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New Footing on Top of Existing

New Footing on Top of Existing

New Footing on Top of Existing

(OP)
Got a job where the contractor completely messed up the footing sizes on my job. For a number of reasons, we would prefer to leave what is there and pour on top (see detail)
Is this ok to simply use the existing footing and flowable fill as a giant spacer down to bearing soil?

RE: New Footing on Top of Existing

I would do this in two stages:

1) Cast the flowable fill up to the elevation of the existing footing.

2) Cast the new footing.

This is probably what you've got in mind anyhow. I'd do it in two stages like this to compensate for any vertical shrinkage that might occur in the vertical column of flowable fill.

Other than that, KootK approved.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: New Footing on Top of Existing

(OP)
Thanks KootK.
So your concern is that the existing flowable fill will continue to shrink? (it has been there about two weeks)
Could that be offset by the initial shrinkage of the new concrete.

I have never seen a mess like this before. The footings are the wrong size, in the wrong places, out of level by 3" in many areas- - it is simply amazing!
Of course none of my vertical rebar for retaining walls exists either.

RE: New Footing on Top of Existing

Quote (XR250)

So your concern is that the existing flowable fill will continue to shrink?

No. My concern is that the new flowable fill below the elevation of the top of the existing footing will shrink relative to the existing footing / flowable fill. If a gap opens up on the left hand side between the new flowable fill and the new footing, it may interfere with achieving uniform bearing. Or maybe I'm just being paranoid. It's a pretty smallish thing overall.

Quote (XR250)

Of course none of my vertical rebar for retaining walls exists either.

Hands over ears...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: New Footing on Top of Existing

Not sure what your loads look like, but you may want to check that there is not a possibility for the pours to separate under lateral or uplift loads if you intend for the pours to work together. Without any reinforcing between the pours you would be relying on bond strength between the pours (which may suffice). If you have reinforcing in the 12" cmu, may need to take a look at how it will be developed in the new pour or anchored in the first pour.

RE: New Footing on Top of Existing

I would agree for the most part, but restrict the top of the flowable fill to no higher than the previous flowable fill level, then your normal weight concrete on top of that with your new rebar as shown. tiphat

Mike McCann, PE, SE (WA)


RE: New Footing on Top of Existing

I like KootK's idea.

One question though, have they already backfilled? Depending on the soil on site, it may be difficult to make the narrow excavation to the left of the recent footing and flow fill.

RE: New Footing on Top of Existing

(OP)

Quote (KootK)


No. My concern is that the new flowable fill below the elevation of the top of the existing footing will shrink relative to the existing footing / flowable fill. If a gap opens up on the left hand side between the new flowable fill and the new footing, it may interfere with achieving uniform bearing. Or maybe I'm just being paranoid. It's a pretty smallish thing overall.

I was not planning on using any more flowable fill. Just pouring the whole thing with concrete. Flowable fill here is about the same cost as concrete and it would just add another step to the construction.

Quote (DetSTRU)

One question though, have they already backfilled? Depending on the soil on site, it may be difficult to make the narrow excavation to the left of the recent footing and flow fill.

That wont be a problem. No backfilling was done or will be done.

RE: New Footing on Top of Existing

Quote (XR250)

I was not planning on using any more flowable fill.

Got it. Same concern though.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: New Footing on Top of Existing

(OP)
How long would you wait between pours?

Let's say we had 2 ft. of concrete and then pour the footing on top of that. I was under the impression that concrete shrinks 1/8" in 20 ft so we should get .0125" of shrinkage? Is that really a problem considering the variability in soil conditions etc.? (wet clay in this case).
The only reason I ask is that the contractor will, of course, want to do it as one pour.

RE: New Footing on Top of Existing

Quote (XR250)

I was under the impression that concrete shrinks 1/8" in 20 ft so we should get .0125" of shrinkage? Is that really a problem considering the variability in soil conditions etc.?

Fine, you've sold me. Single pour.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: New Footing on Top of Existing

(OP)

Quote (KootK)


Fine, you've sold me. Single pour.
Must be because you are working late :>

RE: New Footing on Top of Existing

Probably. But after a night's rest I still find your logic sufficiently persuasive.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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