Encasing an old footing within a new footing it done many times for seismic retrofit, for example.  And while the old footing capacity (piles for example) is usually neglected when designing the new one; there is one area that cannot be neglected and that is the transfer of forces from the old columns and footing.  To facilitate this transfer, the old footing is doweled on all sides and on the top.  I suggest that you consider this transfer action when designing your footing.

hmmm, i think i didnt make it clear enough, anyway the new isolated foot is casted over the old one with neglecting the old footing effect in resist the applied forces, only the new one is resist. i know that will increase the soil  bearing Capacity below this foot, the confusing thing is not all footing will recast, there will be some over the soil directly, and others will cast over RC Footings.

there will be diffrence in streeses between the footings rested on soil and the footings rested on old RC footing is that danger or some thing ??

also is there any recommendations ??

thx again

 Pea$e

IYIeI2cY,

I am assuming the new footing will be larger than the existing footing, so part of the new footing will be bearing on the old concrete and part on soil.

If the new footing area is calculated based on the allowable bearing pressure of the soil, you could neglect the effect of the load on the old footing.  Think of those existing footings as a spacer between the new footing and the soil below.  Really all the footing will bear on soil; some directly, others through the existing footing below.

Care should be taken that the soil around the existing footings, that are going to support the new footings be compacted to the density required to provide for the allowable bearing pressure assumed.

Hope this would help

AEF

thx all for help, hope u both dont get confused in any problem :)

Laterz

I assume you won't have any problem as long as your NEW footings are of size enough to carry safely the load.

Note that the overall response will be dictated anyway by the underlying soil, and it is unlikely underlying isolated structures as footings may importantly enough affect the overall behaviour.

thx for u all :))

Pea$e

It would appear that the reactive pressures would be much different.  The old footing would have a much higher resistance and create an inflection point.  Negative reinforcement would be required to resist the loading.

What boo1 says would be a good complementary measure that can be assessed through assumed different compressibilities (springs, Es etc).

As I read the original question, there are some footings to be cast over old footings and there are others to be cast on previosly unloaded ground.  If this is the case, and the soil is clay you may well have a problem with differential settlement.