Punching Shear on a Footing

[JoshPlumSE]

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

 

[JoshPlumSE]

For programming we treat it entirely consistently... reducing the punching shear demand based on the soil bearing which occurs underneath the pedestal.

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.

 

[miecz]

JoshPlum

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".