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Re: Drilled Pier Minimum Diameter

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I'm not sure if you know the design requirements.  First, this is coastal construction and I have high wind with uplift.  That puts a pier in tension.  I think I would want steel in the pier even if allowable stress showed it didn't crack.  It seems wise.  Second, it is expansive soil and there is uplift from the soil.
 
Thanks for your input.
 
Rich
 
 
On Fri, 15 Oct 2004 15:40:20 -0400 "Jordan Truesdell, PE" <seaint(--nospam--at)truesdellengineering.com> writes:
Is there really a necessity for that #4 bar in what is primarily gravity column?  I've used drilled piers (usu 18-24") for residential projects, and typically allow them to be plain structural concrete,  with no more than 2:1 h/dia above grade (figuring one dia below grade as unsupported).

At 10:16 AM 10/15/2004 -0700, you wrote:
Some of the communities north of here have modestly expansive soil. The local buildings departments provide a "standard" design involving 10" to 12" dia. piers in the range of 6' to 7' with a single #4 bar. The sites are flat & with piers at 6' or 7' o.c. the foundations seem to perform O.K. for them.
Chuck Utzman, P.E.

Scott Maxwell wrote:

Richard:

I don't know if anything that specifies a minimum diameter.  I will
note
that the ACI 336 documents (at least 336.1 and 336.3R) state that they
generally only apply to piers that are 30" in diameter or
greater.  336.3R
states that it is "...difficult to detect sidewall collapse in
small
diameter piers during concrete placement and casing
extraction."  Thus,
it states that small piers are generally only used where there are
consistantly stable soils or the casing are left in place.

I would say that minimum diameter would be largely a function of how 
the
pier will be used (i.e. end bearing or skin friction) and what the 
soils
are at the site in question.  Thus, it would likely be something
that you
would need to get a recommendation from the geotech on.  If they are
going
to be short, end bearing piers, then usually a recommendation from the
geotech on minimum size would be given.  I know that usually most
geotech
reports that I have gotten recommend that strip footings be no less 
than
16" in width.  Thus, it would not suprise me that a geotech
might
recommend against a 12" diameter pier.  But, it will depend on
loading,
how the pier will resist the load, and the soil at the site.

HTH,

Scott
Adrian, MI

On Fri, 15 Oct 2004, richard lewis wrote:

 

I am looking for minimum diameter of a drilled pier.  I was
told by
another engineer that is was 18" by either the ACI or the AASHTO
specifications.  I looked in my copy of ACI 336 .3 "Suggested
Design and
Construction Procedure for Pier Foundation" and didn't find any
minimum
diameter requirements.  My copy is outdated.  I don't have any
AASHTO
specifications.

The reason I ask is I was wondering if 12" diameter was standard
practice.  I have never specified that small before.  It seems
to me the
rebar would be too close in a 12" diameter shaft.

I would appreciate any insight on this.

Thanks.

Rich

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