Raft foundation shear reinforcment 1

Gus14 said:

1) Why is it that ACI specify stirrups arrangement in such narrow rows limited to the columns dimesnions ?

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I don't think that they do. I believe that the [2d] is the max spacing between stirrups, not the maximum dimension of the stirrup group.

Gus14 said:

2) Would making stirrups wider than the column dimesnions help increase the stirrups spacing ?

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Sure. I'd think that you could space the stirrups out as wide as the column, at least. I'd probably abandon the 20 cm column face and just use the 100 cm face as shown below. Do give some though as to how that would impact integrity reinforcing per your local code however.

It's not possible too just locally increase the thickness of the slab? That's usually a much simpler solution for foundation slabs.

The thickness of the raft foundation should be governed by shear .. and determined based on punching shear at critical column and the design for shear is considered before flexure.

I will suggest you to increase the thickness or at least locally increase suggested by JoshPlumSE..

PS: How did you calculate the punching shear ?

Thank you everyone for responding. Thanks KOOTK for explaining the reinforcement detail arrangement. I recalculated the steel area and specified the spacing as per ACI with some fair conservatism the details are attached. The contractor thinks this would be faster and he already poured the cover.

I calculated pushing shear by subtracting the factored load "1800 KN" from (punching area X the soil reaction "250"). I know that the column is huge for such loads but it's under a lot of moments on it's slender side.
This site recommends using shear reinforcement although the details are different.