New footing to be installed over existing footing

[jranderson]

Please see attached. We are looking at installing a new steel gravity column and footing in an existing building. The issue we have is that the centerline of the column is directly over the edge of the existing footing. We are looking at overpouring the new foundation over the existing but can not bear the new footing on the existing because of the increase eccentricity created. My question is whether anyone could provide suggestions in how to install the new footing. Possibly a pile through the existing footing for bearing? Or direct me to articles discussing this installation.

 

[DaveAtkins]

You could pour the new footing under the existing footing. You would need to design the new footing for its own bearing stress plus the bearing stress from the existing footing.

DaveAtkins

 

[Lion06]

What about scabbing on to the existing with drilled and doweled rebar. Size the scab such that the centroid of the new footing is located at the resultant of the axial load for the two columns.

You'd need to make sure bending between the columns is ok - I'd check as plain concrete first and go from there.

 

[msquared48]

I think I tend to agree with Dave here, but I would turn the axis of the new footing by 45 degrees to minimize the undermining, plus go with an elongated, rather than square footing to minimize any underminimg further.

I would also pour the new footing up to the top of the existing footing.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[lexim]

Hello All,

if I am not mis-interpreting the sketch, I think the new footing will be just at the edge of the old footing and above the old footing?

If this is so, my thoughts will be to excavate the new foundation to the base level of the old footing (with 45deg sloping out), and fill with compacted rock gravel or pour mass concrete if the budget can afford this, to bring the bottom of foundation of the new footing to the top of the existing footing.

as per the pressure exerted on the existing footing; i reckon these will be greatly reduced at the corners. it depends however on how nuch load is being transferred to the foundation level.

 

[BAretired]

The attached sketch shows a new footing colored green. If you have enough depth available, the new footing can be cast on top of the existing. Outside the existing footing, fill with concrete to the bottom of existing footing.

The area of the new footing should provide the area required for the new load outside the existing footing and should be centered under the new column.

BA

 

[ToadJones]

pile and cap over existing footing?
too $$$ probably

 

[a2mfk]

Micro piles are not nearly as expensive as you'd think, and think of the demo and concrete labor savings, not to mention they could install 2-4 piles in a day easily. And these can be done with minimal ceiling clearance. What area are you in?

 

[ToadJones]

so my idea not too $$$?

 

[Lion06]

Last job we used micropiles on they were around $15k/micropile. That's not outrageous, but not insignificant. Certainly way more expensive than a spread footing.

 

[a2mfk]

Sorry Toad, not sure I saw your post. You can have the credit if you wish

 

[ToadJones]

Nah, you can have credit.

 

[DaveAtkins]

BA,

You could put styrofoam on top of the existing footing to prevent it from being loaded by the new footing, and design the "green" footing to span across the existing footing.

DaveAtkins

 

[BAretired]

DaveAtkins,

Yes, you could do that. Somehow, the additional pressure from the new footing didn't seem like much of a problem to me. I even wondered about mechanically fastening the two footings together, forcing them to act as a unit.

BA

 

[larsacious]

Have you considered different framing alternatives which may allow you to avoid locating the column where you have it shown? What magnitude of loads are you dealing with? Is your building a single story or multi-story structure?

 

[kieran1]

Could you move the pad foundation away from the existing foundation and provide a concrete stub column and cantilevering beam to support the steel column. This is obviously dependant on having a counter balance foundation as well

Kieran

 

[STRE]

We usually provide Helical Anchors (micro pile as already suggested above)adjacent to the existing footing with the lifting brackets. Cost most probably depends on your locality. I have seen those costing about $1200.00 a piece. You may be looking at another $200.00-$300.00 for lifting brackets and attachments. Reviewing your configuration, usually you may require two of those anchors.

Those anchors will be designed for any additional loading imposed by new column on the existing footing. Hence, you will be able to pour footing right next to existing one. You will certainly be specifying bonding agent on the face of existing footing as well as the reinforcement dowels epoxied into the existing one for continuity of the reinforcement of new one.