Drilled pier design

[BAGW]

Hi,

I have a drilled pier which needs to extend all the way to rock per geotech report for the bearing. The diameter for the drilled pier that I am coming up with is a 4'-0" based on loads. The rock profile is 70'-0" down, which means the drilled pier length will be +/- 70'. IBC 2012, 1810.2.2 calls out minimum 3 piles attached with rigid cap for stability requirements. There is an exception to this stating if the drilled pier is greater than 2'-0" in dia and the adequate lateral support is provided for the entire height and the height does not exceed 12 times least horizontal dims.. I am getting stuck with "height does not exceed 12 times least horizontal dims". Does this mean the min dia I need to use is 6'-0" even though the 4'-0" dia is more than sufficient for load carrying capacity satisfying all limit states.

 

[JLNJ]

I'm bumping this back up.

I know we've used singular piers which exceed the 12x, but maybe this is a new-ish provision. I'm curious about the logic.

 

[BridgeSmith]

We typically use a P-y analysis program (L-pile or All-pile) to design these. Per the commentary in the AASHTO bridge design spec, drilled shafts are considered braced 5' below the ground line.

Fixity is a different matter; that depends on the loads, soil properties, etc. The unbraced length for a column and drilled shaft combination subjected to lateral loads seems to still be a matter of much debate, with some using the max moment/zero shear point to define the point of fixity, while others use the inflection point where the deflection goes to zero.

In any case, 70' is a long way to go to get bearing. Generally, there is enough side friction available to provide the axial capacity needed at considerably shallower depths. If this cannot be achieved due to the necessity of permanent casing, we typically find that driven steel piles become the more economical option.

 

[haynewp]

I've designed isolated piers for single concrete columns to sit on and the slab on grade was used to brace the cols/piers and they went down farther than 12Xhoriz dim. I don't recall seeing a cap with 3 drilled piers below to support a column.

 

[montbIanc]

I'm new to reading the IBC code. When it gives acceptable solutions like quoted by OP, does that mean other solutions aren't acceptable? It doesn't seem to say that in the main section, but the wording of the "Exceptions" seems to imply requirement rather than example.