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!

 

[PEinc]

r13, I have not seen posted if the neighbor's existing wall is a building wall or a site retaining wall. In either case, it may need to be underpinned. Because the property line is on the rear face of the existing wall, it is permissible for the Phillip001's client to dig a test pit behind the wall so that the bottom of footing can be determined and an elevation established for the closest footings for the new addition. Just don't excavate the test pit across the property line or deeper than the existing wall footing.

http://www.PeirceEngineering.com

 

[dik]

I wouldn't hold my breath on that... you did the damage.

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

-Dik

 

[Phillip001]

First of all some more details on the buildings:
- Joists are made of wood (3x8), 28'' c/c, and are 6 inch embedded in the original concrete wall head ;
- Joists are overlaid with 1 in. perpendicular decking;
- Actual concrete wall is topped by a load bearing hollow concrete block wall (Roof load);
- For the backfill, few inches of gravel over footing with 5 in concrete wall. The basement will be finished. As for the neighbors, who knows. Probably the same thing for one, probably few inches of sand over the footing for the shallow one.

There might have been a little misuse of the word underpinning. I have been using it for digging for a new foundation wall near an existing foundation wall. Anyway, there are a few ways to approach this.
1) There is so few to dig that the soil under the neighbor's footing stays vertical and in place;
2) There might be some more digging, but having enough cohesion (clay style), so the soil under the neighbor's footing stays vertical and in place;
Some soil fall off (see red area on picture) so I can :
3) Use a small formwork and try to pack the soil behind with louvers(I wouldn't because the soil would eventually have to properly settle) ;
4) Use a small formwork and try to pour concrete behind (Most likely what I would do);
5) Pour concrete under the neighbor's footing at the same time we pour our wall (Wrong choice but I bet everybody does that, and I bet we will find this on the side where the neighbour also dug his basement)

Then we have the separation problem (blue line). There is a big chance that these walls are traditionally poured one against the other. I do not think, as mentioned far above, that it would be a good idea to ADD a space between them. Whatever you say... theory and reality are not the same, and basically, those are two L-shaped walls back to back, with, maybe, some kind of paper or foam between them. And yes the neighbor's wall probably needs mine, and no I won't sue him. So, in reality, our new wall will push on the neighbour's ground floor. In theory, I might have to use the joists as ties and have to add connectors for that and see what friction can give me. I will then be the first one to ask for straps and anchors in the row house history of the whole world, and lose all credibility.

Last problem, I think, is the fresh concrete pressure on the neighbour's wall:
- I will have to ask the contractor for his methods, but I don't think blindside form is intended, nor reasonable for low height residential purpose. Maybe it's screws, shims and plywood.
- On the bare ground shallow side: If I keep a 2-3 feet of the original wall head, and if the neighbour has a 4-5 feet crawl space, there is not much left under my sawcut and over his soil level to be pushed and pressurised. I understand now that determining these parameters are important. Or maybe a all-in-one design should be adopted.
- On the deep side. There might be a ball joint in the middle, there might not even be rebars underneath the original wall like on this picture. If there are rebars, it makes a nice beam to take the temporary push from the 4 feet poured section. My question is : is there enough concrete shrinkage when it sets so that there is no more pressure on the backing wall ?

Thanks!

 

[BAretired]

So, in reality, our new wall will push on the neighbour's ground floor.

That works, as long as the neighbour's ground floor remains.

My question is : is there enough concrete shrinkage when it sets so that there is no more pressure on the backing wall ?

Who cares? After it sets, any damage which could be done has already been done.

BA

 

[Enable]

There are so many posts in this thread that I am having a hard time following the discussion. However, it seems to me that the main considerations are:
1. Can you rely on the neighbor’s wall as a blindside form? (TL;DR NO! ARE YOU FREAKING NUTS?)
2. Can you solve the rotation problem? (TL;DR Depends on your level of paranoia)

To deal with 1, snap ties were mentioned by DIK but this is absolutely NOT the right product. Snap ties require wedges on the opposite side, which you cannot get to. I’m sure proprietary products exist for one-sided blind construction form pressure but I have never used such a product (check your local formwork supplier. NCA surely has something). Even if not, one could easily make their own solution by welding B7 threaded rod to steel plates and place them inside routed sections on the backside of the blind-side plywood. Conventional plate washers and nuts on the interior side with whalers and strongbacks. I would probably shackle and use tension cables down to the SOG to be super safe.

To deal with 2, you have to gauge your comfort level. Even the engineers in this thread seem to differ quite a bit on how big a concern this is. I suspect that is because the actual rotation is impossible to know. Hell, even attempted “rational analysis” will bear little fruit because of the embedded auxiliary assumptions. In other words, you better be okay with one of two situations: either it might happen, or you say that this is not a YOLO situation and figure out a solution (read: $$). If the latter, probably go full Monty and bring in micropiles as close to the underside of the wall as possible to prevent such rotation. All this other talk is moot.