huangyhg

demolition of existing wall footing to install column
a new column footing needs to be installed at a location where it interferes with an existing wall footing (24"x12" deep). the contractor will have to demo enough of the existing wall footing to install the new footing (5'x5'x12"). the existing wall footing is 16" below finished floor and has 2 courses of 8" cmu on it. the cmu supports a load bearing 2x4 wall, sheathed on the exterior and drywall on the interior. also, 4" brick veneer rests on the wall footing. new footing is 16" below finished floor.
i know this is done all of the time but don't know exactly how they do it. do they cut the footing with a chop saw and try to leave the block hanging? or do the demo the block and leave the 2x wall hanging? i would think with only dead load that the wall sheathing would allow the load to arch over the removed wall footing. or do they need to put in a temporary header to transfer the load? i know it probably depends on the load. the building is only a shell with no occupants yet.
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if the demolition is short, only the footing can be removed and then the new column footing can be built to include the missing wall footing.
how do they remove the footing? do they cut up to the block, then jack hammer the rest? the block is doweled into the footing, if they jack hammer will it crack the cmu or brick? seems to me they would have to remove the block as well....just because it will make things easier and it will probably get damaged.
one option is to leave the footing in place and cast the new footing as a u-shape around it.
most footings will be able to span the 5'.
you then do a basic underpinning construction extending beneathe the ex. footing.
your saying to excavate under the existing footing, place the new footing with the top of new footing to match the top of exisiting footing.
any concern that the existing footing will not have full bearing on the new footing because of air voids?
if both sides of the concrete are exposed and if the concrete comes up above the bottom of the footing on both sides you can eliminate many of the voids by vibrating. there is more of a problem with air voids when you underpin an existing footing and you can only access it from one side. it can be possible, but it becomes more difficult to make sure there are not voids. the other thing to consider is the soil that you'll be doing this in. you need to be able to dig under the footing without having the sides continue to come in.
j
maybe it will be possible to change your new footing from square to rectanglar, with the short dimension set to butt up to the existing footing, and thus avoid cutting anything.
sorry, i should have been a bit more specific.
to ensure adequate bearing leave 2" between the new and existing and either fill it will dry pack or possibly non-shrink grout.
contact your local grout manufacturer for an appropriate product.

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