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standard residential perimeter foundation

standard residential perimeter foundation

standard residential perimeter foundation

it seems like it should be trivial.... but ..

how much load can be put on a standard perimeter foundation?

for example if there is 6000 lbs on a 4x4 post that sits on a 6" x 12" wide footing (with 3-#4 bars)... and the allowable soil bearing pressure 1500 psf... typically on one design a pad under that part of the foundation 6000/ 1500 --> 4 sq ft (2'x2').

This is a bit of a nuisance to draw and for Contractor to build.

so... can the bearing area of the footing be extended to say 4 feet?.... 2' feet either side of the centerline of the post. I am assuming a

As a starting point i looked at Shear Friction, my assumption is that the foundation cracks but there is still resistance to the foundation moving vertically due to Shear Friction:
ACI Ø = 0.75
ACI 11.6.4 Shear Friction Vr = Avf fy µ { Avf =area of steel in shear, µ = 1.4x ¥ , w/ ¥ =1 normal; weight concrete}

3- #4 bars = 0.6 sq in

{0.75} x 0.6 sq-in x 60ksi x 1.4 = 37.8 kips

WOW! 37.8 kips that a lot larger the my 6kip load... so maybe I can extend the loading indefinitely along the length of the footing.

but Maybe this isn't safe?

Next thought -- Just shear strength of concrete--
Shear Strength concrete ACI 11.1 Commentary r 11.1 " Shear strength is based on average shear stress on full effective cross section'..or Vc = 2 √fc’ (bw x d ) {fc’=2500psi, b=6 & d=12} Vc= 7200 lbs.

WOW! 7200 is still a lot more than 4000/2
but Maybe this isn't safe?

Maybe this is abeam bending problem and one has to look at Modulus of sub-grade reaction.... too complicated so maybe on just makes 2ft x 2ft pad under post.

Any thoughts?

Cheers, 580B

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