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Min Area of Steel for Spread Footing
4

Min Area of Steel for Spread Footing

Min Area of Steel for Spread Footing

(OP)
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?  

RE: Min Area of Steel for Spread Footing

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

 

RE: Min Area of Steel for Spread Footing

(OP)
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?

RE: Min Area of Steel for Spread Footing

"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.

RE: Min Area of Steel for Spread Footing

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

 

RE: Min Area of Steel for Spread Footing

(OP)
thanks mike!
For got about that one!

(I hate ACI Code!)  

RE: Min Area of Steel for Spread Footing

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

 

RE: Min Area of Steel for Spread Footing

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.

RE: Min Area of Steel for Spread Footing

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

RE: Min Area of Steel for Spread Footing

3
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.

RE: Min Area of Steel for Spread Footing

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

RE: Min Area of Steel for Spread Footing

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.

RE: Min Area of Steel for Spread Footing

(OP)
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?  

RE: Min Area of Steel for Spread Footing

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.

RE: Min Area of Steel for Spread Footing

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.
 

RE: Min Area of Steel for Spread Footing

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.

RE: Min Area of Steel for Spread Footing

(OP)
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?  

RE: Min Area of Steel for Spread Footing

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

RE: Min Area of Steel for Spread Footing

(OP)
section 10.5.4 refers you to 7.12.2.1

RE: Min Area of Steel for Spread Footing

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

RE: Min Area of Steel for Spread Footing

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.

RE: Min Area of Steel for Spread Footing

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.

RE: Min Area of Steel for Spread Footing

(OP)
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

RE: Min Area of Steel for Spread Footing

(OP)
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.  

RE: Min Area of Steel for Spread Footing

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?
 

RE: Min Area of Steel for Spread Footing

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.
 

RE: Min Area of Steel for Spread Footing

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

RE: Min Area of Steel for Spread Footing

thanks TXStructural

RE: Min Area of Steel for Spread Footing

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.

RE: Min Area of Steel for Spread Footing

Yep  smile

 

RE: Min Area of Steel for Spread Footing

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.

RE: Min Area of Steel for Spread Footing

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.

RE: Min Area of Steel for Spread Footing

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.

RE: Min Area of Steel for Spread Footing

(OP)
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.  

RE: Min Area of Steel for Spread Footing

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.

 

RE: Min Area of Steel for Spread Footing

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.

RE: Min Area of Steel for Spread Footing

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.

RE: Min Area of Steel for Spread Footing

Don't lose faith in building codes.  They are complicated for reasons, even if they may have strayed from the simple formula of "require only the minimum."  Many complications can be avoided by simply going with the conservative solution and simple design.  As with any design, if you get bogged down in the irrelevant minutia, you may never get a working design completed.  Things like lap length can be simplified by using, for example, Class B, top bar Ld for the weakest concrete on the project.  Alternatively, you can complicate/optimize lap splices by applying the available factors.

Minimum reinforcement provisions discussed here are not really all that complicated, it's just the layout and location of the provisions.  JAE pointed out that the minimum applies to tensile reinforcement, and while it appears in the code,  that's based on the principle that if you need flexural reinforcement, you probably need some minimum amount to make it effective.  Rather than get into the mechanics of what happens when concrete cracks under tension, the code simply says "put in this minimum amount."

RE: Min Area of Steel for Spread Footing

TX:

I agree with Hokie.  The ACI318-63 code is the one he and I both learned to use first, and at that time everyone pretty well understood the intent, rational, and the fundamental engineering principles and materials considerations involved in the design process, we were taught that.  And, the next couple editions where pretty straight forward extensions of the previous editions, given what we had learned over the intervening years.  Occasionally there was a free or inexpensive addendum because a significant issue came to light.  We also read the tech. literature to keep abreast of the latest research and thinking.  Without the need for a full blown code revision, every few years, these new ideas, once vetted, could be applied at our (the engineer's) discretion.   It wasn't until code writing became a cottage industry unto itself, and that publishing frequently to support that industry and to provide full employment for some, became an end in itself, that things got out of hand for the practicing engineers.  For all the confusion and complexity, extra expense of new codes and standards, we are not really producing better structures, nor are old structures falling down around us for lack of this complexity having been applied.

I also agree with the type of logic expressed in your last paragraph 'Minimum reinforcement provisions...., so put in this minimum amount.'  We should be teaching this kind of fundamental thinking and logic (common sense) to young engineers, rather than adding another 35 pages of disjointed B.S. to the code to try to cover that; one section referring to four other sections, which refer to...., etc., and then around again, to get to the same logical solution.  The problem with all the codes is that their complexity, arbitrary new factors and variables, use of other codes and standards by reference, pretty much baffles any logic, common sense or engineering judgement and experience factors.  We spend all our time, when trying to use the codes trying to be sure that we have applied and understand ever new phe, phi, pho & phum, and literally end up losing sight of the real engineering principles involved in the solution of the problem.  I suspect many engineers will attest to this, we aren't doing engineering any longer, we are just following an ever more complex set of cookbooks, and then debating how much a pinch of salt really is, without knowing what the hell we are actually cooking.

While codes may be "complicated for reasons," which are not always obvious to the experienced engineer any longer, they certainly have strayed from their intended purpose in their development and that process.  The best codes are the cleanest, simplest codes possible, and that should be the code writers foremost duty and focus; so, clean up "the layout and location of the provisions," and get rid of "the irrelevant minutia."  Their job should not be complexity to assure self employment preservation, or to complicate the practicing engineer's life, or for no real improvement in final product.  Once the intent of various code sections and concepts is taught and understood, the code should become a background document referred to for truly difficult conditions.  There are plenty of times a complexity crops up in one of our designs, and we should be smart enough as engineers, to know that we must dig a little deeper to solve this one to our own satisfaction and for the public safety.  Then the extra complexity is appropriate and brought on by our own design.   But, 90% of what we design should not have to be dragged through this swamp of crap, just to justify a simple spread footing, as in this case.  We shouldn't be working in fear of missing some obscure factor slipped into the latest code, and causing some major problems or legal action.  Rather, our work should be a continuous learning and educational process to really improve the product we design, and to more fully understand what we do and why.

