Spread Footing Extension Due to Column Eccentricity

[RareBugTX]

Hello All:

Working on a project where I designed a framing system (metal deck + Steel Beams and Columns) to a new second story. While on design stage, owner had a safe installed which did not work with column placement. Now i am in a situation where two columns have to be shifted to a side 14" on the already poured footings(4'x4').As expected, Analysis show I can only offset 5" due to soil bearing. Now, am proposing to extend the reinf. footings to a side 18". Question.how do you guys would address bond between new and exist.concrete. Is it realistic to call for rebar to be embedded 18 db? Has anybody seen a contractor actually doing it? Your feedback is valued.

RareBug RA

 

[KootK]

First off, I'd do everything in my power to make the existing footing work, including:

1) Make sure that any components of the load that are concentric to the footing are modelled that way (footing self weight, overburden, etc.).

2) If soil stress is your only issue, you might be able to cheat the system a bit. Usually, geotechs provide bearing pressures governed by settlement concerns. Often, if the average bearing pressure beneath your footing is within the allowable value, it's acceptable to have the peak value go over. Clear this with your geotech however. I've done this often with shear wall footings.

If you go with the footing extension, remember that it's not necessarily enough to develop the new bars into the existing concrete. Rather, you have to effectively lap the new bars with the old. This can require a long embedment. Drilling deep holes into a footing close to the bottom can cause the concrete to spall off beneath the drilled holes. I often place the new bars somewhat above the existing bars to address this issue.

You may find the attached reference useful.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[hokie66]

If it is only a footing which has been cast, with nothing over it, I would just take it out and cast it again. If it is already loaded by superstructure, I would look at underpinning the heavily loaded side.

 

[RareBugTX]

Hokie and kootk. Footing is not loaded yet. Contractor wanted me to okay something that does not work. Both him and owner have been notified. Thanks a lot for your input.

RareBug

 

[pikku14]

KootK,

I was looking through the Hilti document you attached and was wondering if you knew why the minimum anchorage length is greater for a bar in compression than tension? (See page 6)

Cheers

 

[KootK]

I'm afraid that I do not know pikku14. It's a curious thing; intuitively, I would have thought the reverse. Maybe, in the compression case, they want to be able to develop a minimum bond stress capacity to prevent the failure mode from being crushing at the tip of the reinforcing bar? Or maybe the concrete struts connecting lapped bars are assumed to be at at different angle for the tension and compression cases?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[JoshPlumSE]

We had to extend an existing mat slab foundation on one of my very first design project out of school.

We drilled into the existing slab and placed dowels with a chemical adhesive (Hilti?) so that development wouldn't be as much of an issue. Think of it like a post installed anchor bolt, buy horizontal.

If I recall correctly, there was some consideration to avoiding the existing bars when we drilled into the mat. And, obviously, this required special inspection because of the preparation required to ensure proper adhesion dowels. Other than that, I don't recall this being much of an issue cost wise or schedule wise.

 

[hokie66]

The footing in question is tiny, only 4' x 4'. The solution is self evident...start over, which should be done by now. If the problem were more complicated, then more complicated answers might apply.

 

[RareBugTX]

Oh yea!! Question too dumb for certain people who came out with a PE license from the womb. Why bother!!? Thanks to all who contributed. Sincerely appreciate it.
AA

 

[BAretired]

I once had a developer client with a similar problem. His solution...winch the footing over until the c.g. is where you want it. I'm not recommending it, but it seems to have worked in his case. He told me that it was standard practice.

BA

 

[jayrod12]

We lifted a bunch of piers on pads once that were poured on the wrong side of the gridline. I think they moved 12" total. worked like a charm. We were lucky enough that they hadn't backfilled yet and they excavated a larger area than required when they were forming the pads.