plain concrete question
plain concrete question
(OP)
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.
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.






RE: plain concrete question
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.
RE: plain concrete question
RE: plain concrete question
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.
RE: plain concrete question
RE: plain concrete question
My question pertains more to an isolated spread footing under a column such that any crack would be detrimental
RE: plain concrete question
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.
RE: plain concrete question
RE: plain concrete question
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.
RE: plain concrete question
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.
RE: plain concrete question
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.
RE: plain concrete question
RE: plain concrete question
RE: plain concrete question
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".
RE: plain concrete question
RE: plain concrete question
RE: plain concrete question
RE: plain concrete question
RE: plain concrete question
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.