Pile Punching Shear in Pile Cap

[Rdar43]

I was looking at the CRSI pile cap design guide and in the design examples it looks like it isn't accounting for the unbalanced moment for corner piles in it's punching shear check. Does anyone know why this is?

Thanks

 

[KootK]

Which examples are you looking at? With or without lateral load?

 

[Rdar43]

Without lateral load

 

[KootK]

Then I imagine that CRSI simply made the conventional assumption that the joints between the piles and the cap are pins that attract no moment. Obviously, the truth-i-ness of that depends on the relative thickness of the cap and all the other stuff.

 

[slickdeals]

I believe there should be a moment due to the eccentricity between the centroid of the reaction and the centroid of the punching shear perimeter. In my previous discussions with CRSI about it, they haven't been able to find why it's ignored in their calculations. For what it's worth, the PCA handbook also has a design example that ignores it and there are a couple other (think structurepoint's example problem) as well.

 

[BridgeSmith]

I would think that the moment due to eccentricity would be comparatively fairly small, it would primarily be resisted internally by the pile. IOW, if the pile doesn't rotate, there's no moment, just a redistribution of forces (which if I'm not mistaken would be towards the stiffer/stronger side, anyway). Punching shear failure of the cap is an all-or nothing proposition - if it breaks, it breaks along the entire perimeter, so a little higher stress here or there doesn't change when the ultimate failure occurs.

There's also the large variability in the shear capacity of concrete, and the unpredictability in the shape of the ultimate failure surface, which makes any attempt to precisely calculate what the punching shear capacity is, a fool's errand.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Rdar43]

Wouldn't there be still be moment in the slab at the column location that would increase the punching shear stress regardless of the fixity of the column?