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Re: Copy of: Underpinning Existing Footings

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Kate -

I have read the various responses to your posting.  Our firm does lots of
residential remodel work in Southern California and very often use
underpinning.  First of all, despite what the contractor would have you
believe, typically, it is not that big of a deal.  The material costs (the
concrete) are relatively low as are the labor costs because underpinning
should not be confused with finished cabinetry.  The $6/hour guy is digging
the holes.

In regards to using the existing footing, in addition to the width, which may
be substantiated with a higher soils bearing value from a soils report, you
rarely have enough depth to satisfy the minimum 2-story footing requirements.
 So you end up underpinning anyway.

A soils report is a good idea anyway, though, because it gives the owner a
much higher comfort level that the underpinning pads are actually embedded
into good soil.  For jobs of any significant size ($150,000 or more), we
strongly encourage a soils report in any case.

Typically, we use underpinning pads at plus/minus 6' on center.  With a 2'
pad, this only requires the existing footing to span 4' between the pads.  We
check the existing footing for shear and bending to span as a plain concrete
beam between the pads.  The pads are designed to carry all of the load to the
soil.  This is cheaper than a continuous underpin system because you are only
excavating the "A" slots and not the "B"'s.  We detail the pads to be
centered under the existing footing and be wider than the footing.  We show
the concrete to be poured up around the existing footing 2" minimum above the
bottom of the footing to try to encase it.

If you do need continuous underpinning, we usually use screw jacks as
temporary shoring under the footing at plus/minus 6' on center.  These then
get poured in with the new footing.

In regards to the other suggestions you have received, I have seen the
mini-pile and screw anchor systems but have never specified them.  They have
two problems.  First, they do you no good if you have any creep load on your
soil.  Second they are not always approved by the local building department,
i.e. City of LA.

Lastly, do not forget you lateral loads.  Since you are adding a second
story, you need new lateral all the way to the foundation.  You will need to
make new shear and holddown connections to your existing footings.  If the
loads are high this can be a bit tricky.

Good luck.

Bruce Resnick