New building, existing foundation
New building, existing foundation
(OP)
I'm adding an elevator/stairwell addition to an existing 12-story hotel. The addition will extend roughly 12 feet from the hotel. The new columns adjacent to the face of the existing building will bear within the footprint of the existing spread footings.
With the size of footings required for the new columns, I was trying to tie the new footings into the existing and extend it the full width of the addition, to create a combined footing with three column loads (2 new, 1 existing). I can't make it work with our assumed bearing capacity (and assumed existing load).
Short of waiting for the recommendation from the geotech (which may not even help us), I was considering a combined footing (with only the 2 new columns) with a cantilevered stemwall to carry the column over the existing footing, so that the existing structure remains unaffected. I haven’t run numbers on it yet, but assuming I can make it work mathematically, is it reasonable to design a footing where the column is not even within the soil bearing area?
I know they build skyscrapers next to each other all the time, so I’m wondering if anybody has any ideas. Thanks in advance.
With the size of footings required for the new columns, I was trying to tie the new footings into the existing and extend it the full width of the addition, to create a combined footing with three column loads (2 new, 1 existing). I can't make it work with our assumed bearing capacity (and assumed existing load).
Short of waiting for the recommendation from the geotech (which may not even help us), I was considering a combined footing (with only the 2 new columns) with a cantilevered stemwall to carry the column over the existing footing, so that the existing structure remains unaffected. I haven’t run numbers on it yet, but assuming I can make it work mathematically, is it reasonable to design a footing where the column is not even within the soil bearing area?
I know they build skyscrapers next to each other all the time, so I’m wondering if anybody has any ideas. Thanks in advance.






RE: New building, existing foundation
DaveAtkins
RE: New building, existing foundation
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I agree, the bearing stress probably won't work. Is there an obvious solution I'm just not seeing?
RE: New building, existing foundation
It depends on the geotech conditions but perhaps piling will give higher load capacity.
RE: New building, existing foundation
RE: New building, existing foundation
If you can make the numbers work, I don't see why your approach would be bad. You will have to make dead sure the contractor understands not to connect to the existing footing. With the eccentric load on that footing, will you be able to keep it from tilting toward the existing building too much? Normally I wouldn't consider that a problem but I would imagine even a small amount of tilt over 12 floors might create issues.
RE: New building, existing foundation
As for the bearing capacity, we tried to calculate the existing column loads and divide by the footing areas. That gave us 5000 psf. Of course, that assumes from the start that the footings are already at capacity. Three projects within a few blocks from this building--for which we have soils reports--were designed for anything from 4000 to 10000 psf. The owner has agreed to pay for a soil investigation. I'll wait for those results.
Drilled piers and cantilevered grade beam: still a possibility, but the basement floor is 10 feet below grade, and you get into logistical issues, etc.