Underpinning

[mijowe]

A code official has presented the attached detail for underpinning of an existing party wall. The detail was generated to preserve the neighbors ability to develop their property in the future by restricting the underpinning from extending into their property past the face of the party wall.

I am not a fan of the detail, am interested in your thoughts.

 

[ztengguy]

How is that stable?

 

[mijowe]

It is not stable. His response to that was that it is not his job to make it stable, it is mine.

 

[PicoStruc]

You need to have a very good soil for L-Shaped footing,

The triangular pressure under the footing is so high and amplified du to load eccentricity !

 

[ztengguy]

What were you going to propose?

 

[mijowe]

Tieing it into the slab on grade and thickening/reinforcing it is some yet to be determined manor.

 

[FSS]

So why is the underpinning needed? Hard to comment without knowing what started things? Bad soil?

 

[mijowe]

We are building a new building with a basement level, and foundations that are below the existing party wall foundation.

 

[PEinc]

The underpinning piers should be wider, probably at least as wide as the existing wall footing. Then, maybe the toed footing would not be needed. Also, the reinforcing steel is a bad idea. Vertical bars interfere with the dry packing and horizontal Debra's require lap splicing or expensive mechanical splicers. Rebars also interfere with excavating the pits and pouring the concrete. The double pour with the construction joint also needlessly increases the time that the building is unsupported. The detail looks like a detail from a structural engineer with no underpinning experience.

http://www.PeirceEngineering.com

 

[mijowe]

We proposed unreinforced solid piers to match the width of the existing foundation. It was explained to us our approach was overkill and the L shape would save concrete.

He will not budge on extending any underpinning past the neighbors face of the party wall.

 

[PEinc]

It sounds to me like you are talking to someone who hasn't a clue. A little more unreinforced mass concrete is cheaper, easier, and faster than the shown detail.

http://www.PeirceEngineering.com

 

[PEinc]

Show the code guy references on pit underpinning and tell him to seal the design if that is what he wants to see built.

http://www.PeirceEngineering.com

 

[ztengguy]

On a side note, why does your code guy care what it costs, is that not between you and the owner? His job is to make sure its a sound design correct?

 

[a2mfk]

Agree with others that this is odd involvement from a building official.

This looks like a good job for steel micropiles installed under the existing wall. Then you can build your structure to the right side.

 

[bookowski]

I've seen this done exactly as you've drawn it. Typical party wall rules (at least in this area) are that you can do whatever you want to the wall as long as it does not impede the neighbor from future work (building up, going down etc.). They will usually trim back the existing footing after the underpinning is complete. I've never specified this, it's always the SoE engineer - but I've seen it several times. I agree that it doesn't appear very stable - the width of the existing footing is the logical minimum.

 

[bookowski]

mijowe - See attached photo of pretty much the exact detail that you've sketched. This is a typical 4 story historic rowhouse, mid-1800s.

 

[mijowe]

I am willing to put this under the heading of it works even if I cant prove it, but I would not want to sign it.

In your photo, I would want the hooked bar going to the back face.