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Re: Retrofit of Existing RC Spread Footings for Additional Loads

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I have done a number of these over the years and I use different techniques
depending on the exact situation and loads. If the new column load is over 50 to
75 kips, I would prefer to use a grade beam to project out over the existing
footing to pick up the new column load. The grade beam should tie into the nearest
new column footing and cantilever across a new footing (wherever it is convenient
to locate it). Sometimes the depth of the grade beam (above the existing footing)
may be a problem but there are ways around that also (a wide, shallow concrete
beam or steel beams encased in concrete).

Another technique would be to add onto the existing footing. Although I have seen
details where an engineer would just add onto the side of a footing by doweling, I
prefer to dowel into, pour alongside and over the top of the existing. Note that
if you use a bonding agent, it must be water insensitive. The new pier should be
detailed such that it transfers load into both the new and the existing. I have
also found that setting up a spreadsheet to calculate the combined soil bearing
pressures is convenient for this situation. Keep in mind that the bearing pressure
for the original dead load is still on the existing footing. Then you need to
calculate the pressures from the new dead and live loads on the new combined
footing and take into account any eccentricities that might be created. You need
to combined those stresses at various locations of the combined footing and limit
those to your allowable pressures.

Another technique is to cast an new eccentric footing and pier against your
existing foundation wall and take out the eccentricity by having it "lean" against
the existing construction. You must be able to justify and resolve the horizontal
forces that develop to keep the new footing and pier upright. This may be done
with a frost wall or a basement wall as long as you are able to rationalize the
forces and the stresses.

I hope this gives you some ideas on how to handle this type of  problem.

Jim Kestner, P.E.
Green Bay, Wi.

Robert Rogers wrote:

>  I have recently searched the SEAINT E-Mail Archive for any information on
>  modifying existing reinforced concrete spread footings to carry additional
>  column loads.  Although I have found some posts they have provided little
>  information which I could extract for useful purposes.  I have also performed
>  a search of COMPENDEX for similar information.  My success has been limited.
>
>  When placing new columns right beside an existing line of spread footings
>  (interconnected by strip footings) substructure interferences abound.  In
>  one particular case, the spread footing protrude 5 to 8 ft. into a new
>  building area (where new columns are coming down).  I am searching for any
>  good references (journal articles, practice periodicals, etc.) which deals
>  with the design/analysis methodology of increasing the plan dimensions of
>  the existing footings to pick-up and carry the new columns loads.  I know I
>  will end up with a combined mat footing which could dictate a slab on an
>  elastic foundation analysis.
>
>  I am more interested in connection details (new concrete to old, cap system
>  over the existing foundation, etc.).  I have found one article in the
>  Transportation Research Record (No. 1476, Seismic Retrofitting of Bridge
>  Substructures) which was helpful.  Can anybody else point me in a good
>  direction or offer some advice ?
>
>  Thanks
>
>  P.S (use of piles or caissons cannot be used).
>



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