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RE: Pole Embedment Formula

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Oshin-

Be very aware of the basis for the pole embedment formulas in the UBC. The
embedded pole formulas in the UBC are based on specific conditions: 

1.	Not restrained: is assumed to be free to translate and rotate at the
point of load application (i.e., it is a cantilever from the point of
fixity).
2.	Restrained: Some means of translation support is provided at the
surface (such as a slab, etc.). It is not based on the pier not being fixed
by the ground (for rotation) at this point.  The pier will rotate about this
point as if it has a simple support at this location.

If you are framing into a grade beam that provides fixity to the pier,
neither formula applies directly.  You can still use the formulas if you
make adjustments to your model.  For example, a grade beam providing fixity
to the pier forces the pier to bend in an s shape, similar to a column in a
frame fixed at the top and bottom.  To use the "not restrained" formula in
the UBC, you must apply the load at the point of inflection (i.e., zero
moment).  The easiest way to visualize the solution required is to draw the
deflected shapes for the pier.

A simple, more direct way to deal with these problems that I have found
helpful is the method outlined by E. Czerniak in a paper entitled
"Resistance to Overturning of Single, Short Piles", Journal of the
Structural Division, American Society of Civil Engineers, Proceedings Paper
1188, vol. 83, No. ST 2, March, 1957.  The method treats the pier as rigid
for depths to diameter ratios less than 10 (although it also provides a fair
approximation of more slender piers, possible to ratios as high as 15).  It
is a general method in that one determines a moment and shear at the
resisting surface (i.e., the surface below which resistance is provided by
the pier.  If your geotechnical engineer says ignore the top 2 feet, for
example, the restraining surface begins 2 feet below the ground surface.).
For more slender piers, the flexibility of the pier will affect the shears,
moments and deflections in the pier.  However, remember that most values
given for soil or rock resistance are little better than "order of
magnitude" values.  So don't get too hung up on precision.  I usually bound
my solutions to get a feel for the sensitivity of the problem.  The charts
provided in the Czerniak paper simplify this effort.

Regards,
Bill Cain, SE
Oakland, CA


	-----Original Message-----
	From:	SDGSE(--nospam--at)aol.com [SMTP:SDGSE(--nospam--at)aol.com]
	Sent:	Thursday, September 16, 1999 18:03 PM
	To:	seaint(--nospam--at)seaint.org
	Subject:	Pole Embedment Formula

	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.