Point Loads on an Existing Strip Footing

[Apache1]

I had a question come up on a residential basement remodel. As I don't do much residential work (or remodel work in general) I wasn't sure what to do here. As shown below the owner is going to remove a 12' section of bearing wall in their basement and replace it with a beam. The reactions on each end of the beam will be approximately 7 kips. There is an existing strip footing through this location that is 18" wide x 10" deep and has (2) #4 bars continuous in the bottom of the footing (the owner has their original plans from when they built a few years ago). Soil bearing is assumed to be 1,500 psf. If this was new construction I'd put a 30"x30" square spot footing at the locations of the point loads. Another engineer is telling me that it's fine to just bear the new columns onto the existing footing. In my mind 7 kips is too much for the existing strip footing. However, I'm not sure how to model this to show that it will or won't work as-is. It doesn't feel right to say that the existing footing acts as a beam that spreads the load out over the length of the footing. What do you all think?

 

[TheDW]

I'd be fine with it.

You need a 3'-2" length of footing to be 'activated' for 1500psf. I would check that the bottom bars are sufficient for flexure at that length and call it a day.

 

[XR250]

My guess is your soil bearing capacity is greater than 1500 PSF

 

[Apache1]

I'd be fine with it.

Ok, so at what point would you not be fine with it? Let's say the point loads were 12 kips? At that load I'd need 5'-4" of footing. Does that still sounds reasonable? I'm thinking about new construction where sometimes there are point loads of that size coming down on a strip footing. I normally would blow out the strip footing at that location with a 30"x30" square footing (for a load of 12 kips), but maybe that is overkill...

My guess is your soil bearing capacity is greater than 1500 PSF

I'd think so as well, but without a soils report...

 

[phamENG]

I never assume an existing residential footing is reinforced, unless there's a design and detailed inspection records or we've cut it open to find out. Too often the bars are not put in correctly or at all (since they typically aren't required by the residential code here, and it's rare around here for an engineer to inspect houses - it's usually the city inspector just making sure it "looks right").

So I look at these as if they were plain concrete. It usually works out unless I have a really big load coming down.

If you want to do a more detailed analysis, look into beams on elastic foundations. But a simple uniform pressure analysis is typically all that is done (unless you're trying to justify a 1'-6"x8' strip footing for a point load. That would be a bit much.

 

[TheDW]

I've poured/doweled around an existing strip to make a spot footing when I wasn't comfortable with the existing itself. I'd look closer at the 12k load but that would be pushing it for sure.

 

[Apache1]

But a simple uniform pressure analysis is typically all that is done (unless you're trying to justify a 1'-6"x8' strip footing for a point load. That would be a bit much.

But what length of footing should be used for this uniform pressure analysis? I agree that an 8' length would be unreasonable. But what about 2' or 4' or 6'? I thought maybe there was some general rule of thumb that you guys use to come up with that length, but it sounds like there's not. I could save owners quite a bit of money if strip footings can be used to bear point loads up to a certain value. Before this thread I only considered an 18" wide x 10" deep strip footing good up to a point of 3.5 kip or so.

 

[phamENG]

A couple ways to look at it.

1) do a deflection check. with a uniform pressure, how does the footing deflect? Now take that deflected shape, and use it to modify the pressure profile. Assume that you still have the same average bearing pressure, but your highest and lowest pressure lie on a curve the same shape as that deflected shape you calculated. It's not perfect, but I'd say it's more conservative than uniform pressure. I'd do this for point loads with a large quantity of dead load and long term live load and/or settlement sensitive structures.

2) if your loading is mostly short term live load (as it is in a lot of wood framed houses), then I'd use half of the ACI beam depth limits for serviceability. Unless deflection is carefully checked, a reinforced concrete cantilever has to have a depth 'h' of at least l/8. Plain concrete requires you to neglect the bottom 2 inches of footings cast against the earth, so for a nominal 10" thick footing your 'reliable' thickness is h=8". 8x8"=64", or 5'4". So I'd go for a total length of 5'4" (two cantilevers of 2'8") for my uniform pressure assumption before seriously considering beam on elastic foundation or a brute force dig out the existing and add a pad.

For new construction, just widen the footing where it needs to be wider. It doesn't change the cost of a new build by enough to matter.

 

[skeletron]

Run beam-on-elastic foundation analysis for that line of strip.
Run the analysis with existing loads as a baseline.
Run the analysis with the new point loads.
Compare and fiddle with the "k" value of the soil springs to see where your bounds are.