Grade Beams, Drilled Shafts and Expansive Clay 3

[haynewp]

I have a site with 16" wide x 24" deep grade beams supported at 16'-0" on 30" diameter drilled piers that bear at about 10 ft deep with a 60" bell. This is an expansive site so there are 8" void forms under all the grade beams and the structural slab at ground level. The slab will be cast integrally with the grade beams.

When designing the reinforcing for the grade beams, how is the stiffness of the drilled shafts modeled? On page 6-16 here:

http://www.wbdg.org/ccb/DOD/UFC/ufc_3_220_07.pdf

It says: "Steel is recommended in only the bottom of the grade beam if grade beams are supported by drilled shafts above the void space."

Which I do not understand the logic behind at all. There should be significant negative moments in the grade beams that are continuous over the drilled shafts.

So that was my first confusion. Next, I have designed several elevated concrete slab systems before, but I am not sure how to include the drilled shafts in the moment distribution model. It would seem that they are more stiff than typical concrete columns since they are surrounded by the passive pressure of the soil, with a point of fixity at some location below grade. So what is the assumption for distributing the moments into the shafts from the grade beams? I plan on having dowels connect the shafts to the grade beams, so the beams are not just "resting" on top of the shafts.

 

[haynewp]

And the 16ft shaft spacing is just an average. Some of my spans go 23ft-10ft-23ft with skipped 100psf live load.

 

[hokie66]

I would neglect the bored pile stiffness, just assume they are pinned supports. Conservative, but that is the way I do it. As to the recommendation about only bottom steel, that makes no sense. And if you look at the details a little further on in the paper, they show top bars.

 

[csd72]

I always use both top and bottom steel, but I try to avoid shear ties if at all possible (may not be possible in a seismic zone). I have added additional top reo or deepened the footing just to achieve this.

csd

 

[JAE]

hokie66 beat me to it. same comments from me.