalena]

This might be silly question, but how chapter 7.12 apply to footing?
Is says in several places that roh is for structural slabs.

 

[ToadJones]

section 10.5.4 refers you to 7.12.2.1

 

[steellion]

JAE, I see your point and I agree, faces where flexural reinforcement are required should have at least min. reinforcement.

TX, the CRSI Manual uses the 4/3 Flexural As provision in its design examples and tables. Requiring 0.0033bd for every footing would be heavy handed.

Toad, I agree that ACI is frustratingly indecipherable. They don't need to eliminate the commentary, they need to write a clear code. Despite the code, commentary, and equally thick PCA Notes, we still have debates on these fundamental issues.

Summary:
0.0018bH is minimum bottom steel.
Then 4/3 Flexural As <= 0.0033bd
Then Flexural As >= 0.0033bd