New building, existing foundation

[rholder98]

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.

 

[DaveAtkins]

If I understand the problem correctly, the stem wall is perpendicular to the combined footing which supports the two new columns, right? Then the stem wall will create a moment at the combined footing, and I doubt the bearing stress under the combined footing would be acceptable.

DaveAtkins

 

[rholder98]

The plan of my footing would be something like:

+-------------+
| |
| X | X
| |
+-------------+

and in section:

\/ \/
+-----------------+
| |
| +--+
| |
+-------------+
///////////////

I agree, the bearing stress probably won't work. Is there an obvious solution I'm just not seeing?

 

[apsix]

If you taper the footing in plan, with the wide end at the existing building you should be able to obtain uniform bearing stress. Whether the bearing stress is too high unless an impossibly wide footing is used, will depend on column loads and geometry.
It depends on the geotech conditions but perhaps piling will give higher load capacity.

 

[MikeE55]

I have done a similar thing - but for only three floors. If you size things properly, I don't see why your concept is not good. I used a grade beam which cantilevered over a footing to support a column that was located over an existing footing. I used expansion joint material or void forms over the existing footing to keep the load from transferring to the existing. Since the footing you cantilever over will be highly loaded, maybe piles for this component would be your best bet.

 

[UcfSE]

Why are you assuming loads and bearing capacity? It may save you some weight if you get closer to the real loads and bearing capacity, or save your butt.

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.

 

[rholder98]

I have the existing drawings for the first 7 floors of the building. I can get a pretty good estimate of the dead load from that. The next 5 + roof were added later. I pretty much have to take the floor loads from the first 7 and assume the other floors are sufficiently similar. It's not unreasonable, since the addition was built only a few years later, presumably by the same architects. There is to be a full-scale investigation of the remaining floors later, at which time we should be able to verify this assumption.

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.