How to place minimum steel in a rectangular footing? 14

[pob11646]

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?

 

[Lion06]

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.

 

[pob11646]

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?

 

[oneintheeye]

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.

 

[Lion06]

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.

 

[DaveAtkins]

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

 

[Lion06]

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?

 

[ishvaaag]

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.

 

[DaveAtkins]

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

 

[youngstructural]

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!

 

[hokie66]

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.

 

[pob11646]

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.

 

[DaveAtkins]

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

 

[Lion06]

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.

 

[oneintheeye]

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

 

[Lion06]

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.

 

[pob11646]

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?

 

[Lion06]

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.

 

[hokie66]

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.

 

[oneintheeye]

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.