Need a book? Engineering books recommendations...

# RE: Pole Embedment Formula

• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• Subject: RE: Pole Embedment Formula
• From: "Swingle, Mark" <Mark.Swingle(--nospam--at)dgs.ca.gov>
• Date: Fri, 24 Sep 1999 15:38:12 -0700

```On 9/16/99 EDT, you wrote:

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

-------------------------------

1.  If the pole formlae don't apply in rock, then how would one
determine the depth of embedment required to resist the
lateral force P?

2.  Then, how would one determine the point of fixity in rock?
How did you come up with fixity at d/6 below the surface?
I was "taught" by several engineers that fixity is at d/3,
although no justification was given.  Sometimes soils
engineers would allow fixity at 2' below the top of rock,
but not all are willing to give an answer.

Thanks

Mark Swingle, SE
Oakland, CA

```