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Re: Stack Foundation

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Neil Moore wrote:

<<Although you are checking the factor of safety for overturning across the
flat sections, have you checked the maximum soil pressure across the
diagonal?>>

The example was more to compare limit states vs allowable stress design but
you're right the diagonal should be checked.

For low eccentricities the diagonal loading seems to produce higher soil
pressures but when the eccentricity gets to be more than about 43% of the
length of the side it seems that this changes and the soil pressure for
diagonal bending is lower. For the example I did (10 ft x 10 ft footing) I
reckon the peak pressure would be about 9.0 ksf (factored load case with e =
4.41 ft) on the diagonal vs 9.6 ksf on the square.

When the eccentricity reaches half the side length the footing is in an
overturning situation in the square direction and soil pressures are going
to infinity. In the diagonal direction there is still some capacity left.

I guess that's why circular or octagonal footings would be more cost
effective in concrete volume than square footings for this type of
application.

I'm hanging a bit out on a limb on this. Can anyone confirm the above?

Cheers,
John MacLean



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