Modifying Existing Continuous Footing

[Antnyt23]

I drew up a quick detail to modify an existing continuous footing in order to adequately increase the bearing for additional gravity loads coming down on the existing CMU wall/Footing. I was curious on some opinions on issues, improvements, and/or concerns with this modification in doweling to the existing footing in order to increase footing size.

 

[DaveAtkins]

Looks good to me. Developing the #5 @ 12" will be the challenging part--you may want to consider using a hook.

DaveAtkins

 

[DETstru]

You could probably go with #4's if you're trying to develop the bar in as short a distance as possible (I'm assuming you don't need #5's for strength).

Also if you specifically want them to excavate under the existing footing and place concrete you should indicate (+/-) how far to go.

 

[KootK]

When I look at your proportions, what I see is a plain, slightly eccentric, concrete footing supplied in two halves that get stitched together with the dowels. As such, I would make the following adjustments:

1) ditch the under pour sruff.

1a) the dowels will cover shear transfer
1b) I wouldn't rely on that 8", unreinforced, poorly consolidated, uninspectable nib of concrete below to do squat. It'll probably crack off due to shrinkage strain anyhow.
1c) the excavation for the nib may undermine whatever decent soil you've got under the existing footing.

2) I'd add a second row of dowels at the top of the footing.



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.

 

[RHTPE]

The main problem with "improving" the capacity of a footing is that the existing footing has fully engaged the ground below and any settlement has occurred.

Making that footing wider to increase the bearing area does no good until the existing footing settles a bit more so that both the old and the new share the load. Let's be real - compaction of the soil underlying the new footing will be difficult.

I would be concerned about the effect(s) of any additional settlement.


Ralph
Structures Consulting
Northeast USA

 

[Antnyt23]

DETStru,

I agree I need to complete the detailing of everything on it and the #4s may be the way to go.

KootK,

Good points. I was considering doweling the top also and trying to determine if the under pour was worth it or not so I appreciate that look at it and I think you are probably right unless I were to create a large under pour it probably wouldn't serve much of a purpose.

Appreciate the insight.

 

[delagina]

@RHTPE, what's your recommendation in this situation?

 

[RHTPE]

I suppose there really isn't much one can do beyond ensuring that the best possible compaction can be achieved, since it's very difficult to 'preload' the footing.

I think I would first investigate the soil conditions at bottom of existing footing to determine if there is a way to justify a higher allowable bearing so that no modifications are necessary. If that's not feasible then I would investigate the potential effects of some small additional settlement under the existing wall on the rest of the structure. I would then inform the owner that the possibility exists for some future settlement (which may or may not occur).

Much depends on the soil conditions that exist at the site and below the existing footing.


Ralph
Structures Consulting
Northeast USA

 

[KootK]

Short of extreme measures such as introducing piles etc, I think that one has to accept that there will be additional settlement in the existing system. Ralph's concerns over settlement are a big part of why I dislike the nib. Excavation for the nib probably represents the biggest single risk for inducing additional settlememt in the existing structure.

When we renovate, we really need to examine our "fixes" critically (as OP is doing) to ensure that, in the quest for a viable "by the numbers" load path, we're not inadvertently making things worse. It's a bit like the Hippocratic oath. Above all else, do no harm. Well, net harm I guess.

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.

 

[emmgjld]

Unless there is obviously bad soil under the existing footing, Placing the new foundation deeper almost guarantees the soils beneath the existing concrete will be compromised, to some extent. Unless there is obviously bad soil under the existing footing,the 'nib' is usually a really bad idea. Depending on the existing soils condition/bearing, the upper & lower doweling is possible required. I would usually suggest the subgrade soils be very carefully compacted prior to placing concrete.