ACI minimum depth requirement for foundations

[DTS419]

ACI 318-19 13.3.1.2 says that minimum depth of bottom reinforcement for foundations is 6" (not inclusive of cover). Does anyone have any insight on what is driving this?
Could it be justified that a precast spread footing is exempt from this requirement if it can be shown by analysis that the footing is adequate for the design load?

 

[HTURKAK]

IMO , 6" is the minimum depth in order to provide a certain rigidty and even soil stress distribution. In case of PC footing , you may reduce the clear cover ( if blinding layer, water proofing membrane is used ).



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Make it do, or do without.

NEW ENGLAND MAXIM

 

[BulbTheBuilder]

13.3.1.2 Overall depth of foundation shall be selected such that the effective depth of bottom reinforcement is at least 6 in.

Assuming you use a minimum cover of 3" [cast against earth], 3"+6"=9" you are approaching a minimum of 10". As HTURKAK mentioned, I believe is this for shear and flexural rigidity. The footing shouldn't be too "thin" for it to bend excessively due to soil stresses and not too "thin" for shear (one-way and two-way)

 

[Celt83]

It’s been my impression that the 6” min to bottom of reinforcement was there to be able to accommodate the 6” minimum standard hook embedment.

 

[BulbTheBuilder]

You're right, Celt83. I remember reading something similar from CRSI publication where they emphasis on the selection of a minimum thickness based on development of dowel bars from the footing. It all makes sense now.

One thing I still have a hard time figuring out why is the development of compression for foundations.

 

[DTS419]

Why would the bent bars need to be vertical? You could make a thinner footing with bent bars placed so that the hook was horizontal and not eating up thickness.

 

[JAE]

Celt83 is talking about vertical bars used to tie the footing to the concrete wall, stem-wall, or slab above.
The bars are typically "L" shaped bars with a standard 90 degree hook embedded into the footing.
The standard, minimum, hook embedment is 6" for smaller bars - the distance from the top of footing to the bottom of the "L" hook.

 

[DTS419]

That makes sense. I was thinking in terms of develop length of the bottom flexural bars in the footing.