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!

 

[r13]

"snapties", what is that?

 

[BAretired]

Probably should be two words---snap ties.

BA

 

[dik]

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

 

[dik]

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

 

[r13]

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

 

[dik]

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

 

[r13]

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

 

[dik]

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

 

[civeng80]

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.

 

[dik]

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

 

[r13]

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

 

[BAretired]

BAretired[/color]]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

 

[Phillip001]

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.

 

[dik]

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

 

[Phillip001]

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.

 

[BridgeSmith]

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

 

[dik]

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

 

[Celt83]

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

 

[BAretired]

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

 

[r13]

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.