Property line eccentric strip footing 2

[Phillip001]

Hi,

I have allways wondered if eccentric footings create moments in foudation walls. Unfortunately, my bosses would only make me feel like I make to much off nothing and answer vaguely, no mather how many times I would say '' BUT THAT MOMENT HAS TO GO SOMEWHERE!?''. So I have allways secured myself by adding rebars...

Here is a particular case where I am adding load to the building and demolishing the old foundation wall. The contractor wants to keep the old foundation wall head to make the underpinning easier, and drilling the old wall head from under doesn't seem like a good idea.

You will see below that I have chosen a rectangular reaction because this is residential purpose with prabably underestimated soil capacity. I think no rocket engeneering is needed here. I have also considered a possible soil and foundation push from behind with the face rebar that goes from top to bottom.

Simple P/A + Mc/I indicates to me that the load is not high enough to compensate the moment and that traction ''could'' be possible where the old wall sits. Really ?

Thank you very much!

 

[BAretired]

r13,

You call it the "header". The OP's sketch calls it "original wall head". We can't evaluate the capability of the existing "wall head" to resist the applied moment from the eccentric footing, but if the neighbour removes his foundation in the future, which he has every right to do, the eccentric moment must be resisted in order to prevent collapse.

If pressure of wet concrete is the only concern, it can be reduced by slowing the rate of pour or eliminated by using blind side form work.

BA

 

[r13]

We can't evaluate the capability of the existing "wall head" to resist the applied moment from the eccentric footing, ...

I think we maybe able to evaluate it, but shouldn't rely on it as the sole means of support in the analysis and design. I've difficulty to put my trust on such a system. IMO, if the wall have sufficient end supports, and be made composite with the new wall, the situation may improve.

 

[civeng80]

This is a popular thread. I for one have designed many buildings on a boundary and therefore with some eccentricity and have found it difficult in some cases to transfer the resulting moment.

 

[BAretired]

I think we maybe able to evaluate it, but shouldn't rely on it as the sole means of support in the analysis and design. I've difficulty to put my trust on such a system. IMO, if the wall have sufficient end supports, and be made composite with the new wall, the situation may improve.

We can't evaluate it because we don't have sufficient information about it. If we did, why shouldn't we rely on it?
What do you mean "such a system"? We don't know the system, so how can we evaluate it?
The OP and his bosses need to find a way of preventing the wall from rotating outward if the neighbour removes his wall.



BA

 

[r13]

BA,

This is what I called a "system". I've asking information since early on, yes, many information are missing so far. But if we have all information in place, including the length, thickness, material of the existing wall, the depth to remain, and the supports at the far end, we should be able to evaluate. I still have no confidence on the header wall, unless calculation indicate otherwise.

 

[BAretired]

Missing information is the reason we can't evaluate it, r13.

BA

 

[Phillip001]

OK, since some more parameters are requested, here they are !

I also have some answers to BAretired's comments :

Quote (Phillip001, BAretired)
I have allways wondered if eccentric footings create moments in foudation walls. Why would you wonder? Of course they do! Unfortunately, my bosses would only make me feel like I make to much off nothing and answer vaguely, no mather how many times I would say '' BUT THAT MOMENT HAS TO GO SOMEWHERE!?''. Your bosses should tell you what they mean, assuming they know. So I have allways secured myself by adding rebars... Rebars don't help in this case. Adding vertical rebar in the wall or in the column pilaster, especially if we have seismic uplift, certainly helps the wall to take some of that moment we calculate in the footing.

You will see below that I have chosen a rectangular reaction because this is residential purpose with prabably underestimated soil capacity. Could be...we don't know. I think no rocket engeneering is needed here. Well some structural engineering is needed. Well... apparently not because many many have been done without it ! I have also considered a possible soil and foundation push from behind with the face rebar that goes from top to bottom. Can't hurt, but won't likely be stressed. You are probably right about that, but after some settlement, the neighbour's soil foundation might be pushing on the new wall. This push would be counteracting on the footing eccentricity moment.


Simple P/A + Mc/I indicates to me that the load is not high enough to compensate the moment and that traction ''could'' be possible where the old wall sits. Really ? I don't understand this comment. How could a higher load compensate the moment? What do you mean by traction? I might have been unclear because this is basics. On any section, even with plain concrete, a moment will create a compression zone and a traction one. If you have enough axial load on this same section, you don't have traction anymore, but a side lightly compressed with the other one highly compressed.

Another comment: everyone seems convinced that the new wall should be separated from the neighbour's one. Why is that, if my wall can support it's load on it's own, if 4 ft wide wall pouring pressure can't certainly brake the neighbour's wall, and if this is certainly how these walls were built back in the days? Is there pressure on the formwork after the concrete sets? If the formwork is still ''bent'' he is now the one pushing on the concrete..?

 

[r13]

Phillip,

Allow me reiterate my messages.

