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Re: Copy of: Underpinning Ex

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At 15:54 11/26/97 -0700, Nigel Mends wrote:
" ... there is a company called Helicoil that makes a corkscrew affair that
you can "screw" into the ground adjacent to existing footings to provide
added capacity.  As these can be installed without excavating the existing
foundation, they would
eliminate much of the trouble you describe.  They might be just the thing for
solving this problem.

Nigel"


[Bill Cain]  
Chance also makes such a device.  But...
The first thing I usually do is to attempt to evaluate the capacity of the
existing footing.  Often they are sized to minimum code requirements which
constrain dimensions WHEN A DESIGN IS NOT PROVIDED (e.g., '94 UBC, Sect.
1806.2, Table 18-I-D) and have reserve capacity sufficient to support a
second story.   As such, it is often less expensive to verify the  design
(although lack of plans makes it more difficult, test exposures [both rebar
and bottom of footing] and rebar meters can often give an experienced
engineer a pretty good idea of what's there) and consider any reserve
strength (both foundation and bearing capacity) that may be available.
This must be done carefully.  Points I pay particular attention to are:
1. Lack of reinforcing (if the footing is trapezoidal in cross section, it
is usually unreinforced and will require strengthening).
2. Is there any distress in the existing structure (you obviously don't
want to increase loads if it is already failing).
3. Depth of footing.  The UBC depths are a good guide (absent frost
considerations) to the level where seasonal ground moisture variation will
cause doors and windows to bind.  Too shallow and you are asking for trouble.
4.  General character of the soil in the areas (are there problems with
expansive soil, consolidation, etc.).
5.  Will the proposed structure introduce a greatly different loading
pattern on the soil (concentrated loads on compressible soils can be a
problem).
6.  Is there sufficient geotechnical information available or can it be
obtained at reasonable cost?


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BILL CAIN, SE
OAKLAND, CA
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