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How to place minimum steel in a rectangular footing?
14

How to place minimum steel in a rectangular footing?

How to place minimum steel in a rectangular footing?

(OP)
I have got a question about the placement of minimum temperature and shrinkage steel, say for a 35" thick rectangular footing 14 feet long and 5 feet wide. Let's say, that all I need for my footing is minimum temperature and shrinkage steel.

Hence, Ast = 0.0018bh = 0.0018 * (14*12) in * 35 in = 10.58 in2.

Say, I am using #7 bars, thus, I will need 10.58 in2/0.60 in2 = 18 # 7 bars.

Method 1: Or, say I place bars in the top and bottom layers.  For my bottom layer, do I need to place 14 #7 parallel to the short side, and 4 # 7 parallel to the long side.  And do the same for the top layer.

Or, Method 2:  I still place bars in both the top and bottom layer.  For the reinforcment parallel to the do I just need to place a total of 18 #7 in both the top and bottom layers, say 9#7 in the bottom layer, and 9#7 in the top layer.

For Method 2, the Ast required parallel to the long side will be 0.0018 * (5*12) in * 35 in = 3.78 in2, or 7 # 7 bars.  Do I place a total of 7#7 bars in both the top and bottom layers, say 4#7 in the bottom layer, and 3#7 in the top  layer.

Method 3.  Not to confuse matters, but can I place reinforcement in the bottom layer alone, say a total of 18# 7 bars, say 14#7 parallel to the short side, and 4#7 parallel to the long side.

Please advise whether Method 1 or Method 2 is more appropriate.  And is Method 3 practical?

RE: How to place minimum steel in a rectangular footing?

3
For a two-way rectangular footing, you need to put more steel under the column - See ACI 318-05 14.4.4.
Also, there is some difference of opinion on this matter, but I'll give you mine and let others chime in as they see fit.

ACI states that any section in which reinforcement is required by analysis (i.e. the plain concrete section doesn't work), TENSILE reinforcement not less than 0.0018bh shall be provided.  Tensile reinforcement, to me, means at the tension face, not distributed between the two faces.  That is also in each direction, not to be split between the two directions.  

I don't think either method is appropriate.

Also, a 35" footing is pretty thick.  I'm guessing there is uplift on the column or maybe the column base is fixed and there is moment being taken into the footing.  I don't believe that I would use a single layer for a 35" thick footing (even without uplift or moment), and I would use 0.0018bh on each face in each direction while maintaining the appropriate distribution per 15.4.4.  

RE: How to place minimum steel in a rectangular footing?

(OP)
StructuralEIT, thank you so much for your prompt response, as always.

BTW, there is not Section 14.4.4 in ACI 318-05. Section 14.4 is on Page 239.  Did you mean Section 14.3.4?

My 35" footing is for illustrative purpose for this question only.

What would be your best solution for the question posed above?

RE: How to place minimum steel in a rectangular footing?

minimum in tension face, i.e. all in bottom or all in top (or both to keep it simple). Shrinkage and thermal in both faces as required. All in both directions.

Thats how i see it.

RE: How to place minimum steel in a rectangular footing?

My apologies, I meant 15.4.4 (I noted it right at the end of the post and botched it at the beginning).

I would provide (18) #7 in the short direction on the top AND bottom face.  Be sure to check development length of the bars, they may need to be hooked or use a smaller bar.  2' or so isn't much to develop a #7.  I would probably get away from the top T&S steel (as long as it isn't required by analysis) for anything less than, say 24" thick.

For the long direction bars, I would provide (7)#7 on the top AND bottom face.  Again, check development, though it's likely not a problem for this direction.  Again, I would probably get away from the top steel (unless required by analysis) for a thickness less than, say 24".  

I would also meet the distribution requirements in 15.4.4.

RE: How to place minimum steel in a rectangular footing?

Method 1 is incorrect.  Minimum steel should not be divided into perpendicular directions.

I would use Method 3 unless there is some reason you need top steel.

DaveAtkins

RE: How to place minimum steel in a rectangular footing?

Dave-
Method 3 is splitting the total in the two perpendicular directions, just doing it for one face only.
Just out of curiosity - where do you draw the line as to where you want a top layer even if it's not needed for strength?

RE: How to place minimum steel in a rectangular footing?

As per the current spanish code CTE, 0, since one still can make mass footings. CTE clearly states that structural concrete can be mass concrete; provides tensile and flexural design strengths, and Vc contribution to shear (or if not you can state atechnical correct one); no other element in a structure can be more easily targeted to be mass concrete, and there are still current examples out there giving example of this kind of structure. One must conclude the minimum flexural steel in this type of structure is 0.

This said, I have never made one without reinforcement in 32 years of practice. But I may, who knows.

RE: How to place minimum steel in a rectangular footing?

StructuralEIT,

You are correct--my mistake.  You need minimum steel in EACH direction.

I might ask for top steel in a very thick pour.

DaveAtkins

RE: How to place minimum steel in a rectangular footing?

Good, and interesting, thread.  Cudos to StructuralEIT for a very good treatment of the subject...

I'll just add my personal view on the addition of top steel / covering steel when not needed for strength:  I do this routinely for extreeme exposures.  IE: If potential cracking will lead to ingress of harmful liquids (salts, corrosives, etc) or if the surface is to be visible (Mass concrete retaining wall for a hospital I did comes to mind), I include a minimum of 0.00233 steel at the exposure standard stipulated cover (typically 30, 25, or 40mm for New Zealand).

Normally welded reinforcing matt is sufficient for this purpose, and not too difficult to place.  You do get some complaints from the contractors, but usually pointing out that in a structure for an aggresive environment the choices are a little more steel or increased inspections, the contractor ops for the steel.  Oh, and one more thing: I often ask for galv, or passively protected systems, when the environment is REALLY aggressive.  Just my thouths/SOP.

Cheers,

YS

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

RE: How to place minimum steel in a rectangular footing?

3
I agree that if minimum steel controls, you need 7-#7 in the long direction, 18-#7 in the short direction.  If this is a column which only sees gravity load and not uplift, I would only use bottom steel.  I wouldn't use top steel in a footing unless required by analysis.  When you do use top steel in a deep footing, the concrete usually requires revibration to prevent plastic settlement cracking.

It is unfortunate that codes continue to refer to T&S minimums when minimum flexural steel is required.  

RE: How to place minimum steel in a rectangular footing?

(OP)
Thank you very much for all the wonderful responses which are bringing some clariy to a seemingly "easy" topic, but can still be confusing to a lot of engineers.  I have got a follow-up question based on herewegothen's response; which is the tension face of the footing.  I hope all of you, esp. StructuralEIT, herewegothen, DaveAtkins, hokie66, and anyone else interested, will have a go at it.pr

Say, the two way rectangular footing we are discussing is supported two columns which are located towards the ends of the long side.  The usual loads, dead, live, seismic, wind, etc. are being transmitted through the columns. There is some shear at the column base, too.

Now, which is the tension face of the rectangular footing?

Is it just the bottom of the footing?  With gravity and all the other loads acting downwards from the top of the footing, the top of the footing would be in compression and the bottom would be in tension.  So we would need to reinforce the bottom of the footing.

How about the soil pressure acting upwards on the footing?  Visualize an upside down footing, with the columns as supports, and the soil pressure a uniformly distributed load on the footing.  The bottom of this "visualization", i.e. corresponding to the top of the footing would be in tension.

Would not the top of the footing be the tension face then, too, and needs to be reinforced, too?  Your thoughts and feedback, please.

RE: How to place minimum steel in a rectangular footing?

This is a totally different matter.  I thought you were talking about a spread footing with one column in the center.

This footing must be designed for the upward bending between the columns.  In fact, there won't be much tension at the bottom of this footing (a little under each column).

I would use top AND bottom steel in this case.

DaveAtkins

RE: How to place minimum steel in a rectangular footing?

I agree with Dave for your last scenario.  The top will be the tension face, and the bottom will have little (if any) tension - mostly depending on the location of the columns.

RE: How to place minimum steel in a rectangular footing?

unless you analyse it as soil spring then the bottom is in tension. Put it top and bottom.  

RE: How to place minimum steel in a rectangular footing?

herewegothen-
Is that true?  The only way for that to be true is if the footing displaces downwards greater in the middle than at the columns.  That won't happen.  What I think will happen if you analyze it as a beam on an elastic foundation is that the moment may be smaller because the soil pressure isn't constant (or linearly varying) and actually gets higher near the columns, but I don't believe it will cause tension on the bottom face.

RE: How to place minimum steel in a rectangular footing?

(OP)
Sorry for not being more clear earlier.  I do have two columns on my spread footing.  I am providing reinforcement on both the top and bottom faces, in this case, it is minimum reinforcement.  From the discussion above, I will provide minimum reinforcement in EACH direction, TOP and BOTTOM.  Two follow-up questions, though.

1.  I understand that there is significant negative between the two columns (i.e. supports), hence the top face of the spread footing will be in tension.  Need to provide tensile reinforcement in the top face.  Why is there little (or no) tension on the bottom face, or is this not the case?  We still need to provide reinforcement in the bottom face, right?

2. Back to the reinforcement not being allowed to be "split" between the top and bottom faces, what is the main reason for this?  Do the top and bottom bars, say parallel to the long direction, not act together to resist bending about the short direction?

RE: How to place minimum steel in a rectangular footing?

The reason for minimum flexural steel, which Hokie pointed out is misappropriately called T&S steel (mostly because that's the ratio used), is to prevent a sudden failure upon cracking.  If the reinforced section is not as strong as the plain concrete section, it will fail as soon as cracking occurs.  The minimum flexural steel is meant to prevent that.

