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Property line eccentric strip footing
2

Property line eccentric strip footing

Property line eccentric strip footing

(OP)

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!

RE: Property line eccentric strip footing

1) This is the the thread for you: Link. And I must say, it's nice to meet another like minded truth seeker.

2) As far as I'm concerned you're right about all of the bits that matter. Maybe a rectangular distribution, maybe a triangular one.. who knows.

3) Many a boss became a boss by focusing on that which is profitable rather than that which it theoretically correct. Is what it is.

RE: Property line eccentric strip footing

Yes, there will be an eccentric moment that will be resisted by the neighbor's wall. The magnitude of the moment will depend on the ratio of the load to the stiffness of the soil under the footing, and is very difficult to calculate with any accuracy.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Property line eccentric strip footing

Yes, there is moment in the slab, as well as in the wall, but not as severe as not leaning on the neighbor's wall. How you going to prevent damages to the neighbor's wall?

RE: Property line eccentric strip footing

The only way to prevent that wall from leaning on the neighbor's wall is to move it away from the neighbor's wall, so that it has room to rotate and develop that triangular bearing resistance through deformation of the soil under the footing and moment at the connection of the wall to the footing through deformation of the rebar and concrete. If the wall is constructed as shown, the eccentric moment will be resisted primarily as a force couple with the top of the wall pushing on the neighbor's wall and an equal and opposite force carried through the footing to the soil around and below it.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Property line eccentric strip footing

I was going to point out what BridgeSmith said.... Even if you design for the eccentric moment, the contact with the neighbor's wall will prevent rotation of the foundation / wall. Not that you should RELY on that. Relying on it would open you up to liability should the neighbor experience any issues with their house.

That being said, I'd start with a triangular soil bearing profile. Just look at a representative 1 ft width and the standard formulas for a rectangular footing:

SB = (P/A) * (1 +/- (6e/b)) when e < b/6
= 2P / ( 3a * (b/2-e)) when e > b/6

where A = area (b*a)
a = width of the footing (1 ft in my representative footing)
b = length of footing


Alternatively, you could look at the max allowable soil load concentrated in the area directly under the wall. Then taper that down until you get moments to balance out. Not as easy to work out the numbers. But, probably not that hard either. That's not as traditional, but what do we really know about the soil behavior anyway. As long as your assumptions are reasonable, obey statics and don't violate the information given by your geotech, then I'm okay with them.

RE: Property line eccentric strip footing

You shall consider physical separation of the walls, with gap in between walls, or placing compressible material in between to avoid direct contact. Then analyze and design your wall without the presence of the neighboring wall.

RE: Property line eccentric strip footing

Separation of walls is best, but in the real world, the neighbour's wall could be encroaching slightly or totally clear of the property line. It is never a good idea to rely on a neighbour's wall for lateral support of a new wall, because at some time, the existing wall may be removed. I agree with r13 that the wall and foundation should be designed as if the neighbour's wall didn't exist.

In the sketch, the new wall is deeper that the existing wall. This means underpinning is required. The cost of underpinning would be borne by one or both neighbours, depending on the rule of law in the area. Rules vary from place to place.

BA

RE: Property line eccentric strip footing

I'd be a pretty happy camper to have rectifying grade beams tying into the wall foundation perpendicularly in these situations. That said, I do recognize that's costly and seldom done as a result.

RE: Property line eccentric strip footing

Quote:

It is never a good idea to rely on a neighbour's wall for lateral support of a new wall, because at some time, the existing wall may be removed. I agree with r13 that the wall and foundation should be designed as if the neighbour's wall didn't exist.

I agree with that as well. The wall should be designed so that it is stable on it's own.

I was pointing out that, as constructed, without a gap, the neighbor's wall is what's providing the resistance to the overturning moment.

If the neighbor's wall doesn't have sufficient sliding resistance, your wall could move it slightly, when it's loaded. Not a likely scenario if the neighbor's wall is backfilled, though.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Property line eccentric strip footing

Why not just assume a triangular stress distribution under the footing and if the maximum pressure is well below the allowable bearing pressure then the moment can be resisted by the soil.

There is a neighboring building so presumably there is no reason the soil will become wet and lose its strength.

RE: Property line eccentric strip footing

The neighbor can decide to leave and take the wall with him/her at anytime:)

RE: Property line eccentric strip footing

Your neighbour's wall is putting a surcharge on your foundation wall. That ? dimension should be 0 or if not, then the surcharge based on active earth pressure should be applied, or you may be undermining his footing.

I would use a rectangular bearing pressure distribution... the concrete wall is so much stiffer than the soil. In Josh's trapezoidal one shown, the uniform loading should extend from the face of the wall + the depth of the footing, at very least.

How are you separating the concrete pour from your neighbour's wall?

