Adding pad footing to existing strip footing

[mcbooya]

Hello,

I am looking for advice on adding a footing for a post load that is being added to an existing interior basement strip footing. The point load of 7kips would require a 12" width strip footing length of 7' on its own since it is unreinforced. If I pour a footing extension on both sides of the strip footing and dowel it in can I treat the combined strip+extension as a combined area for the new loading condition? Any other concerns to think about?

 

[KootK]

The point load of 7kips would require a 12" width strip footing length of 7' on its own since it is unreinforced.

That doesn't sound so bad given the likely load spread down through a normal height concrete or masonry bearing wall.

f I pour a footing extension on both sides of the strip footing and dowel it in can I treat the combined strip+extension as a combined area for the new loading condition?

That's almost impossible to do when you look at the anchorage / bar lap requirement properly.

If you really need localized bearing capacity beyond what the strip footing can do with load spread, some form of underpinning might be a good option.

 

[SE2607]

That doesn't sound so bad given the likely load spread down through a normal height concrete or masonry bearing wall.



That's almost impossible to do when you look at the anchorage / bar lap requirement properly.

If you really need localized bearing capacity beyond what the strip footing can do with load spread, some form of underpinning might be a good option.

I like an 5' sq. underpinned pad better than a 1' x 7'. Sure, underpinned pads are hard to do, but the continuity of reinforcing KootK refers to is the biggest problem to me. It's like trying to expand an overloaded pad footing by pouring around the perimeter and doweling into the existing pad footing. Enticing, yes. Good idea, no.
If you are in California, the CBC allows 1,500 PSF w/o a soils report. If you have a soils report limiting you to 1,000 PSF, I would question the geotech about that. If you could use 1,500 PSF bearing value, that underpinned pad becomes 2'-3" sq.

 

[XR250]

I usually just put the pad under the existing strip footing. The excavator can get about half of it, the laborers the rest.
Even if the numbers work out, I avoid doweling into footings as it is never done properly.

 

[Ron247]

Is your post setting directly on the new footing or is the post load going to the top of the wall above?

I do not see how pouring a small doweled footing on each side will get the new load applied to one side only, to transfer half of it to the other side unless the post load sets on the wall.

I may prefer the excavate and pour under option already mentioned since you appear to be able to access both sides of the existing footing. Bear in mind, the excavation will make the existing soil bearing in the excavated area shift to the 2 adjacent areas, but it will not return once the concrete is poured.

 

[lexpatrie]

I have a similar project that is seemingly dead in the water, (possibly a contractor issue), but this kind of "underfill" just begs for settlement, if you ask me. That can perhaps be endured, but if it settles, what happens to the strip footing above it? The situation I'm dealing with is an (apparent) strip footing for an interior bearing wall in an area with frost depth on the perpendicular exterior wall. Fundamentally I don't know where the strip footing is, in terms of elevation because the contractor has never "come back" for the final design and told me where things are physically. There's also the plans from the neighbor that show a pad footing in the area that's a conflict, if it exists. Anyway.

It would be almost as much work to shore the existing, rip out the affected area and pour a new footing "flush" with the bottom of the strip footing, and then dowel into the existing footing for some nominal load transfer to prevent undue settlement and actually have an area to compact that's clear for the new footing.

 

[XR250]

You can also just add a helical pier at the point load.