RE: How to place minimum steel in a rectangular footing?

I can't add anything to what Dave and SEIT said.  Pob, you need to put T&S reinforcement out of your mind, and just reinforce this footing as you would any other reinforced concrete element, which means putting the bars where the tension is.  If it only requires minimum steel, that is the amount you use, for tension, not T&S.  Your analogy of turning the footing upside down is good.  It is just a beam with uniform loading, with two supports, in this case the columns.

This is called a combined footing, and I agree with reinforcing it both top and bottom.

RE: How to place minimum steel in a rectangular footing?

structuralEIT I was referring to the area under the columns, either way you analyze it the bottom will be in tension in some area, top in other areas.  

RE: How to place minimum steel in a rectangular footing?

(OP)
Is anyone willing to take a stab, that if all my combined footing requires is minimum flexural steel, then why the reinforcement cannot be "split" between the top and bottom faces?  What is, or are the main reason(s) for this?  Do the top and bottom bars, say parallel to the long direction, not act together to resist bending about the short direction?  Looking forward to your feedback and advice.

RE: How to place minimum steel in a rectangular footing?

2
Structural EIT, I have a detailed discussion with Dr S. K. Ghosh ACI commitee 318 member. As per him 0.0018bh is not meant to prevent sudden faliure, it is only there for temprature and shrinkage, please see ACI 318-08 R15.10.4, it is clearly stated there.  

RE: How to place minimum steel in a rectangular footing?

pob,

The answer to your latest question is definitely NO.  The top and bottom bars do not work together to resist the bending.  Either the top or bottom is in tension, not both, and only the bars on that face resist the tension.  That is the main difference between flexural reinforcement and "T&S" reinforcement.  The assumption in providing T&S reinforcement is that the whole thickness of the section is in the same stress state, so the reinforcement can be evenly distributed.  As stated before, it is unfortunate that the codes use the same percentage of reinforcement for minimum flexural reinforcement as for T&S reinforcement, but the only similarity is the percentage, not the function.

RE: How to place minimum steel in a rectangular footing?

(OP)
Thank you so much for your response, hokie66.  It really helps me to understand the underlying principles behind the footing design.

Ok, you said, "The assumption in providing T&S reinforcement is that the whole thickness of the section is in the same stress state, so the reinforcement can be evenly distributed."

Follow up Q1:
Do you mean that if only Temperature & Shrinkage reinforcement is required, then we can, or that we are allowed to, use the assumption that the whole thickness of the section is in the same stress state, i.e. that the footing is either in tension on both the top and bottom faces, or in compression on both the top and bottom faces, and therefore, we can evenly distribute the reinforcement between the two faces, i.e. that we can split the T&S reinforcement between the top and bottom faces of the footing?  Is this a "Yes" or "No"?  This is basically the gist of my very first post.  Please advise.

You also said, "The top and bottom bars do not work together to resist the bending.  Either the top or bottom is in tension, not both, and only the bars on that face resist the tension."

Follow up Q2:
In this case, do you mean that for the minimum flexural reinforcement, the top and bottome bars do not work together because either the top or bottom face is in tension, not both, and only the bars on that tensile face resist the tension.  Therefore, it is NOT permitted in this case to split the minimum flexural reinforcement between the top and bottom faces.  Also, it is NOT permitted to split the minimum flexural reinforcement in both directions, meaning that minimum flexural steel has to be provided on each face, top and bottom and in each direction?  Again, is this a "Yes" or "No".  Please advise.

Thank you so much to all respondents who have contributed to making this topic a little clearer for all.

RE: How to place minimum steel in a rectangular footing?

Q1...if a concrete element is in direct tension, i.e. in tension over its entire cross-section due to restraint forces, then the T&S provisions apply, and the reinforcement can be distributed equally.  If a concrete element is in compression across the cross-section, then the reinforcement is not for T&S, so other provisions apply.  Other than continuous wall footings, T&S reinforcement does not normally apply in footings, as flexural reinforcement controls.

Q2...Yes

RE: How to place minimum steel in a rectangular footing?

StructGen-
I'm not sure why the addition of 15.10.4 was needed.  I don't see any new information there.  10.5.4 still says that you need a minimum of 0.0018bh of TENSILE reinforcement.  I don't believe that 15.10.4 overrides this.

RE: How to place minimum steel in a rectangular footing?

(OP)
I think I just had a light bulb moment!  All of us have been going on about "minimum steel", starting with my initial inquiry about the placement of "minimum T&S" steel.  However, most of the time, in the subsequent posts, we do not actually specify whether we mean "minimum flexural steel, i.e. minimum main reinforcement steel", or "minimum T&S" steel.

This is going to be a long one, so please bear with me.

StructuralEIT mentioned, "TENSILE reinforcement not less than 0.0018bh shall be provided." That's "minimum flexural reinforcement".

herewegothen referred to, "minimum in tension face, i.e. all in bottom or all in top (or both to keep it simple). Shrinkage and thermal in both faces as required. All in both directions."  Referring to "minimum flexural reinforcement."  herewegothen's later reference to "shrinkage and thermal in both faces as required" confirms that.

Dave Atkins's "Minimum steel should not be divided into perpendicular directions." would refer to "minimum flexural steel."

hokie66's post is the one that really brought this issue to the fore, by stating, "It is unfortunate that codes continue to refer to T&S minimums when minimum flexural steel is required."

Earlier, hokie66 said, " ... if minimum steel controls, you need 7-#7 in the long direction, 18-#7 in the short direction.  If this is a column which only sees gravity load and not uplift, I would only use bottom steel.  I wouldn't use top steel in a footing unless required by analysis.  When you do use top steel in a deep footing, the concrete usually requires revibration to prevent plastic settlement cracking."  Reference to "minimum flexural reinforcement."

ACI 318-08 Section 10.5.1 states  that minimum reinforcement for flexural members shall not be less than that given by As (min) = (3*SQRT(fc')/fy)*bw*dm and not less than 200*bw*d/fy.  For fc' = 4000 psi and fy = 60000 psi, the minimum flexural steel works out to be 0.0033*bw*d.

ACI 318-08 Section 7.12.2.1 (b) states that the "area of S&T reinforcement shall provide at least the following ratios of reinforcement area to gross concrete area, i.e. 0.0018 in slabs where Grade 60 deformed bars or welded wire reinforcement are used. where slabs , but not less than 0.0014.

To add to this, ACI 318-08 Section 10.5.4 (per StructuralEIT's latest post) states that, "for structural slabs and footings of uniform thickness, As (min) in the direction of the span shall be the same as that required by 7.12.2.1, i.e. 0.0018*bw*d.

Section 15.10.4 states, "minimum reinforcing steel in nonprestressed mat foundations shall meet the requirements of 7.12.2 in each principal direction."

Actually, the requirements of Section 10.5.4 and 15.10.4 seem to supersede that of Section 10.5.1, as StructuralEIT pointed out.

I think it is very clear from these ACI 318-08 sections then that the minimum flexural steel is 0.0018*bw*d.

hokie66 did point out it was unfortunate that the "minimum T&S" reinforcement is also 0.0018*bw*d.

Per hokie66's latest post, most situations call for at least minimum flexural steel, 0.0018*bw*d, and this must be placed on each face and in each direction.

However, if only "minimum T&S" reinforcement is required, then we are llowed to, use the assumption that the whole thickness of the section is in the same stress state, i.e. that the footing is either in tension on both the top and bottom faces, or in compression on both the top and bottom faces, and therefore, we can evenly distribute the reinforcement between the two faces, i.e. that we can split the T&S reinforcement between the top and bottom faces of the footing?

The question now is, when do we deduce we need "minimum flexural steel", and when do we say, only "minimum T&S" reinforcement is required?  This is important because it will affect the placement of the reinforcement.

Elaborating further, when can we say we need "minimum flexural steel", and therefore, we must place at least, Ast = 0.0018 * bw * d on the tensile face, on each face and in each direction.

Is there a situation where we can condifently deduce that the footing only requires "minimum T&S" reinforcement, and therefore, we can use the assumption that the whole section is in the same state of stress, and therefore split it between the top and bottom faces?

Sorry for the extremely long post, but I hope it puts everyting in perspective and bring about a discussion that would clarify this matter further.  Thank you so much to all who have contributed to the the understanding of this topic.

RE: How to place minimum steel in a rectangular footing?

Yes, it is long.  And some of it indicates that you understand, and some that you don't.

The amount of flexural reinforcement you require is determined by standard ultimate strength design theory, and is further limited by the minimum provisions you listed above.  I thought you knew how to do that.  If your footing only requires minimum flexural reinforcement, it of course requires that reinforcement on the tensile side, not the compression side.  And that amount of reinforcement can also be considered as T&S reinforcement.  Now, if your code (some do) required a greater quantity of T&S reinforcement due to a high degree of restraint of the element (not likely for footings), then you can count the flexural steel as part of the T&S requirement.  

RE: How to place minimum steel in a rectangular footing?

Structural EIT,

Not any more, please check 05 or 08 code, they have replaced the word "Tensile reinforcement" by "Asmin" further I have discussed this with the person who wrote this clause, he was very clear that 0.0018bh is not flexural min. steel. You can run a quick calculation of 8000 to 12000 PSI conc. beam with 0.0018bh your Phi-Mn will be less then Mcr.
While Eq 10-3 gaurntees that your Phi-Mn will be greater then Mcr.
Code is very clear on this issue, either you use Eq 10-3 or you provide enough to avoid faliure. This thought is also expressed by reducing Phi for compression controled sections and Plain concrete.
If all of this above does not satisfy you let me know I will post the latter from Dr. S. K. Ghosh.