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

-Dik

RE: Property line eccentric strip footing

How do you plan on pouring new wall? Will you use neighbor's wall as formwork on that side? will you be nailing plywood formwork to their wall or put tar paper "paste" to their wall? All of those methods may need neighbor's approval

RE: Property line eccentric strip footing

Where is the property line 'really'?

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

-Dik

RE: Property line eccentric strip footing

As mentioned before, a compressible foam can be placed between the walls as separator. Unfortunately, the neighbor's wall has to be relied upon as form work before the concrete is hardened. After that, your neighbor's wall will not feel pressure, if you have designed the foundation correctly.

RE: Property line eccentric strip footing

Quote:

After that, your neighbor's wall will not feel pressure, if you have designed the foundation correctly.

That's not really accurate, if the wall sees additional is load after it's been placed. Additional load will produce an eccentric reaction from the soil, so there will be some rotation, putting some pressure on the foam. It may be small (or it may be large, depending on the load applied to the wall), but you shouldn't say it's not there at all.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Property line eccentric strip footing

When you designed right, rotation shouldn't be there, since you will limit the potential for, or the amount of, settlement. Also, the foam shall be thick enough to accommodate anticipated movement, but ultimately the system will reach equilibrium as designed (no wall in the front). When the neighbor's wall is removed, the foam may have deformed, but no harm will be done to your wall. I don't see much problem with it. You might have to extend the base slab to increase the leverage though.

RE: Property line eccentric strip footing

How high is the backfill measured to the proposed footing base? Is there any special activities near the wall (add'l load concerns). What is the allowable bearing pressure?

RE: Property line eccentric strip footing

I've seen this done with plywood and snapties... other than deformation of the plywood (if any... it's hidden), there is no load applied to the wall.

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

-Dik

RE: Property line eccentric strip footing

"snapties", what is that?

RE: Property line eccentric strip footing

Probably should be two words---snap ties.

BA

RE: Property line eccentric strip footing

I'm not sure of what the real name for them is... they are metal wires/straps that attach the two form faces together to contain the concrete.

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

-Dik

RE: Property line eccentric strip footing

Quote (Probably should be two words---snap ties.)


that looks better... I knew about them long before I went into engineering... just didn't know how to spell them...
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Property line eccentric strip footing

BA,

Thanks. I don't know its trade name either, usually call it form ties, or form spacer. Anyway, it is well known to me that dik likes to express his point through philosophical short sentence, now even save a space between two words. High :)

RE: Property line eccentric strip footing

Quote (now even save a space between two words)


just frugal...

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

-Dik

RE: Property line eccentric strip footing

Hate to bring up "limit design", but is there a connection.... :)

RE: Property line eccentric strip footing

of course...

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

-Dik

RE: Property line eccentric strip footing

Wouldn't a rigid and stiff footing with an eccentric load still result in a non uniform soil pressure ? Unless it is so massive that its weight offsets the eccentricity.

Just something I was thinking of from this thread as it is assumed that a rectangular distribution is under the footing directly under the wall.

RE: Property line eccentric strip footing

Quote (eccentric load still result in a non uniform soil pressure)


Yup... and the bearing pressure would not be uniform (not quite... but close, just due to the relative rigidity). Using a uniform pressure, the footing moment at the wall will be maximised... just what I want...

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

-Dik

RE: Property line eccentric strip footing

Retaining wall without toe slab is quite difficult to design. It will require long/thick heel slab with backfill to stabilize the wall.

RE: Property line eccentric strip footing

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.

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. Not sure I understand what the contractor wants to do, but it doesn't sound good.

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. 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.


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?

Your detail relies on the existing wall for stability. You need to find a way to support the load which does not rely on the neighbour's wall.

BA

RE: Property line eccentric strip footing

(OP)

Wow so many answers, thank you all for your participation.

I realise some major information was missing on the original sketch : The top of the wall is laterally supported by the beams and the subfloor framing the groudfloor. So yes, technically, with a dowel added to the back rebar and extending to the head of the wall, the wall will be capable of supporting both the moments created by the eccentricity of the footing and also by the possible pressure caused by the neighbour's wall. Isn't it ? They might even cancel themselves.

I do not know how were built theses walls 100 years ago and I pray really hard that the two footings are separated, because if not this will be a nightmare! Although cutting it on the property line and adding dowels to the new wall could be a solution... For wall separation, I haven't thought about that yet. I suppose they are usually poured back to back even today ?

Back then, they also might have been poured back to back with no rebars. I don't know if membranes of any sorts were used between them. They also probably statically rely on each other. I think the best thing would be to separate them if found separated, and to repour the new one back to back if found so.

To minimise the effects on the neighbour's wall, the new wall will be poured section by section, 4 feet or so at a time, so the soil under the neighbour's footing doesn't empty itself and so the pouring pressure isn't generalised on the whole neighbour's foundation system. Also, pressure will be minimised on the other wall since the new wall will be stiffened by it's increased thickness and by the rebars.