1) The neighbor can demolish his wall at anytime, so you need to work out your wall as nothing in the front.
2) You should provide compressible material between the wall to a) prevent the new concrete adhere to the neighbor's wall, which makes it difficult, if one of the wall needs to be separate from the other, for whatever the reason, in the future, b) absorb pressure from wet concrete, and c) depends on the stiffness of the header wall, there is a potential it will deflect some amount, then the compressible material should be able to accommodate the displacement.

There are still some missing information - height of the backfill, length and end supports of the header wall. Are you utilizing the joist as ties?

 

[BAretired]

Please see the sketch below:

The floor joists must carry a tension of F, as shown above (per foot length of footing).
The wall must be reinforced for moment M at top of footing.
The moment at the splice to the existing wall should be F(a + d) where d = 16" (thickness of wall).

For neighbour No. 1, 4'-0 depth of underpinning is required.
For neighbour No. 2, no underpinning is necessary.

Another comment: everyone seems convinced that the new wall should be separated from the neighbour's one. Why is that, if my wall can support it's load on it's own, if 4 ft wide wall pouring pressure can't certainly brake the neighbour's wall, and if this is certainly how these walls were built back in the days? Is there pressure on the formwork after the concrete sets? If the formwork is still ''bent'' he is now the one pushing on the concrete..?

If the new wall is poured against the neighbour's wall without using a form, any liability for excessive deflection or distress to the existing wall claimed by the neighbour would be borne by the Contractor, the Engineer or both. It is an unnecessary risk which should be avoided in my opinion.

On my very first job after starting my own firm, I had a government project with a bank vault having 2'-0" thick walls. One wall was tight against the property line. The contractor used the neighbour's brick wall as a form. This was not a good idea as the wall collapsed from the pressure of fresh concrete. To make matters worse, the wall was part of a law office. The owner was not only a lawyer but also mayor of the town. Not a scenario I would like to repeat.

BA

 

[BAretired]

In the above diagram, if h = 120" - 12" (ftg) - 10" (joist) = 98",
then F = 52,325/98 = 534# per foot.

If joists are spaced at 16"o/c, the tension in each joist is 712#.
It seems unlikely that the existing joists are connected to resist 712# tension.

BA

 

[r13]

Phillip,

I assume the joist is part of a building, if so, suggest to find a way to add strut to counter the rotation, and relief the vertical load on the wall.

 

[PEinc]

Phillip001, what holds up the neighbor's foundation wall and the remaining portion of your existing foundation wall(wall head) when you try to excavate and construct the new L-shaped foundation wall?
You need to provide more information on the existing walls. Bottom of footing elevations? Previous underpinning? If existing underpinning is there now, which building was underpinned? Which existing footing is deeper?

http://www.PeirceEngineering.com

 

[dik]

Just a question... what happens if your neighbour's wall is relying on yours for stability?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[jayrod12]

suggest to find a way to add strut to counter the rotation, and relief the vertical load on the wall.

Are you suggesting that he moves the line of bearing inward? Your suggestion is unclear.

If that is your suggestion, I would agree that this may be a feasible way around it structurally. But I can only imagine that it would not fit within the intended use of the space to have a support system (wall, beam and columns, etc.) a few feet inside of the foundation wall. The space between the two becomes wasted space for the entire length of the wall.

You would be better off continuing with trying to use the floor joist to brace the top of the wall by figuring out/implementing a connection between the new wall and the floor joists capable of resisting the applied loads. 700lbs isn't insurmountable. The existing connection likely is inadequate, but they could upgrade it.

 

[r13]

jayrod,

Yes, I suggested a column, a wall, anything that carries the gravity load down. If it is acceptable. the OP needs to find the best way/material to handle it. The information is not sufficient for me to offer a firm opinion at this time.

 

[BAretired]

Just a question... what happens if your neighbour's wall is relying on yours for stability?

That is an interesting question which could involve the legal profession as well as the engineering profession. What responsibility, if any, does this owner have to prevent the neighbour's wall from collapsing?

Another question: did Neighbour #2 underpin the existing foundation when he placed his footing down to 120" below Main Floor Level?

BA

 

[r13]

PEinc,

What you have addressed are absolute necessities in planning of this project, however, I doubt there is any information available to the OP until excavation. The OP did propose to underpin the neighbor's wall. Do you have good exploratory method, or suggestion for the safety of this operation?

Phillip,

Please let us know the structure above the joist, the height of the backfill, and utilization of this space (between the joist and backfill). Also, the type/material of the joist will help too.

 

[r13]

dik,

IMO, unless there was an agreement between the two neighbors, the neighbor's wall shouldn't rely on this wall, and that's the reason most of the responders suggested not to rely on the neighbor's wall. The OP did intend to stabilize the neighbor's wall by underpinning though.

 

[dik]

It may not matter if there's an agreement... an unforseen consequence, perhaps.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[r13]

dik,

Without agreement, the neighbor might bear 100% responsibility to secure his wall. But it is up to the standard care stipulated in the local building code/ordinance, and/or the lawyers to argue about.