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Re: Pole Embedment Formula

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At 09:02 PM 9/16/99 EDT, you wrote:
>Would it be appropriate to use the restrained embedded pole formula rather 
>than the unrestrained one for the embedment design of a drilled concrete pile 
>in bedrock.
>
>The soil engineer has specified the point of fixity into the bedrock, and I 
>was wondering, if the pile is assumed fixed at that point wouldn't the 
>restrained condition apply? Since, I assume, the pole embedment formula was 
>probably based on embedment into soil rather than bedrock. I always used the 
>unrestrained condition in my designs, but this time I have quite a bit of 
>embedment, so trying to reduce the embedment any way I can.
>
>Any thoughts?
>Thanks
>
>Oshin Tosounian, S.E.
------------------------

Both of the pole embedment formulas indeed contemplate ordinary soils that
are not very rigid compared to the pole, over the pole's depth of embedment.
Embedment into bedrock is quite a change from that.  

The formulas came from work done in the 1940's at Notre Dame and one other
university for the Outdoor Advertising (billboard) Assn, and involved hole
diameters of a foot or two, and depths of ten feet or so. The formulas got
into UBC a long time ago. Several editions later the allowable lateral soil
loads in the UBC table were considerably reduced, making the result more
conservative than originally.

The "Constrained" condition is for when there is a hard resistance to
lateral movement at grade (hard compared to the soil below) and that acts as
a pivot point or fulcrum to the horizontal forces acting on the pole, the
pole being modeled as a rigid free-body.  Max pole moment is at this constraint.

For "unconstrained" conditions (all-dirt embedment) the max pole moment is
at the point of zero pole shear, a depth that varies according to how low or
high above grade the lateral force is, compared to the embedment depth. I
worked up a table of values;
  M = P (h + .17d) is about as bad as they got for small h compared to d.

If you are developing all your resistance in bedrock, I submit that neither
of the UBC formulas applies.

Why not go back to the soil engineer for an interpretation of what he/she
had in mind? Keep principles of mechanics in mind at all times; it ain't voodoo.

Charles O. Greenlaw SE   Sacramento CA