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seismic joint

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The theory is that the earth will move at a constant rate during a seismic event.  The old slab-on-grade and foundation will move at the same rate as the new foundation.  Therefore, there is no need for seismic separation at the slab-on-grade level.
The only reason to dowel the new concrete slab-on-grade to the old is the potential for differential settlement.  Tripping hazards. 
Starting from the slab-on-grade up, the structures will move independently, and sometimes out of phase, potentially causing damage/destruction.  The seismic joint will start in the wall at the slab-on-grade and extend through the structure.
With Joy and Hope,
Bob Freeman, Architect
Structural Designer
Integrated Design Services, Inc.
(949) 387-8500