Thanks  

RE: How to place minimum steel in a rectangular footing?

I would like to see Ghosh's letter.  I agree with SEIT.  If the amount of reinforcing to avoid failure is very low, the minimum amount is referenced to the T&S provisions.  Not logical, but that is the way I read the ACI provisions.  The Australian code AS3600, which did have the same connection between tensile steel and T&S, has now been changed to avoid this confusion.

RE: How to place minimum steel in a rectangular footing?

Let me raise one last counter argument before I present Dr. Ghosh latter, if 0.0018bh is the absloute minimum flexural reinforcement why it is not applicable to beams which are the primary flexural members.
This means that I can design a beam 1mx2m with less reinforcement then 0.0018bh provided that I satisfy 10.5.1, 2 & 3. While if I am designing a two-pile pile cap of 1mx2m of same load I have to satisfy 0.0018bh on tension face.
Do you think its logical?
Hokie66 do you still want me to post it?

RE: How to place minimum steel in a rectangular footing?

I don't think that using beams with such minimal reinforcement is at all practical.  You have to provide 200/fy (or more for high strength concrete), but this can be reduced if you provide at least 1/3 more than required by analysis.  If beam reinforcement is very light, the beam size is wrong.

A two pile cap is not a flexural element, and the minimum flexural reinforcement provisions should not be used.  Truss analogy (strut and tie) is the way to go for pile caps.

In your example above, you mentioned 8000 to 12000 psi concrete.  It is very unusual, if not unheard of, to use such high strength concrete in flexural members.   

RE: How to place minimum steel in a rectangular footing?

I will re-read the '05 and '08 versions, but I believe that the min beam AND slab reinforcement are in the same section (10.5) which talks about minimum FLEXURAL reinforcement. I'll read it again on Monday.

RE: How to place minimum steel in a rectangular footing?

hokie66,

Not sure what type of work you do, but that is the standard material for all mid-rise (600-ft)buildings in US. 5000-8400 PSI for slabs and 12000-8000 PSI for verticals.
Further I have just given an example, a possiable sitituation, I am not debating what method is needed to design pile cap. You have to satisfy all other applicable code requirments when you use Strut-Tie. A footing is a footing you can not get away from shrinkage or min. reinf. requirments by designing it as strut and tie.

Getting back to the topic, StrEIT 10.5.4 is only applicable to footing and slabs of uniform thickness, it is not applicable to beams.

Attached please find my conversation with Dr. S. K. Ghosh, the reason I am posting this here is becuase I want to end this common misperception. This has caused me a lot of headche.

 

RE: How to place minimum steel in a rectangular footing?

StructGen,

I wouldn't call a 600 ft high building mid-rise, but maybe in New York it is.

I agree that 5000 psi and 6000 psi concrete strengths are typical for floors.  You mentioned 8000 to 12000, which I have not used for floors.  I would see no benefit in using such high strength concrete in floors, as the reinforcement percentage is not very sensitive to concrete strength.  The only reason I can think of for using higher strength concrete in a floor is to allow for transfer of column loads through the floor.  The Australian Code AS3600 allows the ratio of column strength to floor strength to be 2.0, provided adequate restraint is provided.  Maybe the ACI code is different in that regard.

I have read the letter from S K Ghosh, and do not agree with his interpretation of Section 10.5.4 (whether he wrote it or not).  He says "Section 10.5.4 essentially says that if you have provided the minimum shrinkage and temperature reinforcement required by Section 7.12, you shall be deemed to have satisfied the minimum flexural reinforcement requirements of Section 10.5".  That is clearly incorrect.  Section 10.5 prescribes Asmin, minimum flexural reinforcement, while Section 7.12 prescribes minimum T&S reinforcement.  Why 10.5 refers back to 7.12 for the quantity is a mystery to me, as it just confuses the issue.  But clearly Asmin is flexural reinforcement, and thus must be placed at the tension face.

RE: How to place minimum steel in a rectangular footing?

Apologies for posting on the end of this thread.

Hokie

Where in AS3600 is there a limit of strength grade ratio between vertical and horizontal elements and does it only apply when columns are offset. AS3600 is only written for strength grades up to 65MPa so it is unlikely that something designed to AS3600 will ever have strength grade ratios greater than 2.0. Although a 600-ft building would be classified as high-rise in Australia and AS3600 would not be applicable in the design of columns where the strength grade would likely exceed 80 or 100MPa.

RE: How to place minimum steel in a rectangular footing?

StructGen-

I still disagree.  Aci 318-08, 10.5.1 says, "At every section of a flexural member where tensil reinforcement is required by analysis (i.e. the plain concrete section is not adequate - that is my interpretation which I think you would agree with), except as provided in 10.5.2, 10.5.3, and 10.5.4, As provided shall not be less than that given by As,min=......."

The way I read this is that everything covered in 10.5 is minimum FLEXURAL reinforcement, 10.5.1-10.5.4 just give the quantities for different scenarios.  The fact that 10.5.4 refers the quantity for minimum FLEXURAL reinforcement back to that for T&S is irrelevant.  What is relevant is that it is minimum FLEXURAL reinforcement as defined by 10.5.1.

I'm not seeing the ambiguity at all.  The first few times I read it I was a little wishy washy, but the more I read it (and I've had this opinion for well over a year and a half now) the more clear it becomes to me that my above interpretation is correct.

RE: How to place minimum steel in a rectangular footing?

I left out a few things I wanted to put in the last post.  First, I never said that 10.5.4 applies to beams.  It is clearly stated to apply to slabe and footings of uniform thickness and that is what this post is pertaining to.  

Second, I disagree with the assertion that you made in your original email to Dr. Gosh that Mcr is almost always greater than the reinforce section using 0.0018bh.  I just did a quick check using a 4'x4'x16" thick footing.  I get Mcr=971K-in, but the nominal moment strength using 0.0018bh with d=12.5" (assume a #8 bar with 3" cover) is 1014K-in.  Neither of these values reflect the phi factor, but it is clear that the values are close.  It's probably less so for thinner ftg, but more so for thicker ftgs.  

Third, I didn't read anything in the email from Dr. Gosh to sway my opinion.  If it's not minimum flexural reinforcement, then it should be taken out of that section.  I have no change in interpretation based on the change in wording from '02 to '05/'08.  Just the phrase, "... in the direction of the span..." shouts flexural reinforcement.  

There is no way in the world that if a plain concrete section were not adequate, that I would feel comfortable providing tensile reinforcement of 0.0009bh on each face, even if it did work on paper.

RE: How to place minimum steel in a rectangular footing?

In light of the Ghosh email to Mr. Ullah, and having reread all the previous threads on this topic, I agree with StructGen, that ACI never intended .0018bt to be the minimum reinforcing on the tension face of a structural slab of uniform thickness.

RE: How to place minimum steel in a rectangular footing?

asixth,

The section I referred to is 10.8 (b).  No, it applies to direct transmission through the floor system.

True, the current Standard only applies up to 65 MPa concrete, but buildings are being built in Australia with higher strength concrete.  This is allowed under Section 1.3.  Subject for another discussion, but I don't know how consistently the various design offices are approaching this interim situation.

RE: How to place minimum steel in a rectangular footing?

miecz,

If .0018bt is not intended as the lower limit on flexural reinforcement in a slab, then what is?

RE: How to place minimum steel in a rectangular footing?

Guys one last time,

Try to understand the intention of code, it expects you to provide minimum reinforcement as per eq 10-3 (0.0033bh) if your design flexure is low you can provide 1.33As-req and can get away with the minimum reiforcement requirement.
Take a pause at this moment.
Code wants you to check your temprature and shrinkage reinforcement. 10.5.4 forces you to check your As-min in the direction of design vs 7.12.
Take a deep breath, think about it, if code's intention is to check flexure minimum in 10.5.4 it will not reffer you to 7.12, it will simply provide you the limit. The way it does for every thing else.

Next why they added 15.10.4, slabs are between 6" to 12" in most cases. typical bottom mesh reinforcement is almost alway more then 0.0018bh, it the footing which can go as deep as 12 feet (for a 700-ft high hurricane zone building). 0.0018bh is a huge number for this big raft that is why it is clearly stated in ACI 318-08 15.10.4

I am amused of the comments, "I dont know why they added 15.10.4" well they added it becuase they feel that need to clarify this issue.
Next is "I dont agree with Ghosh's interperatation", he is not interpreting it, he is telling me what he wrote in ACI 318 and what he wants us to read out of those lines.
another one is "I have'nt read any thing from Ghosh's email to sway my opinion"
What are we trying to establish here we can not have a reasonable discussion?  

RE: How to place minimum steel in a rectangular footing?

15.10.4 just tells you that T&S reinforcement is required in mat footings, etc.  It does not address the flexural requirement.  The two requirements may be the same, but not necessarily.

Again as I said before, it is illogical for the code to refer back to the T&S section for flexural reinforcement.  On that we agree.  But it does.

As to the intention of ACI, it has been that way for a long time, possibly prior to Dr Ghosh's involvement.  I pulled out the 1978 CRSI Handbook, in which slab designs are tabulated based on ACI318-77.  The minimum flexural reinforcement used in the tables was 0.0018bt.  So at least 32 years of confusion, maybe more.  I don't have my older versions ready at hand.   

RE: How to place minimum steel in a rectangular footing?

