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RE: residential pier foundation design

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I called ACI back East one day and asked how I should apply a certain code
provision to the design of the house I was working on.  They said that the
code was not written for houses but for buildings and therefor the ACI code
was not applicable.  My response to ACI was that someone in Whitter (UBC)
must have then made a mistake because they had inadvertently put houses and
buildings in the same group.  The point is that the code is more often
written with 20 story buildings in mind and those of us who do 90% of the
structures built are left with a guessing game
 
I can confirm that what the code says and what reality is are two different
things.  So try this rationality to hang your hat on.  If the pier's axial
load is less than 10% of its capacity (concrete governing not soil capacity)
then consider it a flexural member and not an axial member.
 
George Richards, P. E.
 
.  

-----Original Message-----
From: Jeff Smith [mailto:smthengr(--nospam--at)sirius.com]
Sent: Wednesday, May 09, 2001 8:48 AM
To: seaint(--nospam--at)seaint.org
Subject: residential pier foundation design


I searched the archive and found a similar question but I could not find
quite what I was looking for. I am working on a small frugal 2 story wood
framed plywood shear wall, pier and grade beam residence on a 2 to 1 slope
right next to the Hayward fault. My design is yielding a hefty foundation
that has inspired me to review my approach.
 
Should the R be adjusted for the foundation design even though the building
is 5.5, ie 2.2 for cantilevered fndn piers or 3.5 for a OMRF pier and grade
beam (unlikely because of ductility requirements in grade beams).
 
My pier axial loads are only about 26K of which 13K is the weight of the
foundation, eq lateral loads are from 4-7K including the grade beams, with
soil fixity from 4 to 10 down from grade..
 
Section 1910.8.4 excludes zones 3 and 4 for reduced effective area Ag,
therefore min long. reinforcing is 1% which is 6#6 in an 18" dia pier
regardless of load.
 
I am unclear on how to design the hoops. I come up with #3 spiral hoops at
3" (or something.) 
 
I have never seen hoops like this on a small residential building. I talked
to the soils engineer and he said he had not seen that before either,
usually just stirrups at 12" with a few extra at the top. Of course the
contractor said the same thing but more emphatically. This seems to me like
an example of standard practice for residential construction that takes
exception to the code, or that I do not understand the code. Any thoughts on
this subject would be greatly appreciated.
 
 

Jeff Smith, S.E. 

 


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