Enlarging an existing column footing 2

[kngpenn]

Does anyone know of a calculation procedure for enlarging an existing column footing due to additional loading? At the interface of the new concrete where bars embed in to the existing footing, there should be a calculation to determine the bars size and spacing based on the new loading. I've never encountered this situation.

 

[dik]

If the existing footing has sufficient reinforcing to accommodate the new moment, it's a matter of securing a 'ring' of concrete around the footing or on opposite sides. The existing footing has to be capable of supporting the new load. It may be a matter of providing temporary support and casting a new footing edge (or complete) beneath the existing footing.

Dik

 

[TLHS]

You're trying to widen the footing? It'd probably calculate the reaction pressure from the ground, figure out the load on the new footing area and then dowel in bars to transfer load via shear friction into the existing footing. This is probably pretty easy if the footings are exposed or could easily be exposed.

It might be better though if you could attach a beam or something to the bottom/edge of the column that would then spread the load into the current footing and new footing from the top. Then you don't necessarily have to connect the footings into one integral unit.

Just an idea. I've never done this, personally.

 

[dik]

My first step is usually to contact a geotekkie to see if the allowable soil loads can be marginally increased... if this fails, then look at alternatives...

dik

 

[ron9876]

TLHS that approach neglects the added moment and shear on the existing footing.

If the original footing has shear capacity you will likely have to extend dowels far enough into the existing footing to resist the moment.

 

[JennyNakamura]

Wouldn't you have to jack up the existing footing to relieve the existing stresses? What if the existing footing is heavily stressed; the extension will be unstressed at construction before loading while the existing portion is already stressed. How do you deal with that?

Just thinking out loud -- never worked on something like that before...

 

[hokie66]

The problems with the "donut" approach have been identified above. If dik's approach to the geotech doesn't work, look at building a new footing rather than trying to glue onto an existing.

 

[shobroco]

I think the real answer is "it depends..."

 

[msquared48]

The problem here is that the maximum moment that the rebar is resisting is at the column, away from the edge of the footing. If you increase the moment, then the area of that steel which is very hard to access must be increased - generally not possible unless you drill all the way through the footing.

So do what dik and Hokie said instead.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hawkaz]

In many cases, minimum steel requirements govered the original footing sizes. When this is the case, adding onto a footing is generally not a problem- but you still need to check your moment and shear.

Use TLHS's procedure. I normally add dowels top and bottom for an added comfort level. You will need to widen the footing by at least two feet so they have room to drill the holes for the dowels.