I don't know, I'm still not seeing it.  It is in the code section titled "MINIMUM FLEXURAL STEEL".  That seems pretty clear to me.  The fact that they refer back to the T&S QUANTITY............. the actual code section is in "MINIMUM FLEXURAL REINFORCEMENT".
Additionally, 15.10.4 only applies to COMBINED footings, not slabs or isolated spread footings.
I think it also mentions that the AS,min needs to be placed near the tension face.  If AS,min is per 7.12 (0.0018bh), and it needs to be placed near the tension face, doesn't that mean that you can't split it?  I just can't imagine that there is no lower bound limit for reinforcement for a reinforced footing.
The biggest question I would have at this point is why are slabs and footings even included in section 10.5 if they have no minimum flexural requirements.

RE: How to place minimum steel in a rectangular footing?

I'm reading it over and over, and I'm not doubting Dr. Ghosh's intent, but his explanation is not the way the text reads.  I'll go through my thought process one more time, please tell me where I'm going astray.  

Section 10.5.1 ('08) says, "At every section of a flexural member where tensile reinforcement is required by analysis, except as provided in 10.5.2, 10.5.3, and 10.5.4, As provided shall not be less than that given by ......"
I think we'll agree that a footing is a flexural member.  This, to me, says that any section that doesn't work as plain concrete needs to have a minimum amount of tensile reinforcement.  Do you agree with that statement?  

That minimum amount, EXCEPT as provided in 10.5.2, 10.5.3, and 10.5.4 shall be equation (10-3).  The exception provided in 10.5.2 is for a T-beam with a flange in tension.  The exception provided in 10.5.3 is for a very large section where eq. (10-3) would be excessive.  These are both for tensile steel requirements.  Do you agree with this?

The exception provided in 10.5.4 is for slabs and footings of uniform thickness, and As,min (which is clearly defined as the minimum amount of tensile reinforcement of a flexural member) is the same as that required by 7.12 - which is 0.0018bh.  If just meeting the requirements of 7.12 satisfy 10.5.4, then there are no minimum flexural reinforcement requirements as 7.12 has no requirement about where the steel needs to be placed.  It could be placed all on the compression face and, by your reasoning, meet the requirements of "minimum reinforcement of flexural members".  

Like I said, I don't doubt his obvious intent after reading the response you posted, but the text does not read that way.  I'm also not sure I understand the reasoning for essentially having no desire for minimum tensile steel in footings.  The response posted makes that pretty clear.  Any section that has 0.0009bh (assuming you split the steel between top and bottom) tensile steel will NOT be a reinforced section (because the plain section will have greater capacity - even with the lower phi factor).  
  

RE: How to place minimum steel in a rectangular footing?

PCA MATS program treats minimum tensile steel as 0.0018bh.  If you split this 0.0009bh top and bottom you may provide less steel than required by design.  I proved this to co-workers who used to split top and bottom.  I modeled a mat where the program said min steel controlled. But calculating As req'd based on Mu was greater than 0.0009bh.  So according to PCA MATS minimum tensile steel is 0.0018bh.

RE: How to place minimum steel in a rectangular footing?

hokie66

Ignoring 10.5.2 for the moment, I believe the lower limit of reinforcement is covered by 10.5.1 and 10.5.3,and that the T/S provisions of 7.12 must be met, but not at the tension face, at the whole section.  This has been my position since I joined this debate, in 2007.  See thread507-185223: ACI minimum area of steel and thread167-225434: Min Flex. Reinf for Slabs.

That may not be what ACI Article 10.5 says, but it is what I believe is the intent of the code writers.   

RE: How to place minimum steel in a rectangular footing?

StrEIT, I agree with you untill you assume that As-min in 10.5.4 is the miniumum amount of tensile steel. No it is not the minimum amount of tensile steel. It is Area of steel in the direction of design, let me quote it from ACI ""For structural slabs and footing of uniform thickness, As-min in the direction of span shall be same as that required by 7.12.2.1 ......"
The flexure minimum reinforcement requirements are coverd by 10.5.1, 5.2 & 5.3. While 5.4 ensures that T&S requirments are met. Now here comes your argument why this clause is placed under 10.5 which is flexure minimum reinforcement. Simple reason, it is an additional check so that you dont end up providing less reinforcement then T&S requirement and yes it can be placed at the compression face if you meet the requirments of 10.5.1 & 5.3 for tension face.
Since you reffered to plain concrete, lets look at this issue from a different prespective, you are allowed to design plain concrete but code penalize you with higher Phi, have you ever thought why? to ensure that faliure never happens, same is for the compression controled section. Lets get back to minimum steel, you have to provide min. steel as per Eq 10-3 to ensure a ductile faliure, if you dont want to do that, code forces you to increase your reinforcement 33%. just to ensure that the member will never see faliure (under the intended design loads). Do you see a trend established here?

Now lets take on 0.0018bh, it does not ensure ductile behaviour, it never provide a capacity which is greater gross cross-section capacity Then why on earth it should be flexure minimum reinforcement. Further assuming if it is Flexure min. reinforcement that why its not applied to beams, why beams are exempt of this clause?

Yes I agree with you that text is not black and white that is why I asked this question from the commitee member. Let me tell you some thing, if you don't trust 318-Commitee member then should not trust the code it self becuase these are the guys who write the code not Aliens from another galaxy.

 



 
 

RE: How to place minimum steel in a rectangular footing?

StructGen,

As,min is defined in Chapter 2 as "minimum area of flexural reinforcement".  I don't see how you can consider steel on the compression face to be flexural reinforcement, or all the steel in the direction of the span to be counted as As,min.

Dr Ghosh didn't write this clause, so it is not a matter of trusting him or anyone else.  It is just a matter of what the language says, and to me it is crystal clear.  

The same provision which allows slabs and footings to have a lower percentage as minimum flexural reinforcement has been there for a long time.  I am not sure how long.  CRSI has over the years done a good job of producing design handbooks which run in tandem with the Code, so I attach a certain amount of credence to their interpretations.  However, the CRSI design tables were only for lower strength concretes, and as Rapt said in the earlier thread which miecz referenced, the requirement for strength to be greater than cracking moment is met by .0018bt steel only up to f'c of about 4500 psi.
    

RE: How to place minimum steel in a rectangular footing?

Structgen-

There are plenty of footing sections that have a greater reinforced moment capacity than cracking moment using 0.0018bh.  Run the numbers.  I did it for a 16" thick footing with f'c=4ksi (assuming d=12.5") and Mn/Mcr was about 1.05.  
I don't see how you make the case that a clause that is physically located in the section titled "MINIMUM FLEXURAL REINFORCEMENT" isn't actually minimum flexural reinforcement.  

As for 10.5.3 - Unless you have a ridiculously oversized section, it still provides more steel than 0.0018bh.  You would need a req'd rho (in the beam sense, rho=As/(bd)) of less than 0.0015 to require less steel than that required by 0.0018bh.   That is less than half of the min. per eq. (10-3).

RE: How to place minimum steel in a rectangular footing?

StructGen-

What you are saying, essentially, is that there is no minimum flexural steel requirements for slab or footings.  Do you believe that to be true?

RE: How to place minimum steel in a rectangular footing?

If you were a code writer who wanted the minimum tensile steel to be .0018bh, why would you write "the same as required by 7.12."  You'd have to be an idiot to write it that way. First of all, it would be so much easier to simply say that the minimum tensile steel shall be .0018bh. Secondly, you would avoid any ambiguity, as 7.12 is for tension and compression steel.  Thirdly, you would avoid having to change the code in the event that Article 7,12 changed.  It makes no sense.

RE: How to place minimum steel in a rectangular footing?

miecz-
You have a point, but I still can't get past the title of the section of the code that 10.5.4 is located in.  To have it located there and have it NOT be the steel that is referenced in the title makes even less sense.

RE: How to place minimum steel in a rectangular footing?

Here's how it makes sense to me.  I imagine that I'm writing Article 10.5.  So I write all of it but 10.5.4 and then I say to myself, "but I don't want anyone to forget to check T/S requirements."  So I tack on 10.5.4.

I think this way because I used to write design procedures for production engineers.  Writers have some unwritten cardinal rules.  Never link to an independent Article that may change, forcing you to rewrite your Article, or worse...  Try to be as clear as possible.  Try to be simple to read.  Article 10.5.4, as you read it, violates all that.

RE: How to place minimum steel in a rectangular footing?

You are saying that footings have no minimum flexural steel requirements?  That is the net result of your position.

RE: How to place minimum steel in a rectangular footing?

StructuralEIT

As I read ACI, for slabs with large bending, 10.5.1 provides a fixed minimum, and for slabs with little bending, 10.5.3 provides a variable minimum.  In the case of slabs with little bending, that minimum may be smaller than the cracking moment.  But it is still much larger than the applied moment.

RE: How to place minimum steel in a rectangular footing?

10.5.1 doesn't apply to slabs or footings, right?

RE: How to place minimum steel in a rectangular footing?

The way I read it, 10.5.1 applys to slabs.  10.5.4 does not override 10.5.1 for slabs.  I know some peole read it that way, but I don't.  Again, I read 10.5.4 as a reminder to check T/S for slabs with small bending moments.

If 10.5.4 overrides 10.5.1 for slabs, then, for footings with larger moments, the minimum steel is .0018bh, which, I think, is too small.

RE: How to place minimum steel in a rectangular footing?

The way we read it is Section 10.5 is for beams and not foundations. When code says Min Reinf. of Flesural members, the word members, to me, is the same as saying beams. 10.5.4 says Slabs and footings (which are foundations not members or beams). So after reading these posts I would change the word member in 10.5 heading to beam and delete section 10.5.4.

