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Seismic Joint at foundation

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Will:
 
No, I don't think you are thinking too much.  It is rational thought and experience which forms the basis of the building codes.
 
I am convinced that the coefficient of friction between footing/slab and earth, coupled with the passive resistance of the earth piled against the footings generally allows the concrete to move with the ground motion.  I would guess that with an abundance of water under the slab and under the entire length of footings, there could be some slippage.  This may be the case during liquifaction.
 
We did a lot of earthquake damage assessment, design, upgrades, and repair after the Northridge earthquake.   We worked for several years on structural damage.  I did not see any evidence of foundations slipping on top of the dirt. 
 
We were climbing around in the apartments which partially collapsed, killing several people 2 days after the earthquake.  On the residential streets, every masonry fence and every masonry fireplace was toppled.  Very strange sights.
 
With Joy and Hope,
Bob Freeman, Architect
Structural Designer
Integrated Design Services, Inc.
(949) 387-8500