Entire/Portion of critical section is beyond the footing
Entire/Portion of critical section is beyond the footing
(OP)
This is my first time post, I have simple question.
I have been asked to make thicker shallow footings for a gravel wash plant. The footings will have a larger mass to prevent wheel loaders from digging them out or pushing them.
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Here are my parameters:
12 in column
50k load
depth 12 in.
d 8 in.
soil pressure 3000 psf
f'c 3000 psi
My simple calculations give
A 4.5ft x 4.5ft x 12 in.
actual net soil pressure 2469 psf
beam shear 14in/27in 30psi/60psi ok
punch shear 20in/27in 67psi/109psi ok
flexure 21in/27in 17k-ft/43 k-ft ok
fs 16, R 0.152, j 0.87, Steel area 1.83 in2
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When I increase the footing depth to 36 in I get
36 in depth footing
d 32 in
A 4.5 x 4.5ft x 3ft
beam shear 38in/27in ?
punch shear 44in/27in ?
flexures 21in/27in 17k-ft/700k-ft
Steel area 0.46 in2
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I have scoured the net for the condition of "the entire critical section is beyond the footing" and I have found nothing. Schaum only mentions to reduce the perimeter when "a portion" of the critical section lies beyond the footing. Ambrose says nothing.
Please help. Does this mean no beam shear? Does this mean no punch shear? Is it ok to make footings deeper than minimum?
Thanks, preparing for a grilling....
Sam Ricci Jr in southern NJ
I have been asked to make thicker shallow footings for a gravel wash plant. The footings will have a larger mass to prevent wheel loaders from digging them out or pushing them.
------
Here are my parameters:
12 in column
50k load
depth 12 in.
d 8 in.
soil pressure 3000 psf
f'c 3000 psi
My simple calculations give
A 4.5ft x 4.5ft x 12 in.
actual net soil pressure 2469 psf
beam shear 14in/27in 30psi/60psi ok
punch shear 20in/27in 67psi/109psi ok
flexure 21in/27in 17k-ft/43 k-ft ok
fs 16, R 0.152, j 0.87, Steel area 1.83 in2
------
When I increase the footing depth to 36 in I get
36 in depth footing
d 32 in
A 4.5 x 4.5ft x 3ft
beam shear 38in/27in ?
punch shear 44in/27in ?
flexures 21in/27in 17k-ft/700k-ft
Steel area 0.46 in2
------
I have scoured the net for the condition of "the entire critical section is beyond the footing" and I have found nothing. Schaum only mentions to reduce the perimeter when "a portion" of the critical section lies beyond the footing. Ambrose says nothing.
Please help. Does this mean no beam shear? Does this mean no punch shear? Is it ok to make footings deeper than minimum?
Thanks, preparing for a grilling....
Sam Ricci Jr in southern NJ





RE: Entire/Portion of critical section is beyond the footing
This is not a bad thing - I interpret your calcs to mean that the footing will work very well at the stated conditions with no reinforcement all (plain concrete footing are common in some applications). Be sure to check loading for all conditions, such as wind, seismic, impact, etc. However for an industial settings, which yours seems to be, I would suggest that you include at least a minimal amount of reinforcement even if it is not indicated by calculations.
As an aside sounds like you are getting good general advice to increase the footing thickness (36 in does sound like a little more than I would have expected however). In many industrial applications it is wise make "things" substantially more robust than calculations indicate.
RE: Entire/Portion of critical section is beyond the footing
RE: Entire/Portion of critical section is beyond the footing
Try a mat of #5 bars 4.25" o.c., 12 min.
Design should also be ok without reinforcement as SlideRuleEra pointed out.
RE: Entire/Portion of critical section is beyond the footing
Good Luck, hope this helps.
RE: Entire/Portion of critical section is beyond the footing
I believe there is a provision in ACI318 on PCC - Portland Cement Concrete (unreinforced).
RE: Entire/Portion of critical section is beyond the footing
Thank you for your excellent tips.
I confirmed the "no reinforcement" and "increase allowable soil pressure" threads. I still have to confirm the "increase minimum steel" and "ACI 318 Chapter 11" before I'm happy.
2469 is over exact, but I like to follow the logic before I wave the magic wand...
Thank you.
Sam