So I have multiple situations where I've got these NEW lateral walls between EXISTING gravity columns and the NEW wall foundation overlaps the EXISTING spread footing of the columns to different extents (at both ends). Seismic retrofit is the driving criteria here.
This is one, but I've got sixteen such situations over the footprint of the building:
Anyway, in order to get this to "work," I limit the width of the continuous footing to 6' and then 5' at either end (assuming that's past the end of the wall) which puts those 5' into the EXISTNG 10'x10'x2' footing to the north and 10.5'x10.5'x2' to the south. How do the results from the continuous footing affect the spread footings and vice-versa?
Is there a better way to go about modeling this, foundation-wise?
In this situation the spread footings are designed for the column (pilaster) forces only, the continuous footing is designed for the wall forces only. It's not as accurate as you might need for some combined footings. I recommend to model the whole thing as a Mat Foundation and evaluate that in Ram Concept (if that's an option for you).