Punching Shear on a Footing
Punching Shear on a Footing
(OP)
Take an example of a simple spread footing subjected to an axial load Pu applied through a pedestal. Assume that there isn't any applied moment, though the axial load may be considered to be eccentric. There is also some soil overburden pressure and the self weight of the footing itself.
When you calculate the required punching shear strength, you start with the applied load Pu. But, then you get to reduce it a bit because of the soil pressure below which is counter-acting the applied load, correct?
Obviously, you can only use the amount of force / soil pressure that is within the assumed failure perimeter. This is, after all, the only soil pressure that directly opposes the load Pu.
The question is how do you calculate the force reduction in the punching shear demand? What pressure do you use? Do you base it on the TOTAL applied load (which includes the effects of fooing self weight and the soil overburden)? Or, do you ONLY subtract out the portion of the pressure that directly comes from the applied pedestal load?
I know how I would do this, but I wanted to get other folks opinions on the subject.
Thanks in advance for any input,
Josh
When you calculate the required punching shear strength, you start with the applied load Pu. But, then you get to reduce it a bit because of the soil pressure below which is counter-acting the applied load, correct?
Obviously, you can only use the amount of force / soil pressure that is within the assumed failure perimeter. This is, after all, the only soil pressure that directly opposes the load Pu.
The question is how do you calculate the force reduction in the punching shear demand? What pressure do you use? Do you base it on the TOTAL applied load (which includes the effects of fooing self weight and the soil overburden)? Or, do you ONLY subtract out the portion of the pressure that directly comes from the applied pedestal load?
I know how I would do this, but I wanted to get other folks opinions on the subject.
Thanks in advance for any input,
Josh





RE: Punching Shear on a Footing
RE: Punching Shear on a Footing
RE: Punching Shear on a Footing
RE: Punching Shear on a Footing
My opinion has been that you can reduce the punching shear demand by WHATEVER soil pressure occurs directly underneath the punching shear perimeter. And, that it doesn't matter whether this soil pressure was developed as a result of the pedestal load or the footing self weight or the soil overburden.
If your footing/mat actually experiences this soil pressure then you can use it to reduce the demand. I cannot see anything in the code that would specifically forbid me from being allowed to do this.
Whether or not we feel comfortable as engineers with the reduction in our safety factors when we do this is another question.
Josh
RE: Punching Shear on a Footing
RE: Punching Shear on a Footing
RE: Punching Shear on a Footing
RE: Punching Shear on a Footing
Your method of using the soil pressure that occurs outside the punching shear perimeter results in the same thing as mine.... I'm just looking at it upside down (or you are).
But, to point out a minor difference. The soil pressure due to overburden will have a total magnitude that cancels out the applied over burden. However, that force can be spread out over a different area (because of a triangular soil bearing profile or becasue the area of the pedestal). In most cases the differences between the two methods should be very small.
Like I said earlier, this is a fairly theoretical discussion. But, I work for a software company. Therefore, we try to take into account every minute little detail. Even if my hand calculations wouldn't normally account for this force reduction, I think it is the more theoretically correct way to calculate the demand.
RE: Punching Shear on a Footing
JoshPlum, I would not make a deduction in your model. I believe most engineers, in checking your software, would believe you are in error and would then distrust your results.
RE: Punching Shear on a Footing
Instead, the prevailing argument seems to be "I'm not comfortable that the code provisions can assure an adequal level of safety for punching shear. Therefore, I try to be make sure that my calculations are a bit mor conservative."
I'm not poking fun at that argument. If my mat foundation were at 98% of punching shear capacity, I would personally increase the thickness of the mat... at least if I were to seal the drawings. I just want to make sure that no one has a solid argument against the technique...one either based on code provisions, first principles, or with a solid basis in theory.
RE: Punching Shear on a Footing
RE: Punching Shear on a Footing
RE: Punching Shear on a Footing
RE: Punching Shear on a Footing
I hope it will be useful.
Zafer Geçim
Civil Engineer, M.Sc.
Structoris Engineering Consulting Contracting Co.
RE: Punching Shear on a Footing
However, when the eccentricity is beyond the kern area and I need the footing self weight and overburden to help resist overturning, then I use those forces to calculate soil pressure and thus punching shear.
RE: Punching Shear on a Footing
There are many cases where when you do not include the SW and OB then the eccentricity will be beyond the kern. But, when you do include the SW and OB then the revised eccentricity will be within the kern.
I think this method still makes the most sense for hand calculations, but it is based on some engineering judgment calls about when you use one method or the other.
Therefore, it ends up being easier (at least for a program) to treat all cases the same rather than to try and switch back and forth based on a semi-arbitrary criteria.
RE: Punching Shear on a Footing
RE: Punching Shear on a Footing
For programming purposes, simply include the SW and OB, and the resulting increase in soil pressure.
For footings carrying axial load only, with no moment, the SW and OB result in increased calculated soil pressure, with no net change in punching shear.
It would seem that a footing carrying a moment in addition to axial force would fail sooner in punching shear, as the punching shear stress would be higher on the side with the increased soil pressure. I'm not sure if this is addressed anywhere in the code, and I don't remember seeing a published solution.
RE: Punching Shear on a Footing
I agree, (for programming) it is easier and more consistent to include SW and OB for all cases. Just make sure that you deduct the resulting soil pressure so that it's affect zeros out for the axial only case.
RE: Punching Shear on a Footing
However, this conversation has rightfully raised the issue about whether engineers are comfortable with this reduction. Whether or not it is theoretically correct is somewhat immaterial if that's not the type of calculcation that engineers want to see. Therefore, we're talking about adding a user option related to whether or not puching shear demand can be reduced by this soil pressure.
On another note, footings that experience soild bearing one one side only will fail sooner. I think ACI calls this the "the shear due un-balanced moment". The calculations get a bit trickier, but they're not really all that bad.
RE: Punching Shear on a Footing
I don't really believe there is any disagreement on whether the punching shear can be reduced by the soil bearing pressure. I think that some engineers include the SW and OB to increase the soil bearing, and others ignore all three effects, in which case, it's a wash as far as punching shear is concerned. I can't imagine anyone using the SW and OB to determine the soil pressure, and then ignoring it to determine the punching shear. That just doesn't make sense. You would be causing unnecessary confusion by including such an option in your software.
Please reference the ACI article about "shear due to un-balanced moment".