Spread Footing Extension Due to Column Eccentricity
Spread Footing Extension Due to Column Eccentricity
(OP)
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
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






RE: Spread Footing Extension Due to Column Eccentricity
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.
RE: Spread Footing Extension Due to Column Eccentricity
RE: Spread Footing Extension Due to Column Eccentricity
RareBug
RE: Spread Footing Extension Due to Column Eccentricity
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
RE: Spread Footing Extension Due to Column Eccentricity
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Spread Footing Extension Due to Column Eccentricity
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.
RE: Spread Footing Extension Due to Column Eccentricity
RE: Spread Footing Extension Due to Column Eccentricity
AA
RE: Spread Footing Extension Due to Column Eccentricity
BA
RE: Spread Footing Extension Due to Column Eccentricity