Underpining an Existing Wall Footing or Thickened Edge Slab 2

[dursunlutfu]

Hello Folks,

I am working on an addition to a 1-story wood framed building. I have no information regarding the existing foundation but I am assuming that there is a conventional continuous footing along the perimeter wall of the house. I was proposing to underpin the assumed conventional footing with square pads where there are new introduced point loads coming from the addition of the house to the existing wood sill plate (5kips-6kips). Due to architectural issues (ripping off terrezano floor) , the client does not want to go with that option. Since I have no soils report and info about the foundation, I thought that would be the safest and the most logical option. Would you guys assume the "assumed" existing footing as sufficient or not in this kind of a case ?

Thank you for your time and energy!

 

[JedClampett]

When I use the term underpinning, it's to support a foundation that's being undermined due to excavation activities. It's typically a temporary condition.
If you're adding new loads to an existing foundation, underpinning is probably not the right term. But I see what you mean. If you're worried about adding new loads, can you do a rough calculation of the existing and new loads and get a bearing pressure? Assume a footing width (8 inches? one foot?) and figure out the loads. If the bearing pressure, with the new loads added in, is less than 1000 psf, you're probably all right unless your soil is gumbo.

 

[oldestguy]

If there is a local building code for footing sizes, that's all I would do for the addition and nothing to the existing. If there are some wall cracks resulting, likely during the new final finishing, fixing that will suffice.. Of course during the work if really bad soil (old garbage fill, etc) is found some improvement might be considered at the existing part.

 

[PittEng88]

I would look at distributing the point loads down through the foundation wall at a 1:1 slope and checking the bearing pressure from there using, as Jed mentioned, 1000 psf as an allowable bearing pressure. If it all works out, then you shouldn't have modify the existing footing.

 

[BAretired]

Excavate enough to confirm the existing footing size. Assume it's typical.

BA

 

[dursunlutfu]

Thank you all !