RE: How to place minimum steel in a rectangular footing?

The requirement for ultimate capacity in bending to exceed the cracking moment means a lot more in beams than it does in slabs and footings.  If a footing cracks, then what happens?  It stays where it is, doesn't fall down.  If a slab cracks, what happens?  The crack propagates far enough for redistribution to occur to a greater area of the slab.  So again, it doesn't fall.

RE: How to place minimum steel in a rectangular footing?

hokie66, Compression steel As' also works for flexure and is also the flexural steel, so the total flexural steel in a section is the sum of top and bottom.
So you believe that CRSI which is an independent organization is more credible then ACI-commitee member? what is the reason behind your conclusion?
For all practical purposes 5000 psi is the least strength one can use. See attached comparison at no point 0.0018bh provides a capacity comparable to Phi-Mcr.
 

StrEIT, where did you read this in my statement that there is no minimum requirment for footing and slabs, I believe it is exactly the same as other flexural members + temprature and shrinkage.
Do you know why 10.5.4 is applied only to combined footings, becuase slabs useually have more reinforcement than 0.0018 and isolated footings dont have top reinforcement.


You guys answere one simple question and I will call this debate off, if 10.5.4 is the minimum flexural steel why it is not applicable to beams which are the primary flexural members. dont tell me that I have over sized the section, since 10.5.3 can result a number lower then 10.5.4  

RE: How to place minimum steel in a rectangular footing?

StructGen-

So you are saying that 10.5.1 (and eq. (10-3)) applies to footings?  That seems to be what miecz was saying, too.  I've never read it that way, and this is the first time I'm hearing of someone else reading it that way.  It's certainly possible, and that would make your position more realistic.  Every engineer, program that I've used, and even PCA Notes has used the 0.0018 as a min.

RE: How to place minimum steel in a rectangular footing?

StructGen,

I actually gave an answer to your "one simple question" in the post before yours.

Why do you insist that "For all practical purposes 5000 psi is the least strength one can use"?  What's wrong with 4000?

I just think you are in the minority in this, and I hope all the designs I have done for 40 years using various versions of ACI318 and AS3600 have not been wrong.

I respect the ACI code writers, and I don't think there is a big issue with the way 10.5 is written.  I would prefer a clear division of the provisions for flexural steel from those for T&S steel.  Maybe it will happen next time.

RE: How to place minimum steel in a rectangular footing?

I dont mind being in minority as long as I am correct, what was that post exactly?

RE: How to place minimum steel in a rectangular footing?

StructuralEIT, please check CRSI book they use 10.5.1, 5.3 and 5.4 for min. reinf. in a footing, I dont completely agree with them because again they use 0.0018bh on each face.  

RE: How to place minimum steel in a rectangular footing?

2
I think the key here is that The SUBJECT (i.e. the noun) of 10.5.4 is....ready.....As

As is the flexural reinforcing that goes on the tension side of the member.

So 10.5.4 is saying that the value of As (on the tension side) is equal to the value given in 7.12.

hokie mentioned this earlier

Quote:

As,min is defined in Chapter 2 as "minimum area of flexural reinforcement".  

RE: How to place minimum steel in a rectangular footing?

Again it is flexural reinforcement, where did you get this idea that all flexural reinforcement should go on tension side, compression reinforcement is also flexural steel. Flexural steel = Top + Bott.

RE: How to place minimum steel in a rectangular footing?

So do you count both tensile and compression reinforcement in complying with 10.5.1 for minimum beam reinforcement?  I hope not.

RE: How to place minimum steel in a rectangular footing?

For sure not, because 10.5.1 says "at every section of a flexural member where tensile reinforcement is required....." I am bound to use Ast per 10.5.1 for Tensile reinforcement. I like your sense of humor.

RE: How to place minimum steel in a rectangular footing?

Thanks, but in this instance I wasn't trying to be funny.  My point is that both 10.5.1 and 10.5.4 specify values for As,min, and the definition of As,min does not change from clause to clause.

RE: How to place minimum steel in a rectangular footing?

Sure, "flexural reinforcing" can be considered as compression reinforcing - but compression rebar has its own set of limits totally unrelated to what this section is covering.  I think you are stretching a bit to use compression rebaar as an argument in this matter.

 

RE: How to place minimum steel in a rectangular footing?

Valid point, defination of As,min dont change from clause to clause, that is why ACI 318 uses two different notations for different clauses.
10.5.1 uses notation of "As" which is defined in Section 2.1 "As = Area of non-prestressed longitudnal tension reinforcement"
While 10.5.4 uses notation of "As,min" which is "minimum area of flexural reinforcement."

Thanks for bringing up this point I think we have nailed this issue.

    

RE: How to place minimum steel in a rectangular footing?

Look again.  As,min is used in 10.5.1 and also 10.5.2.

RE: How to place minimum steel in a rectangular footing?

Read the clause 10.5.1 and 10.5.3 not the eq 10-3,  

RE: How to place minimum steel in a rectangular footing?

It is clear to me in 10.5.1, 10.5.2, and 10.5.4 that As=As,min, but the values of As,min and thus As are different in the three clauses.  Clause 10.5.3 is for beams where 10.5.1 and 10.5.2 don't have to be complied with.

If As and As,min are not interchangeable in Section 10.5, how do you explain the use of As,min instead of As in 10.5.2?

RE: How to place minimum steel in a rectangular footing?

Exactly how this is clear to you that As=As,min while code explicitly calls it out as a different notation and further provides the defination of this notation in chapter 2.

10.5.2 is an special case, I will answere your question when you explain me what is your reason to assume As = As,min
   

RE: How to place minimum steel in a rectangular footing?

The whole section 10.5 is about minimum flexural reinforcement.  As,min is the value of this minimum, and varies with the three clauses, which are all "special cases".  As JAE said above, you are stretching the interpretation of As,min to include compression reinforcement.  He was too kind.  I would say you are dead wrong.

RE: How to place minimum steel in a rectangular footing?

Here's another thing to consider, StructGen.  (I was once in your camp but after a few rehashes with co-workers and on Eng-Tips here I changed my mind on this issue.  I hope you will consider this)

1.  The whole section 10.5 is dealing with flexural tension reinforcement.  Read the first paragraph of the commentary of 10.5 (section R10.5)  There it lays out the reason for the whole section - fear of sudden tensile failure of a section in flexure.  In fact, the word "tension/tensile" occurs five times throughout this section of commentary.

2.  The variable As is defined in Chapter 2 as "tensile reinforcement".  A's is the compression reinforcement.  So section 10.5, creating a minimum amount of As, is limiting tension reinforcement.  It does not say A's,min.  It is not limiting compression reinforcement at all.  This isn't even a slightly remote element of this section.

3.  Thus, the value As,min is a minimum value of tension reinforcement.

4.  Section 10.5.4 states that the minimum amount of tension reinforcement (i.e. a limit on As) is "the same as that required by 7.12".  The commentary, just to the right of it states it a bit differently:  The minimum reinforcement required for slabs should be equal to the same AMOUNT as that required by 7.12..  So the key word here is AMOUNT.  It is not saying you go to 7.12 and simply provide temp/shrinkage steel wherever the heck you want.  It is saying, precisely, that you use the AMOUNT of steel indicated by 7.12 as a minimum value for tensile reinforcement.

While this section has created multiple threads, discussions, and arguments, if you read through the logical language of the text, it is pretty clear what is required.

 

RE: How to place minimum steel in a rectangular footing?

hokie, With all due respect you are not logical at all to defend you position, calling me dead wrong is not going to strengthen your point of view. I understand it is difficult to accept that one was incorrect, I am quoting clauses and notations from the ACI and you are coming with what? You have left no room for further discussion with you. Thanks

   

RE: How to place minimum steel in a rectangular footing?

Yes, BA, that is for shrinkage and temperature reinforcement normal to the principal reinforcement.  The provisions for minimum flexural reinforcement are given in Section 911 of the 1963 code, and have remained essentially unchanged through the years.  That was the first code to introduce ultimate strength design and the one I learned first.  Professor Richard Barker knew it back to front, and he led us through all these provisions.

RE: How to place minimum steel in a rectangular footing?

StructGen-

We are all making our case through the logic and language of the actual code.  You seem to be making your case through Dr. Ghosh's email.  

Further, I would say that your idea that compression reinforcement can be considered "flexural" reinforcement and, therefore, counts toward As,min doesn't hold water in this case.  For the small amount of tension steel that we are talking about, the compression block will be very small.  Any top steel, with appropriate cover, will likely NOT be in the compression zone and would need to be 1.) discounted in the tension steel calcs, since it will be close to the NA and essentially worthless, hence rendering it unaccountable for in As,min; or 2.) counted in the tension steel calcs using a strain compatibility analysis, but then it's not on the tension face, and doesn't meet the requirement in 10.5.1 for steel on "the tension face".  The language of 10.5.1 still applies to footings and slabs, it's just that the amount of steel is controlled by 10.5.4.   

RE: How to place minimum steel in a rectangular footing?

JAE, thanks for detail insight, lets go one by one.

1) I suggest you should go through miecz posts above, he has given a great explanation of the flow of thought inside the code and how section 10.5 may have been developed.

2 & 3) I would have accepted your theory if code have not explicitly called out As,min = min. area of flex reinf. I believe if it would be as per your assumption code would have narrated this notation as "min. are of tensile reinf." What I mean is that this argument works both ways.