I certainly hope you are on the code writing committee or have some direct input, because you seem to apply some common sense to the process, and seem to understand our difficulties.  Codes are not an end in themselves, they are a means to an end, and should not overly complicate the process of getting there.  They ill never be made to cover every condition under every possible situation, so let's quite trying to have them do that.  That's what engineering judgement and experience are supposed to cover.  And, maybe everyone won't be qualified to design everything and anything, without some experienced supervision, and more complex codes won't change this situation or dynamic.

RE: Min Area of Steel for Spread Footing

(OP)
Well said dhengr....

I sorry to say that AISC has been traveling this same troubled path.

For shyts and giggles sometime...take a look at Chapter F in the 2005 AISC Code and try to apply it to a built up, unsymmetrical girder section with non-compact elements as often seen in mill building and turbine building crane runways.
It is a complete joke & endlessly frustrating.

Its like code writers went straight from university textbook example problems to writing codes...."look, works perfect for a W30x90!"
I believe it is section F12 that might as well say "go figure it out yourself" ....you could also replace "figure it out" with another word starting with "f".

I will not apologize for hijacking my own thread.  

RE: Min Area of Steel for Spread Footing

(OP)
oh, and your right....my code digging was a CYA.  

RE: Min Area of Steel for Spread Footing

Yes, I am on several ACI committees.  Not on 318, but am well-connected to that group.  ACI code development committees are entirely volunteer.  My employer supports the effort because of its critical interest to our industry.  Many of the code committee members are academics who frequently know a great deal about a portion of the code, but possibly do not really understand how a provision will work in practice.  Very few are practicing engineers, primarily because working engineers cannot take the time needed, nor have sufficient employer support for travel required.

ACI committees meet twice a year at the spring and fall conventions, possibly over a few conference calls, and infrequently, additional face-to-face meetings.  Codes are generally on  6-year cycle, with 3 year minor cycle.  This correlates to the International Building Code cycle.  There are calls for less frequent updates, but every time they try, there are problems identified mid-cycle which require "urgent" fixing.  Even now, there are advances in alternative reinforcing materials and concrete mix design which need to be addressed, at least until a performance standard is implemented.

Even if you do not join ACI and cannot participate in meetings regularly, you can get involved in the process of code development.  All ANSI-accredited standards-making organizations must accept and address public comment for changes.

RE: Min Area of Steel for Spread Footing

Hi List,
Although I have been a member for quite some time, I recently started participating.
I am referring to ACI 318-05.
The minimum reinforcement in flexural members depends on the type of member. For two-dimensional elements like slabs the provisions of Chapter 7 would apply and for one-dimensional elements like beams, Chapter 10:
Typically this is what we follow in our office:
Structural slabs, footing pads, foundation mats - section 7.12.
Pile caps - Guidelines given in CRSI Handbook
Combined footings - Longitudinal Direction Sections 10.5.1, 10.5.3
           Transverse Direction - 7.12 (Distribution varies)
Strap beams in footings - Sections 10.5.1, 10.5.3
Cantilever slabs like balconies which are statically determinate and where there is no possibility of redistribution of moments, we
use sections 10.5.1 and 10.5.3.
 

RE: Min Area of Steel for Spread Footing

I agree completely with Dhengr but do not see any hope that the practicing engineers would ever be able to bring about any meaningful change in this money-making code industry that has taken root.
We as engineers just don't have the capability/mentality to bring about this needed change.
So we are left to whine or whimper and feel on some primitive level that we have accomplished something.
 

RE: Min Area of Steel for Spread Footing

Sail3:

One small way that we, as practicing engineers, can have some effect on this code mania is to talk assertively and often with our legislators, or local officials who pass the laws and regs. which force/allow the adoption of the latest codes every cycle.  And, in particular, for us, that means the IBC and its various brothers, which then adopt all the other, almost latest eds. of, codes by reference.  We have to explain to them that buildings won't start falling down around them if we skip the next couple, or every other, edition of the IBC.  They don't know any better, they assume the newest must be the bestist, and that is the code writers pitch.  And, we have to educate them and explain that there really isn't much improvement or advancement in the latest edition as relates to a better structure for them.  As part of our standard practice, we keep up with changes in the industry, and the current code allows us to apply these, without a full blown code revision, and all the other complex changes that entails.  Maybe we should also explain to them that they should give some of this authority and responsibility back to the EOR, rather than relying on a bunch of new minutia in the form of a new code.  We have to explain to them that this current rate of code change is just adding confusion and commotion in the entire construction industry, actually costing more money, and not providing better infrastructure for their dollar.  Most of the state and local building depts. will actually work with us on this, since they are as confused and put-upon, as we are, by not actually learning to use and interpret the codes, before they change again.  The fact is, that this rate of change and commotion likely adds to the potential of error, since nobody really learns to use, interpret, or inspect to a version of the code, before it changes, and starts all over again.

The outcome of this would be that the code writers could change and print to their hearts content, we would follow the research in tech. literature, and apply it as we saw fit and were willing to assume responsibility for it; BUT they wouldn't sell many of their newly printed editions, every other cycle.  This would get their attention, because if we don't have to buy it, they don't have anything to sell.  

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