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?

 

[TXStructural]

The minimum steel requirement for footings is 10.5.4 (the 0.0018 minimum in 7.12.2.1). The minimum steel equation in 10.5.1 (eqn 10-3) does not apply directly to footings. Where subject to flexure, the intent of 10.5 is "As" tensile reinforcement, not T&S. The T&S minimums are only a surrogate.

Obviously, the code has become convoluted beyond imagination. Part of this is the result of a decision by ACI that an equation or value should not appear more than one place in the code, or in multiple codes. This has created a complicated web of references. With the -14 revision, 318 will no longer have chapters on flexure, shear, or torsion, but will be "member based" where beams, columns, slabs, etc. each have their own chapter, probably with overlapping provisions, or possibly with a clearly defined set of equations referred to as needed. I do not expect a well-oiled code, but the committee is working hard to eliminate confusion brought about by the way things are now organized.

The good news is, if it goes as planned, when you open up the "beam" chapter, you will know when you can stop designing and start building.

ACI 314 is in the process of setting up a website with flowcharts that describe the process of design. Rather than the stepwise, linear method we should be using, the process under 318-08 is a series of bifurcations and references.

 

[PUEngineer]

TXStructural...The commentary of 7.12.2 which you get to via 10.5.4 says that the reinforcement can be distributed near the top or bottom faces of the slab. I read this as footing also, since the section 7.12.2 references slabs only and not footings. Do you interpret this differently or not find it applicable?

I should point out that this is ACI 318-08, not sure if the language is exactly the same in ACI 318-05.

 

[ToadJones]

TX-
If I read 10.5.4 correctly, AS min for footings is never based on the provisions for "flexural members" rather is based solely on the provisions of 7.12.2.1

 

[ToadJones]

I put this spreadsheet together a few years ago and tried to cover all the provisions of the code that I usually run into with the notes at the bottom of the page...its never been checked.
Maybe folks here would like to take a look and offer somethign to add or point out shortcomings.

 

[ToadJones]

oops...try again

 

[ToadJones]

Wont let me upload.

 

[ToadJones]

last try

 

[bmagdalena]

I went thru my notes from concrete class, and min steel in footings was based on 10.5.1 not 10.5.4.

I agree that flexure might not be a concern, but it is present and 10.5.1 apply to min steel where tensile reinforcement is required by analysis, even if is not controlling.
so whay wouldnt min steel from 10.5.1 be used?

 

[TXStructural]

10.5 is "Minimum reinforcement of flexural members" so a footing subject to bending is required to comply with the relevant provision(s).

10.5.1 states, in part, that eqn 10-3 applies "except as provided in 10.5.2, 10.5.3, and 10.5.4..." Since 10.5.4 applies, we ignore 10.5.1.

10.5.2 does not apply to footings of uniform thickness.

10.5.3 does not apply since 10.5.1 and 10.5.2 do not apply.

10.5.4 is the provision for minimum reinforcement of footings and structural slabs (and mats and similar structures cast on ground to transmit structural loads directly to soil.) This requirement points directly and ONLY to 7.12.2.1. (0.0018, in this case)

"As" refers to longitudinal tension (flexural) reinforcement (see the page 19, Chapter 2, of 318-08). So I read 10.5.4 to say that "As, min" is in the direction of the span (consistent with the definition of "As") and also exclusively referring to flexural tension reinforcement.

R10.5.4 states "The minimum reinforcement required for slabs should be equal to the same amount as that required by 7.12.2.1 for shrinkage and temperature reinforcement." This is why I called 7.12.2.1 a surrogate. I believe that technically, the requirement is completely different, but that the numbers were the same so ACI 318 (probably at the behest of ACI Technical Activities Committee, or TAC) removed 0.0018, et al from this section and referred the user to 7.12.2.1, in an effort to avoid repeating a number which was similar. If you look carefully at 7.12.2.1, the 0.0018 applies to all sections (in all directions), but the 10.5.4 provision only applies to the longitudinal reinforcement, not transverse. Of course, if the footing is a typical two-way footing, it would apply both ways since both ways would be resisting flexure to more or less the same extent.

 

[JAE]

TXStructural did all the work - I just am posting to say I agree. Good post TXS.

 

[bmagdalena]

thanks TXStructural

 

[hokie66]

JAE,
Does this thread remind you of anything? Maybe if TXStructural had participated in that one, we would have been able to convince that NY engineer.

 

[JAE]

Yep

 

[TXStructural]

You just about have to be half lawyer to read these codes.
--

NY. Pfft. They won't listen to a Texan, since "there's nothing worth seeing west of the Hudson." (rolleyes)

My dad told me that about New Yorkers. And then I had a pilot say that to me, after he flew into an obviously severe thunderstorm in west Texas while traveling through. He flies into an isolated hail storm and then blames it on geography.

 

[apsix]

And what is wrong with spat's suggestion; 'Design as structural plain concrete using ACI 318 Section 22.7. Then throw in some reinforcing if you want to'?

If it's a stumpy pad footing that's the approach I would use.

 

[dhengr]

TX:

Gone are the good old days when you had to have a vague idea what the hell you were doing to solve an engineering problem. You needed a vague idea of how the material acted and what the basic design concepts were based on. If ACI eliminates the commentaries, then nobody will have any idea of the logic (or illogic) or rational they used in arriving at their approach to solving a given problem. And, that elimination certainly can’t be (won’t be) based on the fact that they are making the codes clearer or easier to use. So, now lets introduce as 36x48 fold out flow chart to clear that all up. The idea that you don’t want to repeat the same formula or number to many times does have some merit, particularly when they discover the printing errors in the formulas and then try to get them all corrected in the next printing. But, the idea that code section X refers you to three other sections, each of which refer you to some other sections, in a never ending circle, ultimately has you forgetting what the original problem was, in this never ending chase. Now, throw in a flow chart which runs you around in circles too and they have set up a situation where a simple design problem will never get solved, and nobody will understand what they are doing in the process either.

 

[ToadJones]

dhengr-
The way the ACI code is written flies in the face of logic.
It is ridiculous really.
I merely had to review on of thee simplest footings the world has ever seen but I have to be thorough given the client and it requires a stamp...(I wish I could name the client & State). So I journeyed deeper into the code than I usually would and it turned into something like what your post describes.
The ACI code is terrible.
If not for PCA notes it would be even worse.

 

[TXStructural]

The major revision to ACI 318 is intended to fix this. And I would disagree that it no longer requires an engineer. That said, 318 removes certain "first principles" from everyday engineering design. It is efficient to use load and resistance factors, rather than determining material reliability and factors of safety on your own. The "complex" simplifications, such as development length, are intended to make uniform the mundane details which are required for safe buildings and do not need to be examined each time you design.

The complications arose over time, as each code change cycle added a variable, and another paragraph of "except when..." Hopefully, this time they reset those hideous provisions in the name of unifying the code provisions relevant to each member.

318-14 should not require a flowchart to use, which is the point.

 

[hokie66]

I have an idea...let's reinstate ACI318-63. We never had much problem with interpretation, and the structures designed with that code seemed to do just fine. 6"x9", about 5/16" thick. Add in the beam torsion provisions, and voila, a workable code, with both WSD and USD. But ACI318-08, in my opinion, is not unworkable. For concrete folks like me, it is just a progression on previous codes. The codes that I find hard to follow are the steel codes, of all countries, I think.

 

[steellion]

Being a younger engineer, I envy those that worked at a time when the code was actually based on reason and engineering mechanics, rather than the endless array of arbitrary factors that every code (not just ACI) has become.