There are multiple old row houses in the city digging basements... but I don't know the secret of how they do it! I have seen the plans of an other engineer with no rebars at all :S. I am definitely an expensive engineer ... and it's not my salary!

Thanks again.


RE: Property line eccentric strip footing

For separation, you can use asphalt impregnated fibreboard and plywood with snap ties to keep it from loading the wall and providing a positive separation if the other building 'goes'.... careful what the existing soil is... if cohesive, then likely OK... if granular, you could have some issues with underpinning. You may want to ask if you can do a damage assessment before starting... photographing cracks, etc. in the adjacent building. Look into the corners of door and window openings for diagonal cracks, etc. Also look at the existing basement slab for cracking. If neighbour refuses... then simply send him a letter stipulating that you wanted to undertake this, but he was not accommodating.

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

-Dik

RE: Property line eccentric strip footing

(OP)
Yes, damage assessment before starting will be done. Thanks for the suggestion. I am not sure letting a plywood that could decompose and keep moisture between the two buildings is a very good idea...although I have nothing else to suggest.

RE: Property line eccentric strip footing

Quote:

I am not sure letting a plywood that could decompose and keep moisture between the two buildings is a very good idea

The plywood decomposing and leaving a gap would be the aim, I think. My issue would be, plywood would probably take too long to decompose, and wouldn't provide the gap for movement. Trapping moisture between 2 concrete walls isn't really a problem. Concrete gains strength better when it's moist, or even under water.

Most of the issues are rendered moot, now that we understand the top of the wall is restrained. If you want to be sure there's no pressure exerted on the neighbor's wall, you can form it with foam and use a chemical to dissolve the foam afterwards, or cardboard gap filler that get saturated with water after it concrete sets up.

Rod Smith, P.E., The artist formerly known as HotRod10

RE: Property line eccentric strip footing

Quote (I am not sure letting a plywood that could decompose)


Use PWF material... it will almost outlast the aluminum beer can, if you are concerned. The purpose of the fibreboard is to help form a bond break... even foam. The purpose of the plywood is to provide a back form to minimise the loading on the adjacent wall.
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Property line eccentric strip footing

blind side form work should be used so that no pressure is placed on the adjacent property wall: Link

Be careful on the material used to fill the gap as it will need to be compressible to meet seismic separation requirements. Some jurisdictions may also limit the selection of material so as not to attract or harbor rodents.

My Personal Open Source Structural Applications:
https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:
https://github.com/open-struct-engineer

RE: Property line eccentric strip footing

The issue is not resolved by applying blind side form work. It is necessary to resist the moment of the eccentric footing by preventing the wall from rotating; and it must be done without reliance on the neighbouring wall.

BA

RE: Property line eccentric strip footing

BA,

The OP is relying on the header, the remain after partially demolishing the old wall (see OP's sketch), to provide rotational restrain. I would concern with the flexibility/stiffness of the header beam to achieve the goal, without additional considerations.

If designed correctly, the pressure on the neighbor's wall will come from two sources - 1) fluid pressure during concrete placement, and 2) small rotation, that causes tilting of the wall as anticipated in typical retaining walls. The pressure from the former will drop very fast after concrete start to set. The latter will have some effect that needs to be addressed.

RE: Property line eccentric strip footing

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

RE: Property line eccentric strip footing

Quote:

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.

RE: Property line eccentric strip footing

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.

RE: Property line eccentric strip footing

Quote (r13)

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

RE: Property line eccentric strip footing

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.

RE: Property line eccentric strip footing

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

BA

RE: Property line eccentric strip footing

(OP)
OK, since some more parameters are requested, here they are !

I also have some answers to BAretired's comments :

Quote (BAretired)

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..?

RE: Property line eccentric strip footing

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?

RE: Property line eccentric strip footing

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.

Quote (Phillip001, OP)

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

RE: Property line eccentric strip footing

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

RE: Property line eccentric strip footing

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.

RE: Property line eccentric strip footing

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?

www.PeirceEngineering.com

RE: Property line eccentric strip footing

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

RE: Property line eccentric strip footing

Quote (r13)

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.

RE: Property line eccentric strip footing

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.

RE: Property line eccentric strip footing

Quote (dik)

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

RE: Property line eccentric strip footing

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.

RE: Property line eccentric strip footing

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.

RE: Property line eccentric strip footing

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

RE: Property line eccentric strip footing

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.

RE: Property line eccentric strip footing

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.

www.PeirceEngineering.com

RE: Property line eccentric strip footing

Quote (the neighbor might bear 100% responsibility)


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

RE: Property line eccentric strip footing

(OP)
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!

RE: Property line eccentric strip footing

Quote (Phillip001)

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.

Quote (Phillip001)

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

RE: Property line eccentric strip footing

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.

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