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Re: Retrofit of Existing RC Spread Footings for Additional Loads

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Mr. Rogers,

Noting that you are more interested on the details aspect of your
situation/references, Caltrans (California Department of Transportation) has
some useful details for retrofitting existing spread footings to account for
addition loads (usually moment).  They usually add piles, but you can see if
you can get away without using them (soil bearing capacity, etc.).  See
Memo-To-Designers 20-4, " Earthquake Retrofit Guidelines for Bridges".
(website:???) It is basically based on Priestley's work at San Diego, CA.,
among others of course (See "Seismic Design & Retrofit of Bridges" by
Priestley, Seible, and Calvi, 1996).  Also, see general details in
FHWA-RD-94-052, " Seismic Retrofitting Manual for Highway Bridges"  May 1995
for FREE!!.  Phone:????

They basically 1) Roughen the surface of the existing footing 2) Uncover the
existing positive moment reinforcement at the bot. (approximate demo height &
depth called out) 3) extend these bars by splicing (mechanical or welded) 4)
Add dowels along the face of the existing footing to handle the shear btw the
new and the old along vertical planes  5) Add hooked dowels at top of footing
5) Add top layer of reinf. conc. over both new and old footing (min. 1 ft.) 6)
Add the necessary main reinforcement for positive and negative moments at top
and bottom, as required.  When its all done, it looks like a C-shape section
cupping over a square section.  

For ductility purposes, they also include hair pin stirrups (90 deg. one end,
135 deg. other end) as well as bundle bars at each face (say rectagular for
example) of the column coming in, within the new conc. top layer. Also, they
add #4/#5 bars around the column embedded in the new top layer (mech. spliced
or welded)  At times, post tensioning is also used.  If welding is used for
bars, Caltrans' recommended welding requirements are strictly followed.

Hope this helps
ed gonzalez

>>> "Robert Rogers" <> 12/21/98 10:23AM >>>
 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 ?
 P.S (use of piles or caissons cannot be used).