enlarging existing footing

huangyhg

enlarging existing footing??
has anyone enlarged an existing concrete spread footing by doweling reinforcing into the sides and pouring a larger footing around the 4 edges of the existing? (adding floors to existing building)
not my idea, i would like to remove the existing and build a new footing. the builder suggested this to save shoring costs and minimize disruption to on-going operation at the footing level.
the owner of course thought this was a wonderful idea!
i'm cross posting to geotech also.
see my post in foundations.  
building footings are small and usually redundant compared with major bridge footings which i have encased in new footings for seismic retrofit.
removing bridge footings would significantly impact traffic, there are fewer of them (less redundant) and they are usually fairly large.  this typically makes them good candidates for full enclosure or encapsulation.
regards,
qshake
eng-tips forums:real solutions for real problems really quick.
this will work fine and is the usual practice. the only thing  you have to make sure is
1.  proper bonding of old and new concrete surface.
2.  recalculating the pad thickness and rebars for shear and increased moments
3.  proper lap distances for dowels.
keep in mind that the existing footing will still have the stresses from the existing dead load plus a portion of the new loads. this may affect how you size the additional concrete. i prefer to pour on top of the existing footing, if possible, in addition to doweling the pier into the new concrete.
if you use a bonding agent, make sure that it is not a "reimulsifible".
jike i like your idea but, i suggest to also include 4 sides connection with dowels to put all existing footing in total interaction with the new top and side concrete. is very important to install the dowels with the hilti technical recommendation books accordingly with the footing moment resisting reaction.
this is fairly common, but one thing to keep in mind (and i am not sure all engineers know this) is that the new dowels must lap with the bars in the existing footing.  just embedding the new dowels per hilti recommendations (6" or 8", for example) develops the new dowels, but not the bars in the existing footing.
daveatkins
seraymond:
i certainly agree with doweling the existing footing to the new concrete on 4 sides even though i did not mention it. thanks!
you will need to know what rebar is existing to know if it is sufficient to withstand the additional moment from the additional footing.
i would also make sure the bond between the new and old concrete as well as the doweling issue.
on the other part, i would assume that the existing building is say two or three stories high and that you want to add say a couple more. an important consideration besides the footing capacity issue is to make the additional stories lighter.
from a different point of view and if you like the idea, what we have done in the past is to excavate immediately below the existing footings and pouring new footings right there and size them according to your analysis and design. we have framed with steel the new floor additions and has worked very well.
regards