4) I believe you are looking at Pre-2005 code, 318-O2 used to have this syntex, 318-05 10.5.4 states "For structural slabs and footings of uniform thickness, As,min in the direction of span shall be the same as that required by 7.12..." and yes the "amount" is still there in commentary but why you assume it is tensile reinf. It is the total amount in the direction of span.

Again all these issues can be argued for ever, as StrEit said it is not black and white, my point is if something is not written clear enough what would you do? you will go to the source. That is what I did, I contacted ACI-318 commitee member. What I dont understand is why it is so hard for some people on this board to accept that they were wrong, why on earth one thinks that his interpretation of code over rules the explanation provided by ACI 318 Commitee member.
 

RE: How to place minimum steel in a rectangular footing?

hokie,

The provisions of ACI 318-63 Article 911 "Minimum reinforcement of flexural members" does not apply to slabs of uniform thickness.  For beams, the minimum percentage of tensile reinforcement is 200/fy or 1.33 times that required to resist the moment as you stated in an earlier post.  

A footing 35" x 5' x 14' is more like a beam than a slab, so I would be inclined to reinforce it in accordance with the recommendations for beams, not slabs.  However, I would probably use nominal transverse steel as the footing is eminently capable of resisting those moments without reinforcement.

The OP should be aware that, if the columns are some distance from the ends of the footing or if they are stiff enough to develop significant rotational restraint, the footing will have two points of contraflexure and will need to be reinforced both top and bottom.

BA

RE: How to place minimum steel in a rectangular footing?

We all make mistakes, and it is just possible that Dr Ghosh's language in this case is imprecise.  I suggest that if you want a definitive answer, you should copy this entire thread and forward it to him as evidence of how practicing engineers interpret the provisions of Section 10.5.  You might even invite him to join Eng-Tips and our discussion.

RE: How to place minimum steel in a rectangular footing?

Str.EIT, Yes I am making my case on the basis of ACI-Commitee members provided explanation, is that incorrect?
I am not agruing about the contribution of top steel for flexure, my whole argument is that As,min = total area of steel in the direction of span.
Let me question this why you assume that language of 10.5.1 applies to 10.5.4, if this would be the case ACI have placed a genralzied statement right under 10.5.
Now lets look at section 10.5 heading "Minimum reinforcement of flexural members". I have accepted your opinion if the heading would be "minimum flexural reinforcement"  

RE: How to place minimum steel in a rectangular footing?

1.  What may have been developed in various person's speculation is meaningless.  What the codes says is fact.  My post above is based solely on what the code says.

2 & 3.  I don't agree.  I think you are just wrong to bring compression steel into this.  It doesn't apply, doesn't make sense, and doesn't fit in the section's overall theme and concern - per the commentary.

4.  I'm looking at 318-05.  I've also looked at 08.  I quoted the text AND the commentary.  It says "amount".  I assume it is tensile reinforcement because THE WHOLE SECTION IS ABOUT TENSILE REINFORCEMENT.

Finally, your reference to Dr. Ghosh really defeats your argument.  In your own letter to him you note that the 318 states the following:

"ACI 318-02
10.5.4 "For structural slabs and footings of uniform thickness the minimum area of tensile
reinforcement in the direction of span shall be same as that required by 7.12...."

ACI 318-05/08
10.5.4 "For structural slabs and footings of uniform thickness, As,min in the direction of span shall
be the same as that required by 7.12...."

And Dr. Ghosh then states this:

I wouldn't want you to read too much into the change in the wording of Section 10.5.4 between ACI 318-02
and ACI 318-05. For ACI 318-05, it was decided to use notation in place of verbiage, whenever that is practicable.


So he's telling you that nothing changed substatially between 02 and 05, just wording to be consistent with the use of variables.  So he reinforces what 318-02 states, that it is all about tensile reinforcing.

I believe that Dr. Ghosh didn't understand your question.  I think he thought you were suggesting that 0.0018bh might have to be placed "per face".  He says you don't have to put 0.0018bh per face.  (his quote is: "it is not 0.0018bh per face").  

Finally, he refers to 15.10.4 commentary where it says,"Min. reinf. steel MAY be distributed near the top or bottom of the section, or MAY be allocated between the two faces of the section as deemed appropriate for specific conditions....".  

The language states clearly that it should be placed as appropriate for specific conditions.  Why would there be specific conditions that affect where the 7.12 rebar goes?  I mean, according to you it can go anywhere.  So this section clearly indicates that specific conditions can affect WHERE the reinforcing should go.  To me, that clearly means in tensile areas since combined footings and mats usually have varying positive and negative moment regions.





 

RE: How to place minimum steel in a rectangular footing?

StrucGen, I would second the idea of again communicating with ACI - specifically the persons who wrote section 10.5.4 or reviewed it in the recent code editions.  I don't know whether Dr. Ghosh is the right person.  How did you come to choose him to communicate with?

I've also done a few preliminary calcs on this and found that 0.0018bh does indeed meet or exceed the value of Mcr for 7 and 8 inch thick slabs.

b = 12"
d = 7"
f'c = 4 ksi
fy = 60 ksi

0.0018bh = 0.173 in^2
#4 @ 13" o.c. = 0.185 in^2/ft of slab

fr = 7.5 x sqrt(f'c) = .474 ksi
Ig = 512 in^4
y = 4"
φ = 1.0

Mcr = frIg / y = 60.7 in-kips

As calculated to resist 60.7 in-kips = 0.147 in^2/ft of slab

Therefore, 0.0018bh provides greater than Mcr.


 

RE: How to place minimum steel in a rectangular footing?

The chairman of the "Flexure and Axial Loads; Beams, Slabs, and Columns" committee is Robert Frosch.  I'll post a question to him on the ACI site.



 

RE: How to place minimum steel in a rectangular footing?

JAE, You are only reading what supports your opinion from Ghosh's email, he is very clear, I am amazed how come you come up with this argument that he might not have understood my question. Read again I am copying it below for you.

"0.0018bh is the total amount of reinforcement you provide in each orthogonal direction of a structural slab or footing - it is not 0.0018bh per face. This becomes abundantly clear if you refer to the new commentary we have placed under Section R15.10.4, which applies specifically to combined footings and mats. Section 7.12 gives you shrinkage and temperature reinforcement. There should be no expectation that this reinforcement would prevent sudden flexural failure. I do not believe that ACI 318 has tried to produce such an impression. Section 10.5.4 essentially says that if you have provided the minimum shrinkage and temperature reinforcement required by Section 7.12, you shall be deemed to have satisfied the minimum flexural reinforcement requirement of Section 10.5."
 

RE: How to place minimum steel in a rectangular footing?

Heh - "it is not 0.0018bh per face".   Obvious to me that he thought you were suggesting 0.0036bh total.

When he then states that there should be no expectation that it prevent sudden flexural failure, I just don't see it with the calcs I did.  It clearly CAN exceed Mcr.

I've posted a technical question to ACI and I'll see what they respond with.  I'll also talk to a few other profs I know who sit on ACI committees, specifically Robert Frosch's committee.

RE: How to place minimum steel in a rectangular footing?

JAE, I would appreciate if you pose this question to the chairman of ACI Committee 318-0D, I am hopefull that Mr Frosch will enlighten us.

RE: How to place minimum steel in a rectangular footing?

Dr Ghosh says "I do not believe...", which means just that this is his opinion.  He did not write it, as it has been there since 1963.  He may agree with you, although I am not sure of that based on his response to your enquiry.  If he does, it will be interesting to hear the opinion of Robert Frosch.

RE: How to place minimum steel in a rectangular footing?

JAE, If you are posting your question to ACI technical advisery (free service for members) please keep in mind that your question will never reach to Mr. Frosch, please do check the disclaimer of that service, it clearly states that "opinion expressed by the technical staff is not the official position of ACI" and you know how these tech. support works, couple of grad students from Northweastren or University of Chicago doing there internship in skokie illinois.

RE: How to place minimum steel in a rectangular footing?

Check out the attached...this is from a recently updated concrete text book by MacGregor & Wight.  MacGregor is on the ACI Subcommittee.  It is an example problem for a structural slab design and refers to 10.5.4.  

Note that clearly the use of 7.12 reinforcement is applied on both top and bottom surfaces of the slab for, respectively, negative and positive moment regions.

This textbook, at least, verifies that the 7.12 reinforcement is a minimum TENSILE reinforcement requirement.

 

RE: How to place minimum steel in a rectangular footing?

Also, I don't think there's any discrepancy between the wording of As or As,min.  10.5.1 says As provided shall never be less than As,min.  Then in 10.5.1 it gives the basic equation for As,min.  Then 10.5.2, 10.5.3, and 10.5.4 gives exceptions to the equation in 10.5.1 (i.e. exceptions to the quantity of As,min, not exceptions to the presence of As,min).

RE: How to place minimum steel in a rectangular footing?

Wow!  When I read Dr. Ghosh's email, I thought it was pretty clear, and that it supported what I believed before I read it.  After reading other posts, I guess this is a case of reading what you already believed.

I have to take issue with my previous post being "meaningless."  Article 10.5 is not clear.  The committee that wrote Article 10.5 could have written it differently.  I think it is beneficial to speculate as to why it is worded the way it is.  Did they word it that way because:

A.  The research on flexural behavior of slabs coincidentally came up with the same amount of minimal tensile reinforcing as is required for temperature/shrinkage of the entire section, so they decided to save ink and paper by referring to section 7.12?

B.  The research on flexural behavior of slabs found a causal relationship between the amount of minimal tensile reinforcing as is required for temperature/shrinkage of the entire section, so they decided to point out the relationship by referring to Article 7.12?

