plain concrete question 3

[Lion06]

Here goes a question about plain concrete. There are just some questions that I wish I would have come up with in school, but I didn't. If you design a footing as plain concrete and it meets the strength requirements of chapter 22, how do you ensure that it doesn't crack from shrinkage considerations (or a combination of shrinkage and residual stresses from shrinkage)? I guess I am thinking that as this unreinforced section tries to shrink, that even if it doesn't crack there will be residual tensile stresses in the concrete that may (or may not) be located near the critical section for moment. How would you go about accounting for this in your design?
I understand this is mostly academic as I haven't seen a plain concrete footing called for and I would never call for one myself. The code does have this section available, however, and I am sure some people do use it.

 

[jike]

With regard to isolated column footings, they are generally small enough that shrinkage stresses will not be sufficiently large enough to cause cracking. With regard to continuous wall footings, they normally just let these crack (transversely) where they will since there usually is a wall with some longitundinal reinforcing above which will tie the two cracked sections back together.

Were you aware that during the WPA days, (a long time ago) when reinforcing steel was hard to obtain, many unreinforced gravity retaining walls were constructed for grade separation projects in about 20 foot long sections.

 

[shin25]

Why don't you check chapter 22 of ACI 318, which is for plain concrete. See if the limitations apply for your footing.

 

[UcfSE]

Check ACI 318 section 22.3 and R22.3. You should find your answer there. Basically you provide joints often enough to address the problem.

How much do you think a footing will shrink, with the relative dimensions it would have to be to make plain concrete work? A wall footing may try to shrink significantly longitudinally. That's why its a good idea to provide all those continuous bars. Using the plain concrete provisions are good for checking that your footing doesn't need cross bars. These are commonly left out in my area for residential footings. They are a hassle to put in there.

 

[hokie66]

For a wall footing under a concrete wall, there is often no need for any reinforcement. It depends on the dimensions of the footing and whether transverse bars are required for bending reinforcement. In the longitudinal direction, there is no reason to prevent restraint shrinkage cracking.

 

[Lion06]

I am not talking about under a wall footing. A crack perpendicular to the wall wouldn't really matter since the bending is about the opposite axis in that case.
My question pertains more to an isolated spread footing under a column such that any crack would be detrimental

 

[shin25]

StructuralEIT,

You cannot have your footing unreinforced. Per ACI you cannot have this kind of isolated footing unreinforced. Minimum required reinforcing for a footing of this sort is .002Ag.

 

[Lion06]

Where exactly do you find the 0.002Ag? T&S is 0.0018Ag. Also, I didn't read anywhere in chapter 22 that this chapter is prohibited for isolated spread footings. If you can point me to where this is prohibited, I would greatly appreciate it.

 

[miecz]

The Commentary R22.2.2 states that the "use of structural plain concrete members should be limited to members that are primarily in a state of compression".

I would say that, generally, an isolated footing does not meet that criteria. Unless the plan dimensions are small, there will be significant tension at the bottom of the footing.

 

[jike]

The only times that I have ever seen plain concrete isolated footings used have been:

1) in houses construction under basenment posts in non-seismic areas

2) one time in an existing warehouse on the west coast and I question their use in a seismically active area.

As others have mentioned, it is fairly common to use unreinforced foundation walls and footings in light commercial construction but then longitundinal bars are added to control cracking.

 

[Lion06]

Alright so let's say, hypthetically, that a contractor calls you and says, "I forgot to put the rebar mat in this footing. It was supposed to be 6' x 6'. I did over-excavate it and the in place footing is actually 9' x 9'. Will this work?"
Do you automatically say, "No, rip it out!"? Or do you check it per chapter 22?
If you do check it per chapter 22, then I have to ask the original question again.

 

[Lion06]

sorry, hypothetically.

 

[BigH]

Wow! Might you all look into many of the "old" texts on foundation design and constrution (1930s and earlier). I believe that there were a great many foundations that were cast as plain concrete (seconding jike's comment on the WPA). I'll be searching for examples over the next several days. I remember reading this in many of the older texts. Footings were typically quite thick though in order to ensure that the stresses stayed within the footing width and didn't lead to tension.

 

[UcfSE]

StrlEIT, no it doesn't work unless they cast it much much thicker as well. As you know, the moment in the footing increases with the increased width. Casting is bigger like that and keeping the specified thickness would not work. That only made it worse.

You need a big section modulus to get concrete to work in tension. Remember that we are are saying you can use plain concrete but that doesn't mean you'll get section dimensions similar to what we commonly see with reinforced footings.

Besides, they forgot the steel anyway. Yes, rip it out. You always say "let me check first".

 

[Lion06]

Yes, I realized that after I posted it that the moment is affected by L^2.

 

[hokie66]

An unreinforced column footing is one whose depth is great enough that bending is not a consideration. In general, deeper than it is wide.

 

[civilperson]

All concrete is to be assumed cracked, and the plain concrete will work just fine with a crack. Any isolated footing which extends out from the supported limits less than the depth will work as a plain concrete foundation. Walls, columns, pedestals can all be supported on plain concrete.

 

[Tomfh]

Concrete needn't be assumed cracked in plain concrete design.

 

[vincentpa]

Shrinkage and Temperature stresses will not be as large a problem on spread footings as say walls because there is not a lot of restraint for a spread footing. It can shrink in all directions. The dimensions are usually relatively small for unreinforced spread footings too. Both of these factors combined usually result in an uncracked section.

As for ACI stating the spread footing should be primarily in compression; I believe their intent was that the footing should not be part of the MWFRS. A spread footing will always have some tension. I have seen isolated spread footings for exterior columns supporting canopies for strip malls. Generally when I see isolated unreinforced spread footings, they are lightly loaded and small in plan dimensions i.e. 3'x3' or 4'x4' with a relatively large pier. The reinforcement will not be able to develop beyond the pier anyway even if you put it there. Therefore the spread footing is for all intents and purposes unreinforced. I have designed many spread footings as unreinforced even though I have reinforcement in them. Why do I put light reinforcement (0.0018Ag w/o considering development length) in them? Just in case, but I still design the small footing as unreinforced.

Another place where realatively large piers exist with smaller footings is PEMB end wall column foundations. The size of piers has grown proportionally with the thickenss of ACI 318. Appendix D has increased the need for a larger pier but the footing size remains the same. So you might have a large pier on a small footing.

For examples of unreinforced design, see PCA Notes on 318.