Concrete Torsion

Toad,
what ACI are you using, in my version it talks about shear and there are no clauses ACI 11.6.2.1 or 11.6.2.2

An expert is a man who has made all the mistakes which can be made in a very narrow field

How do you get torsion in a footing? Don't the soil stresses offset the applied torsion? Maybe a sketch.

I'm with ron here.. not sure how you get torsion in a footing..

Toad: are you talking about an footing with an eccentric load that you are trying to resist moment by having a grade beam/strap beam?

ACI 318-02 is what I am using...I think 08' is the same still. We have it here in the office, just have to find it.
I have a combined footing with two very tall pier stems that have high moments in opposing directions.
I was wondering myself, now that I have had more of a chance to look at it, how much torsion is actually there. The footing "footprint" in the direction that would offset the torsion through soil pressure is narrow due to space constraints.

My thinking is that some of torsion along the longitudinal axis of the footing has to be transferred through the footing itself since the soil is an elastic material.

excuse the poor sketch....

Non-redistributed torsion.

Ron9876-
care to elaborate?
Thank you

Consider a condition where the combo of moments and axial loads result in uniform soil pressure under the rigid footing.

Now cut a section between the piers and statics will show there will be a torsional moment at the section. If you allowed the section to crack and redistribute the torsion you would change the soil pressures to some hard to calculate value. (I guess you could do some detailed finite element analysis and determine something but that probably isn't justified.)

Do that calculation and compare the value to ACI 318-05 11.6.1(a). Unless you have some very unusual condition I wouldn't put torsion steel in a footing. I would change either the width or depth until it works.

By the way what kind of condition do you have that will give you moments in opposite directions on the same footing?

Ron-
Thank you for the explanation. It is clear to me now that the footing, while loaded quite unusually for this case, really does not experience as much torsion as I originally (hastily) thought.

Without giving too much information, this footing is a combined footing for what are essentially flag-pole type columns that are loaded in opposite directions with. Typical "pole footings" are not possible in this case due to various construction constraints and in-place utilities. A deep combined spread footing was the preference of the owner.

..also,
For the case of this footing, ACI 11.6.1a allows for considerably higher torsion than the torsion experienced by the footing.

Thanks again for the input.