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!

 

[KootK]

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.

 

[BridgeSmith]

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

 

[r13]

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?

 

[BridgeSmith]

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

 

[JoshPlumSE]

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.

 

[r13]

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.

 

[BAretired]

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

 

[KootK]

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.

 

[BridgeSmith]

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

 

[civeng80]

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.

 

[r13]

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

 

[dik]

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

 

[AskTooMuch]

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

 

[dik]

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

 

[r13]

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.

 

[BridgeSmith]

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

 

[r13]

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.

 

[r13]

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?

 

[dik]

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