C.  The committee writng Article 10.5 found no relationship between the amount of minimal tensile reinforcing as is required for temperature/shrinkage of the entire section, but they wanted to tell designers that Article 7.12 also needed to be checked?

These are the only three scenarios that I can think of, and "C" seems a lot more plausible to me than "A" or "B."

Finally, as some have brought up other references, allow me to bring up ACI 350.  This code is very similar to ACI318, but is specifically geared toward tank walls, i.e. structural slabs.  The wording is ACI350 is slightly different than ACI318.  It is more precise.  I think ACI350 clearly has T/S steel checked in addition to minimal steel required for flexure.  

 

RE: How to place minimum steel in a rectangular footing?

JAE, let me give you a better one from Macgragor, take a look at the attachment, here he clearly spell out what he bleives. But I dont think he is correct reason is this simply dont work, please take a look at the calculation I posted above, think for a moment ACI dont write code for a specific concrete strength or size of section. Code clause has to be applicable universally unless a limit is defined. In this case it just dont work for any thing above 5000 psi, even for 4000psi you have to rig up the Phi-factor in your calcs.
 

RE: How to place minimum steel in a rectangular footing?

StructGen,

I read your attachment and it states exactly what we've been suggesting all along - that the As,min from 10.5.4 is a minimum tensile reinforcement.

I did a comparison for slabs with f'c = 4000 psi and 6000 psi using phi = 0.9.   See attached.  What this shows is that the 7.12 steel very closely compares with the cracking moment when placed near the tensile side of the slab.

One thing to note here too is that Mcr isn't necessarily a good measure of prevention of catastrophic, sudden failure as all concrete essentially cracks under load at very low moments.  What this shows is that 7.12 steel, even up at f'c=6000 psi, gets you within a narrow range of the cracking moment.

I'm also surprised that you were critical of others for questioning Ghosh's opinion and directly above you question MacGregor's.

RE: How to place minimum steel in a rectangular footing?

JAE, Let me explain this contradiction, yes I was critical of questioning Ghosh's statement becuase none of the question came up with any logical backing or calculation. Forum participants were giving there own opinions, well its not the matter of vanilla vs strawberry vs choclate.

I have added 4000psi conc in my calc and I am attaching it for you, slabs all over united states are done with min. 5000psi conc. and conc strength for slabs go up to 8000 psi. only metal decks are poured with 4000psi.

Why I think Macgragor is incorrect becuase his statement dont reflect the fact, when I crunch the numbers it dont work. Again if the intention of ACI is what Macgragor is writing in his book then phi-Mn should be greater then Mcr by a margin of 1.1 to 1.5 for all concrete strengths. But this is not the fact, it just dont work. Either Macgragor is incorrect or he lives in the world of 2000psi where he get a 1.5 ratio of phi-Mn/Mcr for 0.0018bh  

RE: How to place minimum steel in a rectangular footing?

JAE, also you are trying to go above and beyond to defend your opinion, if Mcr isn't necessarily a good measure of prevention of catastrophic, then you are not agreeing with your own argument.
Let me tell you, I work on commercial buildings, office, high end condos, I have not used 4000 psi from last 5 years. I have used concrete up to 12000 psi and have seen pour breaks of 16000 psi, Macgragors statement is not from our world.
I am attaching a calculation based on Eq 10-3 check this out you always have a constant ratio (for each section) regardless of your concrete strength, this is called min. reinforcement.

RE: How to place minimum steel in a rectangular footing?

So using a 1/3, 2/3 distribution the 0.0018 reinforcement, the bottom of footing steel could be as little as 0.0006bt? It seems unlikely though I can see how one might read it that way.

Maybe we are splitting hairs here because under real world loading conditions the minimum flexural reinforcement calculated (x 4/3) will almost always be greater than 0.0018bh.

RE: How to place minimum steel in a rectangular footing?

StructGen-

I think you are "not in our world" if you never see f'c<5ksi.  Additionally Mcr is NOT a good measure of prevention of catastrophic failure because there is no warning, no ductility.  The member cracks, and bang...... failure occurs immediately.

I'll look at your calcs tomorrow.

RE: How to place minimum steel in a rectangular footing?

...laptop powered out before I completed the thought.

I see where StructGen is coming from with pro rho min argument, but I think the logic in the Code predates the argument he is presenting. For most isolated 3-4 ksi footings history has found that the rho min (as for beams) is overkill. Unless of course your client's last name is Steinbrenner.

New materials and non-traditional uses may mean new thinking for certain code provisions. Maybe this is one of them.

RE: How to place minimum steel in a rectangular footing?

StructGen - I don't know of any reference where minimum tensile reinforcement is intended to be above Mcr.  

You are basing your opinion on Ghosh's statements (which I think are confused) and on your presumption that Mcr is the limit state for catastrophic failure.  

Your statement of using f'c in the high end may be true in the general high office building world you work in but in the vast number of projects in the US, f'c = 3000 psi or f'c = 4000 psi is very often used.  f'c = 5000 to 6000 is used for prestressed concrete more times than not.

The more I've looked into this the more I'm convinced that the 7.12 reference is to the AMOUNT of TENSILE reinforcement in a flexural member.  I'm convinced.  The text books I've looked at all conform to this.  The exact English language logic agrees.  Nothing you've said has been a convincing argument to me at all so I'm done here.

I'm weary of this back-and-forth thread so until I hear back from ACI and another professor I've contacted, I'm letting this go.  We'll just have to agree to disagree.   Still with respect to you and everyone on this thread of course.

 

RE: How to place minimum steel in a rectangular footing?

Footings don't catastrophically collapse because of too little steel.  The soil retains the cracked sections until the steel picks up the load.  Ductile behaviour then occurs.

Flexural reinforcing in slabs is provided in the direction of the span, which can be either in one direction or two.  If a crack forms, the steel in the other direction, whether flexural or T&S, distributes the load in the transverse direction, therefore slabs with minimum reinforcement don't collapse catastrophically either due to too little steel, as ductile behavior occurs due to the redistribution.  No "bang" happens.  Beams bang, slabs sag.

The argument about varying reinforcement with concrete strength raises another question.  If a slab or footing is designed for 3000 psi concrete and the test results show 8000 psi, is the concrete to be rejected?  I think not.

Using high strength concrete in slabs is a poor use of the material.  

RE: How to place minimum steel in a rectangular footing?

JAE, looks like we lost each other in the middle of discussion, the comparison of Phi-Mn/Mcr is mentioned in the reffrence I posted from Macgragors concrete design book 4th edition.

ACI in R10.5 also explains the same reason of minimum reinforcement. "With a very small amount of tensile reinforcement, the computed moment strength as a reinforced concrete section using cracked section analysis becomes less then that of corresponding unreinforced concrete section computed from its modulus of rupture, faliure in such a case can be sudden"

   

RE: How to place minimum steel in a rectangular footing?

results, who is right??

RE: How to place minimum steel in a rectangular footing?

When egos run high on a sensitive topic, I don't think we will each a conclusion on it.

RE: How to place minimum steel in a rectangular footing?

It should not be a sensitive topic.  And egos should not play a role in it.  The argument is not of great importance anyway.  Anyone who believes that temperature steel specified in the code is inadequate to prevent sudden catastrophic collapse is free to provide additional reinforcement.  If unsure, take the conservative route.

BA

RE: How to place minimum steel in a rectangular footing?

It's egos which can make any topic sensitive, regardless of the nature of the topic.

The one thing that is clear is that ACI318 is unclear on this subject.

RE: How to place minimum steel in a rectangular footing?

BA,

For some reason I missed your post of 30 August, 21:13.  Section 911(b) of ACI318-63 does cover minimum reinforcement of structural slabs of uniform thickness.  It is essentially the same provision which we have now been arguing about.  All those years, I thought it was clear, but maybe it isn't.

RE: How to place minimum steel in a rectangular footing?

hokie,

The meaning in ACI318-63 seems clear enough to me.  The reason for treating slabs of uniform thickness differently from other flexural members is not clear to me.

I do not have the current ACI318 so cannot comment intelligently on its minimum reinforcement provisions.

   

BA

RE: How to place minimum steel in a rectangular footing?

BA,

The argument I made 31 Aug 17:49 about slabs vs beams is not mine originally.  I read it in a text, but can't put my hands on it now.

RE: How to place minimum steel in a rectangular footing?

OK - I sent a question to ACI and I got a response I got back from them today.

My question to ACI was this:  
I have been discussing an issue related to ACI 318-05, section 10.5 with a number of other engineers on an engineering forum website.  The issue that has many of us confused is as follows:

Section 10.5.1 through 10.5.4 presents minimum areas of reinforcement for flexural members.   10.5.1 provides a formula for As,min for tensile reinforcement.

10.5.2 provides for T-beams with flanges in tension.

10.5.3 provides an out for very large sections, using the 1/3 greater than As(calc).

So all of these are very clear.  No problems.  

However, we come to 10.5.4 and it states that for "structural slabs and footings of uniform thickness" we are told that As,min is the AMOUNT (commentary) required by 7.12.

The confusion we have here can be resolved into two different interpretations:

Interpretation 1.  
For structural slabs and footings - 10.5.4 points you to section 7.12 and you calculate 0.0018bh and insert that into your structural slab or footing in any place within the thickness you wish.  7.12 doesn't require positioning of the min. steel in the tension area.  You can put it in the middle, in the top, split it between faces, etc.  Also, since 10.5.4 refers to 7.12, we don't need to meet 10.5.1-3 at all for slabs and footings.  In addition, the 318-08 commentary for 15.10.4 seems to imply that the 7.12 reinforcing can be place anywhere within the section.

