Modification of Existing Spread Footing resting on soil

[allimuthug]

Hi,

I have a existing Isolated foundation. The existing compression load and moment on the plinth where increased. Hence the foundation is modified as shown in the picture attached. If the bars where do welled in tho the existing foundation as shown in the picture below.
It is like an hinge which only transfers shear from the existing foundation to the new attached piece. If any body has opinion that the connection transfers moment please justify your answer. Also if I have to transfer moment and shear what kind of arrangement or detail is necessary, please explain in detail with sketch please.

 

[KootK]

1) as you're envisioning this, your outer ring of concrete would be in torsion and you'd probably need closed stirrups there.

2) in my opinion, there's no way to trick this connection into not attracting moment. You've got a force applied eccentrically to the.comnection, and the capacity to resist some flexural tension and compression at the joint. I think that you'll pick up moment at the joint until the bars in tension yield or breakout the concrete.

3) One solution is to effectively lap the new rebar with the existing rebar. Call Hilti for additional information. They have design guides and software that cover this.

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.

 

[TLHS]

I'm not clear what you're doing. Are you not trying to transfer moment, or do you want to transfer moment and don't think it's justified? If you're trying to limit moment transfer, you might be able to do it with some tricky detailing using dowels. I suspect, though, that you'll still transfer some moment, even if it's just from concrete bond and bearing. You could limit that bond and maybe try to force a slip plane. Whether that's a big deal or not would take more information.

If you are trying to carry moment, then the conceptual detail will likely do that. You have to detail your post installed rebar to make sure you can actually transfer the amount of load you need into the existing structure.

 

[WARose]

I'm not 100% sure what the reason for the modification is. (I assume to accommodate a bearing pressure increase?) But I have modified footings as well......and you appear to have done something similar.

A modification that I would suggest though: to be sure the additional load/pressure goes to the entire bearing area, we always use to have two "beams" running on top of the new footing for each orthogonal direction (for a total of 4). (It sort of look like a inverted double T beam with shallow webs.) They tied into the RC pedestal on the sides.....and the new and existing footing on the top. We always felt this assured any new load would get distributed as evenly as possible. (Remembering the fact that the soil under the existing footing is likely well consolidated.)

The reinforcement for the "beams" would look like any other: flexural reinforcement (top and bottom), stirrups, and rebar/Hiltis tying into the previously mentioned elements.

 

[dhengr]

Allimuthug:
Don’t forget that when you extend the dimensions of the existing footing and increase the load on the footing, you also increase the bending moments and shear in that footing, near the pedestal. How are you going to increase the depth and rebar in the existing footing to accommodate these increases?

 

[Coty]

Yes, the post-installed bottom bar will transfer moment up until the point it yields. Was that the question?

Only by using slip dowels will you not transfer moment, but if you don't transfer moment in this case, you don't have stability in your new foundation extension unless you treat the extension as some sort of ring with torsion and bending parallel to the rings longitudinal axis.

Expanding on what KootK mentioned, if you need to transfer moment, then you can develop rebar now with epoxy. Hilti has some good literature about it online, too. You better have a long drill bit and room for access all around the footing to go that route. That way, you can develop the tension required in the bottom bar. Since it is the end of the footing, where the moment will be relatively small, you may not need to fully develop the bar, but simply analyze an epoxy anchor for use as a tension dowel via concrete breakout strength assuming no reinforcement in the footing.