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Adding Load to Existing footing

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Berniedog

Structural
Dec 19, 2005
200
Help.

I have some experience with under pinning footings but not with adding 100 kips to an existing footing. See attached PDF. They want to add a crane column to an existing footing. Can I just dowel into the existing footing and add footing size and check the dowels for shear and bending? Or do I need to underpin?

It is complicated.
 
 http://files.engineering.com/getfile.aspx?folder=6874436f-2359-4d57-b680-0a1de31ea233&file=Conlift_F-2_Found.pdf
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for the added eccentricity alone it looks like something will need to be done, either a footing extension, grade beam to redistribute force to a new foundation element, etc. Not to mention the fact that the change in bearing pressure may result in differential settlement.
 
Looks like a metal building foundation you have there (I may be wrong). When I design these usually uplift controls the design so I end up with footings that are much larger than they need to be. If you can get the original reactions you might want to check to see if the existing footing will work. 100k isn't exactly a small load so I wouldn't be surprised if if doesn't work.

Your section shows a footing that is 2'-10" thick (3'-6" - 8") are you sure it's that thick? I usually shy away from "adding to the footing size" because it's hard to increase the punching shear or fluexural capacity of an existing footing. Not to mention in your scenario you might need top reinforcing in the footing.
 
Just had a similar problem. I designed a grade beam to span over the existing footing and then provided a spread footing at each end of the grade beam. It worked because the existing footing was low enough to construct the grade beam over the top of it. You will want to make sure that the grade beam does not bear on the existing footing.
 
The solution, often, will depend upon the physical size of the existing footing, the required bearing area for the new load (including eccentricity), the clearance above the existing footing, nearby columns/footings, etc. As structSU10 previously indicated, solution could be new footing or footings with grade beam or footing extension. There may even be times where removing the exising footing and pouring a new one may be more cost effective.
 
1) I agree with strutSU10 that more foundation, of some sort, will almost certainly be required.

2) I don't see how underpinning would help you here unless:

2a) There's a kick as bearing stratum not too far below the existing footing or;

2b) Pouring an entirely new footing under the existing one would somehow be more cost effective.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
I designed the original footing. It is indeed about 2'-10" deep to get below frost depth. (see the PDF file) THe top of foundation is -0'-8" so I cannot place a grade beam over the existing foundation. It sounds like placing a grade beam under the existing foundation is the way to go.
 
Berniedog said:
It sounds like placing a grade beam under the existing foundation is the way to go.

Do you mean footing under existing?

BA
 
Yes. Under the existing which doesn't seem practical
 
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