Interpretation 2
Section 10.5.4 is located within section 10.5 which is clearly dealing with flexural reinforcement minimums in tension.  Therefore, 10.5.4 is referring to 7.12 to give us an AMOUNT of As,min, and this should be placed in position where tension occurs.  It should never be split between faces, placed in the center of the section, or especially placed in the compression area of the section.  
In addition, section R15.10.4 states that this 7.12 reinforcement should be placed "as deemed appropriate for specific conditions" - therefore, the only specific condition that would sway me where to place the reinforcement would be where tensile stresses occur.

This section has produced numerous lines of communication on our website and I know it is a very confusing section of 318.  I would ask if you could clear this up for us.  

1.  Does 10.5.4 imply that 7.12 is simply an AMOUNT of reinforcement and that it still should be placed in tension areas?  

2.  Does 10.5.4 then negate the use of 10.5.1-3 for structural slabs and footings or should structural slabs and footings also meet those sections in addition to 7.12?

3.  Should the 10.5.4 reinforcement from 7.12 be a second check on As,min.  In other words, do we check 10.5.1-3 for a structural slab and THEN also check to see if it meets 7.12?

Thanks for your help.


Here is ACI's response:

ACI publishes codes, specifications, and reports for the concrete user.  This message is in response to your technical question/inquiry.

ACI 318 is a consensus document and thus language is debated until all committee members can come to an agreement. This may not lead to the most concise set of requirements as many voices are trying to relay their thoughts.

Question 1. Attached is a copy from MacGregor and Wight book addressing this issue. see the attachment below

Question 2. Section 10.5.4 directly sets the minimum requirement for a subset of flexural members, structural slabs and footings of uniform thickness.

Question 3. See answer to Question 2.

Sincerely,
Technical Staff  


So it appears that they refer to MacGregor's book and in it he clearly indicates that the 7.12 steel is a tensile reinforcing and should therefore be placed in the tensile area of the footing or slab.

They also indicate that 10.5.4 "directly" specifies the min. steel for slabs and footings in response to my question on whether slabs and footings also need to meet 10.5.1, 10.5.2, etc.

Hope this helps - sorry for the long post.

 

 

RE: How to place minimum steel in a rectangular footing?

Funny how ACI still, even in the response, never actually uses the word "tensile" but refer you to somebody else who did.  Why can't they just say to use that as a check for min tension steel?  I never questioned that this was their intent but it's nice to hear it anyways.  Thanks JAE.

RE: How to place minimum steel in a rectangular footing?

So is the mystery solved?

RE: How to place minimum steel in a rectangular footing?

Well, to me having ACI refer to MacGregor's book where he clearly states that 10.5.4 rebar (from 7.12) is TENSILE reinforcement, and MacGregor's examples show the 7.12 steel is placed wherever tension exists whether top or bottom clears it up for me.

 

RE: How to place minimum steel in a rectangular footing?

So if you put all the T&S steel on the tensile side of a 2' thick footing,  no need to worry about T&S cracking on top side?

RE: How to place minimum steel in a rectangular footing?

no dont you put all the min tensile on tension zone and whatever is required in the top (or bottom) for T and S? Half total or more/less if thick or thin.   

RE: How to place minimum steel in a rectangular footing?

The lesson I take from this is that one needs to be very clear when writing, because the reader's interpretation is going to be clouded by what he/she thought before reading it.

I am impressed by the clarity of JAE's question to ACI.  However, the response is disappointing.  Why, if this is what they meant, couldn't they have answered question 2 with something like: Yes, 10.5.4 negates the use of 10.5.1-3 for structural slabs and footings. Case closed.  How about a little discussion as to how two seemingly unrelated phenomena have the identical requirement?  I wish they had written something logical, rather than authoritative.

RE: How to place minimum steel in a rectangular footing?

miecz,

I feel the same disaapointment in ACI's reply - but as was mentioned earlier, these are tech questions that are answered by non-committee members.  But their reference to MacGregor's book helped.  MacGregor is on the 318 committee.

RE: How to place minimum steel in a rectangular footing?

Quote:

In building construction the amount of such steel is usually arbitrarily chosen as 0.002 to 0.0025 of the slab area

The above is a quotation in reference to Temperature and Shrinkage Reinforcement from "Introduction to Reinforced Concrete Design" by Hale Sutherland and Raymond C. Reese (Copyright 1943...Second edition 1953.  Mr. Reese was Chairman of ACI 318-63.

BA

RE: How to place minimum steel in a rectangular footing?

I agreeed with BA, the top face of thicker footings shall not have without any reinfcrcement, or otherwise make the footing tapered (If not uplift) at the end or otherwise expect cracks.

But what is the limit at which footing acts as thicker?  

RE: How to place minimum steel in a rectangular footing?

I don't think BA said that.  There is no reason at all to place reinforcement in the top of a footing if the top is in compression.  Invariably, top reinforcement in thick footings CAUSES plastic settlement cracking due to inadequate consolidation.

RE: How to place minimum steel in a rectangular footing?

JAE, as I have said again in my previous post, the answere you have got from ACI is not the official standing of ACI please do read the disclaimer on website. This is some one's interpretation of code not the opinion of ACI-318 Commitee. What you got is nothing more then a personal opinon of you me or any one else.
Have you had a chance to talk to Mr Frosch of 318-commitee member?

I am post disclaimer of ACI for you guyes

TERMS AND CONDITIONS OF USE

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ALL INFORMATION IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR NON-INFRINGEMENT. Neither ACI nor its subsidiary corporations, affiliates, principals, officers, employees, agents, and their successors ("Releasees") shall be liable for damages of any kind, including any special, indirect, incidental, or consequential damages, including without limitation, lost revenues or lost profits, which may result from the use of any Information.

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Do you accept the above Terms and Conditions of Use?


 

RE: How to place minimum steel in a rectangular footing?

We have to remember that the ACI code (like almost all codes) is the work product of a committee.  Like the old saying, "a camel is a horse designed by committee".

RE: How to place minimum steel in a rectangular footing?

StructGen, no - I haven't had time to contact anyone else such as Mr. Frosch.

I understand your mention of the disclaimer - but when I find in MacGregor's textbook an application of that section that concurs with the tech response, and two places in his book that confirm a tensile use of 7.12, then I'm convinced.  You can believe what you want, OK?

I appreciate the debate here in this thread and on Eng-Tips in general.

 

RE: How to place minimum steel in a rectangular footing?

Must say this is a great debate and very interesting. This could go on forever I think what needs to be taken into consideration is:

1. The supporting sciences for codes do not always provide crisp answers to engineering problems, building requires the use of judgment. Judgment has soft boundaries and is influenced strongly by what is considered to be acceptable risk.

2. THE PURPOSE OF THE CODE: the main function of the Code is to keep people out of trouble, to make structures safe, to make it difficult for somebody to design an unsafe structure.

As Hokie point out, the committee that developed the code decided what was acceptable risk and this committee also decided what science to use to develop the rule.

Thus even if the committee chairperson said that "Clause 10.4 is ....". You could still debate this interpretation, based on other knowledge ect.

Based on what I consider acceptable risk for my designs, I would use definition used in MacGregor book. and sleep well.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

RE: How to place minimum steel in a rectangular footing?

Stru gen you are rejecting mcgregor approach on the basis he is only one member of the commitee and so not neccessary the commitee view but still hold fast that Dr ghosh is absolutely correct despite only being one member? I would suggest going with mcgregor as also the technical help states as this is the more conservative route and rest easy. If you were in court and a lawyer asked ACI for technical help they would get that response, maybe worth remembering.  

RE: How to place minimum steel in a rectangular footing?

herewegothen

Problem is, in many cases, the Macgregor approach is not the conservative route.  At this point, I'm left with doing it both ways, and picking the more conservative of the two. Oh, well, it's not my money...

JAE

Not only does Macgregor have it that way, but every textbook I've looked at has it the same way as Macgregor, except for PCA's "Rectangular Concrete Tanks", which follows ACI350.  My wife says I can be very stubborn.

RE: How to place minimum steel in a rectangular footing?

How does ACI318 define flexural cracking moment?  What area of steel is required to resist 1.1 to 1.5 times flexural cracking moment?   

BA

RE: How to place minimum steel in a rectangular footing?

AS3600 only deal with concrete strength of up to 65 mPa. Having said that its only a matter of months before the new AS3600 issue is out which i understand caters for up to 100 mPa concrete.

Going back to footing reo,be careful with placement of tensile reo at correct face as footing with two columns require flexural steel at top face! (inverted beam).

RE: How to place minimum steel in a rectangular footing?

The 35" deep footing supporting two columns described by the OP is more of a beam than a slab.  Minimum reinforcement should not be taken from the slab provisions of the code but from the beam provisions.    

BA

RE: How to place minimum steel in a rectangular footing?

BA,

We would have to agree to disagree on that.  A footing is a footing.

RE: How to place minimum steel in a rectangular footing?

Hokie,

See this attached example. It's copied from "Design of Concrete Structures" by Arthur H. Nilson & David Darwin (Cornell and Kansas Univ. Professors), it's a text book for engineering graduates in some of the universities.

Anyhow the idea and the way he is treating the slab as beam spanning between supports make sense.  
 

RE: How to place minimum steel in a rectangular footing?

The Nilson and Darwin example supports BA's opinion as to which minimum reinforcement provision should be applied in the case of a strap footing.  At least the example doesn't show reinforcement being used on the compression face where none is required.

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